Calibration of the photon spectrometer PHOS of the ALICE experiment
EEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN-EP-2019-0208 February 2019c (cid:13)
Calibration of the photon spectrometer PHOS of the ALICE experiment
ALICE Collaboration ∗ Abstract
The procedure for the energy calibration of the high granularity electromagnetic calorimeter PHOSof the ALICE experiment is presented. The methods used to perform the relative gain calibration, toevaluate the geometrical alignment and the corresponding correction of the absolute energy scale, toobtain the nonlinearity correction coefficients and finally, to calculate the time-dependent calibrationcorrections, are discussed and illustrated by the PHOS performance in proton-proton (pp) collisionsat √ s =
13 TeV. After applying all corrections, the achieved mass resolutions for π and η mesonsfor p T > . / c are σ π m = . ± .
03 MeV/ c and σ η m = . ± . c , respectively. ∗ See Appendix A for the list of collaboration members a r X i v : . [ phy s i c s . i n s - d e t ] N ov alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration The ALICE experiment [1] is one of the four major experiments at the Large Hadron Collider (LHC) atCERN. Its primary goal is the study of the properties of the hot and dense quark–gluon matter createdin ultrarelativistic heavy-ion collisions. This dictates the unique features of the ALICE detector design:ability to register and identify both soft particles, reflecting collective behavior of the hot matter, andhard penetrating probes, i.e. jets, direct photons, etc., carrying information about the inner, hottest partof the created fireball. The ALICE experiment incorporates detectors based on a number of particle iden-tification techniques. The tracking system is able to detect and identify relatively soft charged particleswith transverse momenta p T > −
100 MeV/ c and process high-multiplicity events. ALICE includes anelectromagnetic calorimeter system: the PHOton Spectrometer (PHOS) [1, 2] and the ElectromagneticCalorimeter (EMCal) [3] with the Di-Jet Calorimeter (DCal) [4]. The PHOS calorimeter is designed tomeasure spectra, collective flow and correlations of thermal and prompt direct photons, and of neutralmesons via their decay into photon pairs. This requires high granularity as well as excellent energy andposition resolution. The electromagnetic calorimeter EMCal/DCal is used for the measurement of elec-trons from heavy flavour decays and the electromagnetic component of jets, spectra and correlations ofisolated direct photons and spectra of neutral mesons. This requires a large acceptance but less strictrequirements on the energy and position resolution. In this paper, the methods used for the calibrationof the PHOS detector during the LHC data taking campaigns of 2009 − − π peak, using invariant massdistributions and the minimization of event-by-event variables [6, 7]. The electromagnetic calorimeter(ECAL) of the CMS experiment [8] was pre-calibrated with laboratory measurements of crystal lightyield, and the gain and quantum efficiency of the photodetectors. These were followed by beam testswith high-energy electrons and cosmic-ray muons. The absolute calibration was determined by usingthe Z -boson mass and channel-by-channel relative calibration. The relative calibration involved the mea-surement of transverse energy and the use of ϕ -symmetry, the π and η meson invariant mass fit, and acomparison of the energy measured in the ECAL to the track momentum measured in the silicon trackerfor isolated electrons from W − and Z -boson decays [9, 10]. The longitudinally segmented liquid-argoncalorimeter of the ATLAS experiment [11] was calibrated by using a multivariate algorithm to simulatethe e / γ response [12]. The absolute energy scale was calibrated by using electrons from a large sampleof Z → e + e − decays and validated with J / ψ → e + e − decays.The energy calibration of PHOS includes four mutually dependent aspects: relative gain calibration, ab-solute energy calibration, nonlinearity correction, and time-dependent calibration correction. The PHOSdetector will be briefly described in section 2. The relative gain calibration is presented in section 3,including the pre-calibration using the LED monitoring system and the calibration using the π peakposition which are described in sections 3.2 and 3.3, respectively.Fixing the absolute energy calibration of a calorimeter using the π mass peak suffers from systematicuncertainties due to the geometrical alignment of the calorimeter and the energy scale. Because ofthat the absolute energy calibration is validated using the electron E / p ratio, as described in section4.1, and the detector geometrical alignment is checked as described in section 4.2. The estimation ofthe nonlinearity correction is described in section 5 and the calculation of the time-dependent energycalibration correction is discussed in section 6. The final calibration results are presented in section 7.2alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration The PHOS is a single arm, high-resolution electromagnetic calorimeter which detects and identifiesphotons and electrons in a wide p T range from ∼
100 MeV/ c to ∼
100 GeV/ c at mid-rapidity and, ad-ditionally, provides a trigger in case of a large energy deposition by an energetic particle. The mainparameters of the detector are summarized in Tab. 1. PHOS is located inside the solenoid magnet pro-viding a 0.5 T magnetic field. The TRD and TOF detectors are designed to have windows in front of thePHOS modules to reduce the material budget in front of the PHOS down to 0.2 X [ ? ]. The PHOS issubdivided into four independent units, named modules, positioned at the bottom of the ALICE detectorat a radial distance of 460 cm from the interaction point (IP) to the front surface of crystals as shown inFig. 1. It covers approximately a quarter of a unit in pseudo-rapidity, | η | ≤ . ◦ in azimuthalangle. Its total active area is 6 m . Table 1:
General parameters of the PHOS detector
Coverage in pseudo-rapidity − . ≤ η ≤ . ∆ ϕ = ◦ Distance to interaction point 460 cmModularity Three modules with 3584 and one with 1792 crystalsMaterial Lead-tungstate (PbWO ) crystalsCrystal dimensions 22 × ×
180 mm Depth in radiation length 20 X Number of crystals 12 544Total area 6.0 m Operating temperature − ◦ CThree PHOS modules are segmented into 3584 detection elements (cells) arranged in 56 rows of 64elements each, while the fourth module has 56 rows of 32 elements. A part of a cell matrix is shown inFig. 2, left. The PHOS modules are numbered counterclockwise in Fig. 1 [1]. Each detection elementcomprises a 22 × ×
180 mm lead-tungstate crystal, PbWO [13], coupled to a 5 × AvalanchePhotoDiode (APD Hamamatsu S8664-55) whose signal is processed by a low-noise preamplifier. TheAPD and the preamplifier are integrated in a common body glued onto the end face of the crystal withoptically transparent glue with a high refractive index, see Fig. 2, right. The PbWO was chosen as anactive medium because it is a dense, fast and relatively radiation-hard scintillating crystal. Its radiationlength is only 0.89 cm and its Moli`ere radius is 2.0 cm. It has a broad emission spectrum with bandsaround 420 and 550 nm [13].The light yield of PbWO crystals is relatively low and strongly depends on temperature (temperaturecoefficient of − / ◦ C). In order to increase the light yield by about a factor 3 compared to standardconditions, the PHOS crystals are operated at a temperature of − ◦ C. The energy resolution of a PHOSprototype measured under these conditions in beam tests [14] is described by a parametrization as follows σ E E = (cid:115)(cid:16) aE (cid:17) + (cid:18) b √ E (cid:19) + c (1)where a = .
013 GeV, b = . / and c = . crystals isstabilized with a precision of 0 . ◦ C. Temperature monitoring is based on resistive temperature sensorsof thickness 30 − µ m inserted in the gap between the crystals. For the purpose of temperature sta-bilization, a PHOS module is subdivided by thermo-insulation into “cold” and “warm” volumes. Stripunits, comprising two rows of eight detection elements, are mounted onto the main mechanical assemblypoints in a module. The crystal strips are located in the cold volume, whereas the readout electronicsare located outside, in the warm volume. The APDs belonging to one strip unit, and their associated3alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration Figure 1: [Color online] ALICE cross-sectional view in Run 2, PHOS modules are located at the bottom of thesetup. preamplifiers, provide 2 × F ) circulating through the pipes on the inner panel surfaces. Moisture condensation is prevented bymaking airtight cold and warm volumes ventilated with nitrogen.Every channel in the PHOS detector is monitored with an LED system that is driven by stable currentinjectors [15]. The system consists of LED matrices for each PHOS module, having one LED per PHOScell with controlled light amplitude and flashing rate.The PHOS electronic chain includes energy digitization and trigger logic for generating trigger inputs tothe zero (L0) and first (L1) levels of the ALICE Central Trigger Processor (CTP) [16]. In order to coverthe required large dynamic range from 10 MeV to 100 GeV, each energy shaper channel provides twooutputs with low and high amplification, digitized in separate ADCs. The upper limit of the dynamic4alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationranges in high- and low-gain channels are 5 GeV and 80 GeV, with the ratio of these amplificationsvarying slightly from channel to channel with a mean of approximately 16.8. The gain of each APD canbe set individually, by adjusting the bias voltage through the voltage distribution and control system. Toequalize the energy response of all cells, the bias voltage of each APD can be set to a precision of 0.2 V,which corresponds to a ∼ .
5% gain variation (see Fig. 5, left for more details). The timing informationis derived from an offline pulse-shape analysis.
Photons and electrons hitting an electromagnetic calorimeter produce electromagnetic showers with atransverse profile determined by the Moli`ere radius of the calorimeter material. When the transversecell size of the calorimeter is comparable with the Moli`ere radius, such as in PHOS, the electromagneticshower is developed in several adjacent cells around the impact point. The group of cells with commonedges, containing the electromagnetic shower generated by a photon, is referred to as a cluster (see sec.4.5.2 of [2]). The sum of energies deposited by the shower in each cell of the cluster, is the measuredphoton energy [17]. With the PHOS granularity, the energy deposited in the central cell of the cluster isabout 80% of the total cluster energy.The amplitude of the signals measured in the cells of the cluster is proportional to the deposited energyin the cells. A set of calibration procedures is necessary to convert these data to an appropriate energyscale. Relative energy calibration means equalization of the response of all channels to the same energydeposition. In the case of PHOS, calibration at the hardware level via adjusting the APD bias voltage iscomplemented by refinement of the calibration parameters in an offline analysis. In order to ensure theuniformity of trigger response over the PHOS acceptance, the amplification in all channels was adjustedto make the trigger efficiency response turn-on curve as sharp as possible. This adjustment was performedonce during the PHOS commissioning in LHC Run 1 and just before the start of the LHC Run 2 datataking period. The final calibration is done in an offline analysis described hereafter in detail. In order todisentangle calibration effects from effects related to cluster overlaps in the high occupancy environmentof heavy-ion collisions, the calibration is performed in low occupancy events provided by pp collisions.At first, two approaches were tested: calibration using the Minimum-Ionizing Particle (MIP) peak andequalization of mean energies in each channel. The minimum ionization signal of charged particles inthe PHOS detector has a most probable value of about 250 MeV which is close to the lower end of thedynamic range. The calibration based on the MIP peak has a poor accuracy because of several effectssuch as relatively low number of counts of charged particles per cell, low signal-to-background ratio
Figure 2: [Color online] Left: Part of a cell matrix of one module; Right: A detector element comprising a PbWO crystal, APD photodetector and preamplifier. π peakequalization described below was deployed in all subsequent papers [19–23].Our final strategy of the PHOS relative calibration is based on APD gain equalization as a pre-calibration(see section 3.2) and the π peak adjustment as a final step (see section 3.3). HG/LG ratio
15 16 17 18 19 N u m be r o f c e ll s ( c oun t s ) ALICEPHOS mean = 16.76RMS = 0.46
Figure 3:
The ratio of high-to-low gains, for all cells.
The LED monitoring system, with its capability to emit signals at high rate and with variable amplitudescovering the whole dynamic range of the PHOS, was used to measure the high-to-low gain ratio. The gainratio distribution for all active PHOS cells is shown in Fig. 3 and spans from 15 to 18 with an average ofabout 16.8. The gain ratio is used for high energy amplitudes exceeding the high-gain dynamic range. Inthis case, the energy is the product of the high-gain calibration parameter and the high-to-low gain ratio.The high-to-low gain ratio is stable and does not need to be frequently measured and updated.The ratio of high-to-low gain is defined by the electronics components of the amplifiers and may varyfrom channel to channel. Therefore it is considered as one of the calibration parameters to be determined.The calibration methods discussed in the section 3.3 of this paper are based on data collected with beam,and ensure a good calibration of high-gain channels within the high-gain dynamic range, E < π peak adjustment method described in section 3.3,because of the limited statistics of high-energy clusters. Therefore the ratio of high-to-low gain has tobe measured independently using signals of amplitudes which are detected simultaneously in both high-and low-gain channels. Each APD has a particular gain dependence on bias voltage. At low bias voltages, the APD gain isassumed to be unity. The APD gain is calculated as the ratio of the measured amplitude at a given voltageto a reference amplitude at 20 V where the dark current in the APD is negligible. The dependenceof the APD gain on the bias voltage was measured using the PHOS LED monitoring system, whose6alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationprogrammable light output was shown to be very stable over several hours, a period far longer thannecessary for gain measurements. The amplitude distribution from the LED flash is measured at severalvalues of APD bias voltage in the range from 20 to 395 V. Figure 4 shows the LED amplitude for differentvoltages for one example channel.
Amplitude (ADC counts)
100 200 300 400 500 600 700 N u m be r o f e v en t s ALICE
PHOS LED monitoring system= 20 V
APD
U =100 V
APD
U =200 V
APD
U =250 V
APD
U =300 V
APD U Figure 4: [Color online] The amplitude of the LED peak for different APD bias voltages, for one example channel.
Figure 5 (left) shows the gain-voltage dependence for three channels illustrating the spread of the gainsat a given voltage. An APD gain of 29 was set for all channels in order to provide the designed dynamicrange of the energy measurement in PHOS. The bias voltages are required to cover a range from from290 to 395 V, as shown in Fig. 5 (right). (Volts)
APD U AP D ga i n ALICEPHOS
Channel 1Channel 2Channel 3 (Volts)
APD U
300 320 340 360 380 N u m be r o f c e ll s ( c oun t s ) ALICEPHOS
Figure 5: [Color online] Left: The dependence of the APD gain on applied bias voltage, for three differentchannels. Typical and two extreme cases are presented. Right: The distribution of the APD bias voltages, for allPHOS cells, for an APD gain of 29.
After the equalization of the APD gains, the calibration needs to be further refined to take into accountthe specific light yield of the different crystals. However, the spread of light yields of the differentPbWO4 crystals is about 12% [13], which is relatively small compared to the initial pre-calibration, andhas been neglected. The APD gain equalization can thus be considered as a first step towards the energycalibration based on physics signals from collision events such as the π peak.The invariant mass of photon pairs is constructed as follows: m γγ = (cid:113) E γ , E γ , ( − cos θ ) , (2)7alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration ) c (GeV/ gg m ) c C oun t s / ( M e V / ALICE=13 TeV s pp c – = 113.8 æ m Æ c – = 13.8 m s Figure 6: [Color online] Invariant mass distribution of cluster pairs after APD gain equalization in pp collisionsat √ s =
13 TeV for p T > . / c . The red curve is a fit of the spectrum with the sum of a Gaussian and asecond-order polynomial function. The green dashed line is the background contribution only. where E γ , i is the energy of the reconstructed photon i , and θ is the opening angle between the twophotons. The invariant mass distribution of cluster pairs detected in PHOS was measured in pp collisions,at √ s =
13 TeV, with a cut on the cluster pair transverse momentum of p T > . / c . Fig. 6 showsthe invariant mass distribution after APD gain equalization. The choice of the low- p T cut is driven bymaximizing the signal-to-background ratio and minimizing the energy nonlinearity effects which will bediscussed in Section 5. A clear π peak above the combinatorial background is observed. The invariantmass distribution is fitted in the range 35 −
210 MeV/ c with the sum of a Gaussian and a second orderpolynomial. The extracted π peak position (cid:104) m (cid:105) ≈ . ± . c is ∼
15% lower than the PDGvalue [24] and its width σ m ≈ . ± . c is approximately 3 times larger than the expectedresolution of 5.5 MeV/ c for an ideally calibrated PHOS as described in GEANT-based Monte Carlosimulations [17]. However, these values are an acceptable starting point for the final relative PHOScalibration based on π peak equalization described in the following section. π peak position The calibration procedure calculates the calibration coefficient α i relating the energy deposition E dep andthe measured response amplitude, A , with E dep = α i · A , for each detector channel. To find the coefficients,the di-photon invariant mass distribution is constructed, see Eq. (2). One of the two photons must directlyhit the detector channel under consideration. The second photon can be anywhere in PHOS.The invariant mass distribution shows a peak corresponding to the π meson at m i with some mass shiftdue to miscalibration. A correction to the calibration coefficient, which relates the measured amplitude A and corrected energy E corr as E corr = α i · c i · A , is defined by the following equation: c i = (cid:18) m π m i (cid:19) n , (3)where m π is the true neutral pion mass and n > α i obtained at iteration j being updated to α j + i = α ji · c i , until no furtherimprovement of a calibration is found. If we assume that the calibration coefficients (for all cells wherepartner photons are registered) fluctuate around some mean value, and therefore their energies are correcton average, then the shift of the peak position can be attributed to the miscalibration of the currentcell. From Eq. (2) E , γ = m / m / ( E , γ ( − cos θ ) , the correction coefficient for the current cell i is c i = E c orrect / E i = m DG / m and one can expect that the most appropriate power is n =
2. However, thisassumption is not completely true. To illustrate this, the procedure is applied to a toy model implementingseveral values of n as described in the next section. 8alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration The toy model describes the influence of the simultaneous calibration of different cells of a calorimeter.In a real calorimeter a photon cluster includes a cell with a dominant energy deposition plus a fewadditional cells. The simplified model assumes that the entire photon energy is deposited in one cell ofa calorimeter. In the model, the calorimeter covers a pseudorapidity | η | < ×
100 cells in the ϕ and η directions. Each cell has an independent calibrationcoefficient which initially is randomly assigned according to a Gaussian distribution with mean 1 and awidth of 20%.The particle generator is tuned to produce neutral pions with a flat rapidity and azimuthal distribution anda realistic p T spectrum as measured in pp collisions at √ s = π mesons areforced to decay into photon pairs. The photon energies are smeared according to Eq. (1). A cut on theminimal reconstructed photon energy E γ > E min = . σ c , defined as the RMS of the differencebetween estimated and true calibration coefficients α i − α true i for all cells of the toy simulation, versusiteration number. All calibration procedures start from the same initial de-calibration of cells and use Iteration c s - - ALICE simulation E s =2, n E s =1, n E s =1.6, n E s =2, 2 n E s =1, 2 n E s =1.6, 2 n Figure 7: [Color online] Study using a toy Monte Carlo simulation of the convergence of the iterative calibrationprocedure based on equalization of the π peak position. The residual de-calibration σ c is shown as a function ofthe iteration number. Two values of calorimeter energy resolution are considered, standard ( σ E ) and twice as poor(2 σ E ). the same sample of π mesons. The final precision of the calibration depends on the accuracy of thereconstructed pion peak position for a typical cell, which in turn depends on the peak width (defined bythe energy and position resolution) and the available statistics. In the model, the number of the simulatedpions is defined by a requirement to have 10 reconstructed photons per cell after a p T cut of 1 . / c on the reconstructed photon pairs. This corresponds to the calibration using real data, as described insection 3.3.2.To study the dependence of the final calibration accuracy on the energy resolution, the default energyresolution of the toy calorimeter was decreased by a factor of 2; these simulations are marked as 2 σ E .For powers n <
2, the residual de-calibration stabilizes at values corresponding to the final precision ofthe calibration. In the case of n =
2, the residual de-calibration rapidly decreases at the first iteration, butafter 2 − n < n , the RMS of the de-calibration distribution is studied as a functionof iteration number for several values of n , see Fig. 8 (left), and versus n for several iterations (right).For large values of n , only a few iterations are needed to reach saturation. However, better accuracy is9alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationobtained for lower values of n . Since each iteration, in an analysis with real data, is very time-consumingwe chose a value of n = . − Iteration c s - - - =1.0 n =1.1 n =1.2 n =1.3 n =1.4 n =1.5 n =1.6 n =1.7 n =1.8 n =1.9 n =2.0 n ALICE simulation n c s - - - It 1It 2It 3It 4It 5It 6It 7It 8It 9It 10
ALICE simulation
Figure 8: [Color online] Left: Residual de-calibration in the toy model simulation with default energy resolutionversus iteration number for several values of power n . Right: Residual de-calibration versus power n for severaliterations. π calibration using pp collision data The procedure described above is used in the final step of the calibration of the PHOS detector. Thecalibration is performed using physics data from pp collisions at √ s =
13 TeV recorded in 2017. Thesample contains 7 . · minimum bias (MB) events and 5 · events recorded with the PHOS L0trigger [25, 26], corresponding to an integrated luminosity of L int =
12 nb − and 5 . − , respectively.The calibration correction is only applied to the central cell of a cluster. Clusters that are close to a deadcell are not removed. Instead, the standard approach is extended to such clusters. As a result, the showerleakage to bad cells is compensated by higher calibration coefficients in adjacent good cells. A set ofcuts are applied: on the minimum number of cells in a cluster, N cells >
2, the minimum cluster energy E clu > E min = . D = ∑ w i (( x i − x ) + ( z i − z ) ) / w > . , (4)where x i , z i are the coordinates of the cell i , x , z are coordinates of the cluster center of gravity in thePHOS plane and the weights w = ∑ w i , with w i = max ( , log ( E i / E clu ) + . ) are calculated using theenergy deposition in a cell E i and the total cluster energy E clu . These cuts are used to select photonclusters and reject rare events induced by hadron interactions directly in the APD which result in dispro-portionally high signals [27]. A minimum pion transverse momentum cut p T > . / c is imposed toreduce the combinatorial background.At each iteration the correction for the calibration coefficients is calculated using power n = .
6. Figure 9shows that about 3 iterations are sufficient to reach an almost final calibration. This is in good agreementwith the predictions of the toy Monte Carlo. The width of the peak in modules 2 and 3 is close towhat is expected from Monte Carlo simulations by taking into account the PbWO response and idealcalibration. In modules 1 and 4, the width is larger because of a batch of front-end electronics cards withsomewhat higher noise characteristics. Fixing the π peak position to the PDG value is not sufficient to fix the absolute energy scale of thecalorimeter. As shown in Eq. (2), the measured mass depends both on the cluster energy and on theopening angle. 10alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration Iteration ) c ( G e V / ps - · Module 1Module 2Module 3Module 4
ALICE=13 TeV s pp Figure 9: [Color online] Dependence of the π peak width on the iteration number for photon pairs with p T > . / c in four PHOS modules. The reconstructed opening angle is dependent on the distance of the detector to the IP. An evaluationand check of the detector geometry is discussed in section 4.2. To study possible biases to the absoluteenergy scale, an independent cross-check was performed using the E / p ratio using identified electrons.The electrons were identified using the ALICE central tracking system, consisting of the Inner TrackingSystem (ITS) and the Time Projection Chamber (TPC) [28, 29], and by matching tracks with PHOSclusters. Using electrons for the absolute energy calibration of an electromagnetic calorimeters is a widely usedapproach [10]. In the PHOS, electrons and photons effectively deposit all their energy in the calorimeter.It is possible to compare the energy measured in the calorimeter with the momentum of an electronreconstructed in the tracking system upstream of the calorimeter. There are two advantages of thisapproach compared to the calibration using the π mass peak. First, only single clusters are consideredand no iterative procedure is necessary. Second, the method does not depend on the exact position of thecalorimeter. The geometrical mis-alignment, appearing in the calculation of the opening angle θ in theEq. (2), is not mixed with the energy calibration. The disadvantages of this method concern the limitednumber of reconstructed electrons and the effects of the material budget in front of the calorimeter.Furthermore, this method can be used as a cross-check for the calibration using the π mass peak.The E / p calibration was carried out using pp collisions at √ s =
13 TeV in 2017, i.e. the same data setas that used for the π calibration. Charged tracks were reconstructed with the ALICE central trackingsystem. Figure 10 shows the E / p ratio distribution for two ranges of cluster energies in a PHOS module. E is the energy of the cluster in the calorimeter and p is the reconstructed track momentum. Electronscan be identified in the region around E / p = 1 independently from the d E /d x method provided by thetracking system.An optional cut is applied on the cluster dispersion, using Eq. (4), that corresponds to the expectedelectromagnetic shower transverse size. These E / p distributions are marked as ‘EM clusters’ in Fig. 10.This reduces the background from hadrons both at low and high p T , and keeps the efficiency close to100%.To improve the accuracy of the peak reconstruction, the signal-to-background ratio was further improvedby selecting electrons that were identified through their specific ionization energy loss, d E /d x , in theTPC [28, 30]. This method is efficient at low p T . However, in the region of relativistic rise for pions, p T (cid:38) / c , a separation of pions and electrons becomes increasingly difficult. The available statisticsis not sufficient to perform a channel-by-channel calibration for all 12 544 channels with good accuracy.11alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration E/p ) ( c oun t s ) E / p / d ( N d · all tracks, all clustersall tracks, EM clusters tracks, all clusters – e tracks, EM clusters – e ALICE=13 TeV s pp < 2.5 GeV clu E E/p ) ( c oun t s ) E / p / d ( N d · all tracks, all clustersall tracks, EM clusters ALICE=13 TeV s pp < 15 GeV clu E
10 <
Figure 10: [Color online] Distribution of the cluster energy to track momentum, E / p ratio, for two ranges ofcluster energies E clu in one PHOS module. A peak around unity due to the electron contribution is visible. (GeV) E æ E / p Æ DataMonte Carlo
ALICE=13 TeV s pp ) c (GeV/ T p D a t a / M C (GeV) E E / p s DataMonte Carlo
ALICE=13 TeV s pp ) c (GeV/ T p D a t a / M C Figure 11: [Color online] Mean (left) and width (right) of the E / p peak position in data and MC for electroncandidates. Figure 11 shows the E / p peak position and the peak width, after fitting the E / p distributions with thedispersion cut applied, as a function of cluster energy. The data are from the two middle PHOS modules.These modules have the best energy resolution. Note that the non-linearity corrections, discussed insection 5 are applied in this analysis for comparisons with Monte Carlo simulations. At high p T , themean is close to unity, but gradually decreases towards smaller p T , reflecting an increased relative energyloss of lower energy electrons. Figure 11 also shows the results from Monte Carlo simulations withthe PYTHIA8 event generator [31] using the standard ALICE software framework for the analysis ofreal data. The simulation includes a remaining small mis-calibration describing an inaccuracy of ourcalibration to reproduce the π mass peak position and width and their dependence on p T . The agreementis better than ∼ .
2% providing an independent estimate of the absolute energy scale precision in thePHOS.
The precise measurement of the distance between the IP and the calorimeter surface, R , is a difficulttask because of the detectors installed in front of PHOS. Uncertainties in the measurement of R directlytranslate to uncertainties in the energy scale. 12alibration of the photon spectrometer PHOS of the ALICE experiment ALICE CollaborationEquation (5) shows the dependence on R and the distance between the clusters, L , in the calorimeterfor the calculation of the two-photon invariant mass: m γγ = √ E E | sin ( θ / ) | ≈ √ E E L R , (5)The alignment of the PHOS was measured via the photogrammetry procedure [32]. In addition, an inde-pendent estimate of the PHOS alignment is performed by matching tracks reconstructed in the trackingsystem with clusters in PHOS. To study the alignment it is convenient to use the local coordinate systemof the PHOS module where z is the coordinate along the beam and x is the coordinate perpendicularto the beam direction. The alignment in the z and x directions is straightforward, unlike checks for theradial distance. true RR PHOS z PHOS z track z q IP B
Figure 12: [Color online] An illustration of the dependence of (cid:104) dz (cid:105) , from equation (6), with z , in a radially shifteddetector. The magnetic field of 0.5 T is along the z direction. Figure 12 shows the geometry and variables used to establish the radial distance of the PHOS from theIP. The difference between the z coordinate of the reconstructed cluster position in the calorimeter, z PHOS ,and the point of the track extrapolated to the surface of the calorimeter, z track , through the ratio of true( R true ) and expected ( R ) radial distances is: dz = z PHOS − z track = z PHOS − R tan θ = z PHOS (cid:18) − RR true (cid:19) . (6)In this analysis, the depth of the shower maximum for a photon is used as a reference point [17]. Acorrection for this depth is introduced to the cluster center of gravity so that the x and z coordinatescorrespond to those of the photon at the front surface of PHOS. In contrast to photons and electrons,because of the large nuclear interaction length of the EM calorimeter, the center of gravity of a hadronicshower is almost uniformly distributed in the depth of the calorimeter and therefore hadronic tracksare not suitable for such calibration. Electron showers reach their shower maximum about one unitin radiation length X earlier than photons. The difference between the photon and electron clustercoordinate in the z direction can be written as: δ z e = − X sin (cid:18) arctan z PHOS R true (cid:19) . (7)Figure 13 (left) shows the (cid:104) dz (cid:105) versus z dependence. The data from the two modules are very similar,with the same slope of − . · − . There are some oscillations around the linear dependence. The slope13alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationis slightly larger than the expected slope, B e , from Eq. (7), of − . · − . This difference correspondsto ∼ (cid:104) dx (cid:105) versus x analysis that are not present for the (cid:104) dz (cid:105) versus z analysis. Figure 13 (right) shows the (cid:104) dx (cid:105) versus x dependence. The data for positive and negative charges have similar slopes, but oppositeoffsets, because of the track bending in the magnetic field. This results in different incident angles, forelectrons and positrons, with respect to photons. These angles strongly depend on the particle p T , makingthis analysis much more complicated than the (cid:104) dz (cid:105) versus z study. Therefore, only the (cid:104) dz (cid:105) versus z dataare used in the final PHOS alignment procedure. (cm) z - - ( c m ) æ z d Æ - - -2 – Module 2: (-0.23 -2 – Module 3: (-0.23
ALICE=13 TeV s pp, (cm) x - - - ( c m ) æ x d Æ - -2 – : (1.96 + Module 2, e -2 – : (1.71 - Module 2, e -2 – : (1.87 + Module 3, e -2 – : (1.89 - Module 3, e
ALICE=13 TeV s pp, Figure 13: [Color online] Dependence of the mean distance between track extrapolation to the PHOS surface andcluster position in the cluster coordinate on the PHOS plane along (left) and perpendicular (right), to the beam andmagnetic field direction. In the left plot contributions of electrons and positrons are combined. The dependenciesare fitted with linear functions and the resulting slopes are shown in both legends.
There are several effects that may influence the linearity of PHOS energy measurement. At low energies,light attenuation in the crystals, electronic noise, electronic thresholds and amplitude digitization areimportant. At high energies, shower leakage contributes to a nonlinear response. For the physics analysisit is sufficient to reproduce the observed nonlinearity of the detector in the Monte Carlo simulations, butpractically, it is more convenient to correct real data for the nonlinearity in order to reduce the massresolution of a neutral meson peak in wide p T bins.The nonlinearity is corrected through a recalculation of the cluster energy E by the following parameter-ization: E corr = (cid:26) aE + b √ E + c + d / √ E + e / E , E ≤ E α E + β √ E , E > E (8)where free parameters a , b , c , d , e , E are chosen to provide a p T -independent reconstructed neutral pionmass m π in pp collisions at √ s =
13 TeV and parameters α and β are fixed to ensure a smooth functionat the point E = E .Figure 14 shows the ratio of the PDG π mass to the measured π peak position as a function of meanphoton energy, E γ . The data were restricted to symmetric π decays with | E γ , − E γ , | < . ( E γ , + E γ , ) .A fit with the function E corr ( E ) / E (Eq. 8) is shown by the red curve.However, this method is not reliable at very low energies where systematic uncertainties for the π signalextraction are large because of the limited PHOS acceptance. The same is true at high p T where photons14alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration (GeV) g E m ea s m / p m ALICE=13 TeV s pp decays p Symmetric decays p Fit to sym. Final non-linearity correction
Figure 14: [Color online] Estimation of PHOS nonlinearity using symmetric π decays defined by | E γ , − E γ , | < . ( E γ , + E γ , ) . Data fit with function (8). The final tuned nonlinearity is shown with a dashed curve. from symmetric decays start to merge into one cluster. To improve the nonlinearity parameterization, aset of invariant mass distributions were calculated as a function of p T , without requiring symmetric de-cays. Each mass distribution was corrected for nonlinearity with different sets of nonlinearity parameters( a , b , c , d , e , E ). Figure 15 (left) shows examples of the dependence on d and e , on the peak positionversus p T . Note that parameter a sets an absolute normalization and can be factorized in this analysis.To find the best set of parameters, a fit of the peak p T -dependence with a constant function is performedin the range 0 . −
25 GeV / c . The resulting χ value for each set of parameters is shown in Fig. 15 (right).In this plot we fix optimal values of parameters a , b , c , E and vary only parameters d , e . The optimal set,obtained by minimizing χ , is ( a = . ± . b = − . ± . / , c = . ± .
001 GeV, d = − . ± . / , e = . ± . and E = . ± .
01 GeV). The nonlinearitycorrection corresponding to this set is shown with a black dashed line in Fig. 14. This parameter set,corresponding to the filled red circles in the left plot of Fig. 15, is used in the offline reconstruction. ) c (GeV/ T p ) c ( G e V / m ALICE=13 TeV s pp d=-0.472, e=0.125d=-0.477, e=0.125d=-0.472, e=0.120d=-0.477, e=0.120d=-0.475, e=0.123 / ND F c ) d (GeV - - - ) e ( G e V Figure 15: [Color online] Left: the π peak position as a function of the transverse momentum for several valuesof nonlinearity parameters ( d , e ), with default values for a , b and c . Right: the deviation from a constant value ofthe π peak position expressed in χ / NDF as a function of the nonlinearity parameters ( d , e ). The light yield from the crystals, and the gain in the APDs, are strongly temperature dependent [2, 33]. Tominimize this dependency on the PHOS energy scale, the PHOS crystal matrices were thermo-stabilizedto within 0 . ◦ C. This temperature variation results in a change of about 0.6% in light yield and APDgain. Another effect that may influence the long-term stability of the amplitude measurement in thePHOS detector is the crystal transparency dependence on the radiation dose. A run-dependent calibration15alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationcorrection, common for all channels in each PHOS module, was implemented to account for all theseeffects. In order to estimate this correction, the standard calibrations and corrections were applied. Foreach run, the mean value of the π mass peak in each module was extracted, using only photon pairs inthat module.The correction is calculated using the data sample collected with the PHOS L0 trigger since it has betterstatistics at high p T , where the signal-to-background ratio is larger. Figure 16 shows the reconstructed π mass peak versus run number, for 400 sequential runs, from pp collisions at √ s =
13 TeV, recordedduring 3 months of data taking from June to September 2017, with stable running conditions. The dataare for the two middle PHOS modules. These have the largest acceptance and the best energy resolution.On average the peak position is stable to within ∼ c in both modules, but reveals several corre-lated and uncorrelated trends in these two modules. Correlated trends are related to the powering of thePHOS front-end electronics in both modules, and therefore to the variation of the heat deposition andtemperature of the crystal matrix. Uncorrelated trends may have different reasons: switching on or offisolated front-end cards, formation of ice jams in the cooling pipes of the cooling system, etc. There is novisible global correlated trend of a decrease of the peak position in all modules, which would indicate aradiation damage in the crystals and a decrease of their transparency with time. The total integrated dosein the PHOS crystals accumulated during 3 years of running with pp, p − Pb and Pb − Pb beams duringRun 2, is estimated to be less than 0.1 Gy. The total hadron fluence was about 2 · cm − . Run index ) ( G e V / c p m Module 2
ALICE=13 TeV s pp
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Run index ) ( G e V / c p m Module 3
Figure 16: [Color online] Example of the dependence of the π peak position on the run number for 400 sequentialruns recorded during 3 months of the 2017 data taking campaign. In the calibration procedure the mean value of the peak position over the whole period is calculated anddeviations with respect to this value are estimated. If the peak position in a module is known with uncer-tainty better than 1 MeV, all calibration coefficients in a module are corrected by the ratio m mean / m run . Ifa run is too short and fitting is not possible, the mean value over the whole period is used. The invariant mass spectrum of cluster pairs, after applying all calibration corrections, is shown in Fig. 17in the region of the π (left) and η -meson (right) peaks. All four PHOS modules were considered. Itreveals a much narrower π peak and better signal-to-background ratio compared to the pre-calibratedresult shown in Fig. 6. The improved calibration allows to resolve details of the shape of the π peak,therefore the mass distribution is fitted with a sum of a Crystal Ball function [34] for the peak description,and a polynomial of the second order for the combinatorial background. For the η meson a sum of Gaus-sian and second order polynomial is used. Both the π and η meson peak positions are consistent withtheir PDG values of m π = .
98 MeV / c and m η = . / c within the statistical uncertaintiesshown in Fig. 17. The agreement of the η peak position with the PDG values provides a cross-check16alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaborationof the correctness of the description of the PHOS alignment in the ALICE setup and therefore, of theabsolute energy calibration. ) c (GeV/ gg m ) c C oun t s / ( M e V / · ALICE=13 TeV s pp c – = 134.95 æ m Æ c – = 4.56 m s ) c (GeV/ gg m ) c C oun t s / ( M e V / · ALICE=13 TeV s pp c – = 547.7 æ m Æ c – = 15.3 m s Figure 17: [Color online] Invariant mass distributions of cluster pairs for p T > . / c in the π (left) and η (right) mass region after calibration with per-channel π peak equalization. For the π data, the solid curveshows the fitting function using the sum of the Crystal Ball and a polynomial function. For the η data, the solidcurve shows the fit function composed of a Gaussian and a polynomial function. The dashed lines represent thebackground contributions in both plots. Figure 18 shows the peak positions and peak widths of the π and η mesons as a function of transversemomentum. The width of the π peak reaches a minimum value σ ≈ / c at p T = − / c .The reconstructed mass remains approximately constant up to p T ∼
25 GeV / c , and increases with p T af-terwards. This is due to a considerable fraction of overlapping cluster pairs. The reconstruction softwarehas a bias towards clusters that are better separated due to fluctuations in the energy deposition, thusincreasing the extracted pion mass. This effect is not corrected for, instead MC simulations are used toaccount for it in the efficiency calculations. In the case of the η meson, the peak position is stable sincethe influence of the overlap in this case only appear above p T ∼
80 GeV / c . ) c (GeV/ T p ) c ( G e V / p m DataPDG value ALICE=13 TeV s pp ) c (GeV/ T p ) c ( G e V / ps ) c (GeV/ T p ) c ( G e V / h m DataPDG value ALICE=13 TeV s pp ) c (GeV/ T p ) c ( G e V / hs Figure 18: [Color online] Peak position and width for π (left) and η mesons (right) as a function of transversemomentum. Vertical error bars represent fit uncertainties. In this paper all the steps of the calibration of the ALICE electromagnetic calorimeter PHOS from a com-pletely uncalibrated state to the final set of calibration parameters are presented. The results are equiva-lent to Monte Carlo simulations with an ideally calibrated detector. Pre-calibration, with the equalizationof the photodetector gains, is provided by the use of the monitoring system with light-emitting diodes.17alibration of the photon spectrometer PHOS of the ALICE experiment ALICE CollaborationThis preliminary calibration serves as a starting point for the energy calibration based on adjusting thereconstructed π mass from data collected in high-luminosity proton-proton collisions. The calibrationcoefficients averaged over a large period of data taking are obtained with this relative calibration proce-dure.The absolute energy scale is verified by analyzing pp data with electron tracks reconstructed in theALICE central tracking system and matched with PHOS clusters. An accurate correction of the PHOSgeometrical alignment in the radial direction, also achieved using electron tracks, is necessary for theabsolute energy calibration. Further refining of the calibration is performed by correcting the PHOSresponse for energy nonlinearity effects. Finally, the calibration is corrected for time variations in per-formance due to changes in running conditions and power dissipation in the front-end electronics of thedetector.The resulting time-dependent calibration parameters of the PHOS spectrometer ensure a stable responseand the best possible resolution of the detector over a large time span. After applying all calibration stepsin the reconstruction of pp collision data at √ s =
13 TeV, the π and η meson peak positions are close totheir PDG mass values over a wide p T range. The achieved mass resolution is σ π m = . ± .
03 MeV/ c and σ η m = . ± . c (for p T > . c ). Acknowledgements
The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable con-tributions to the construction of the experiment and the CERN accelerator teams for the outstandingperformance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources andsupport provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration.The ALICE Collaboration acknowledges the following funding agencies for their support in building andrunning the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute)Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia;Austrian Academy of Sciences, Austrian Science Fund { FWF } : [M 2467-N36] and Nationalstiftung f¨urForschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technolo-gies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Cient´ıficoe Tecnol´ogico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudose Projetos (Finep) and Fundac¸ ˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP), Brazil; Min-istry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC)and Ministry of Education of China (MOEC) , China; Croatian Science Foundation and Ministry of Sci-ence and Education, Croatia; Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN),Cubaenerg´ıa, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Repub-lic; The Danish Council for Independent Research — Natural Sciences, the Carlsberg Foundation andDanish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland;Commissariat `a l’Energie Atomique (CEA), Institut National de Physique Nucl´eaire et de Physique desParticules (IN2P3) and Centre National de la Recherche Scientifique (CNRS) and Rl´egion des Pays de laLoire, France; Bundesministerium f¨ur Bildung, Wissenschaft, Forschung und Technologie (BMBF) andGSI Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Germany; General Secretariat for Researchand Technology, Ministry of Education, Research and Religions, Greece; National Research, Devel-opment and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE),Department of Science and Technology, Government of India (DST), University Grants Commission,Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; IndonesianInstitute of Science, Indonesia; Centro Fermi - Museo Storico della Fisica e Centro Studi e RicercheEnrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Scienceand Technology , Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Sci-ence (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology18alibration of the photon spectrometer PHOS of the ALICE experiment ALICE Collaboration(MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnolog´ıa, through Fondo de Coop-eraci´on Internacional en Ciencia y Tecnolog´ıa (FONCICYT) and Direcci´on General de Asuntos delPersonal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek(NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technol-ogy for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Cat´olicadel Per´u, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; KoreaInstitute of Science and Technology Information and National Research Foundation of Korea (NRF),Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Min-istry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for NuclearResearch (JINR), Ministry of Education and Science of the Russian Federation, National Research Cen-tre Kurchatov Institute, Russian Science Foundation and Russian Foundation for Basic Research, Russia;Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National ResearchFoundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallen-berg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; NationalScience and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT)and Office of the Higher Education Commission under NRU project of Thailand, Thailand; TurkishAtomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Scienceand Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the UnitedStates of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP),United States of America. References [1]
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26 ,58 , P. Cortese ,M.R. Cosentino , F. Costa , S. Costanza , J. Crkovsk´a , P. Crochet , E. Cuautle , L. Cunqueiro ,D. Dabrowski , T. Dahms
103 ,116 , A. Dainese , F.P.A. Damas
113 ,136 , S. Dani , M.C. Danisch ,A. Danu , D. Das , I. Das , S. Das , A. Dash , S. Dash , A. Dashi , S. De
85 ,49 , A. De Caro , G. deCataldo , C. de Conti , J. de Cuveland , A. De Falco , D. De Gruttola
10 ,30 , N. De Marco , S. DePasquale , R.D. De Souza , H.F. Degenhardt , A. Deisting
104 ,102 , K.R. Deja , A. Deloff ,S. Delsanto , P. Dhankher , D. Di Bari , A. Di Mauro , R.A. Diaz , T. Dietel , P. Dillenseger ,Y. Ding , R. Divi`a , Ø. Djuvsland , A. Dobrin , D. Domenicis Gimenez , B. D¨onigus , O. Dordic ,A.K. Dubey , A. Dubla , S. Dudi , A.K. Duggal , M. Dukhishyam , P. Dupieux , R.J. Ehlers ,D. Elia , H. Engel , E. Epple , B. Erazmus , F. Erhardt , A. Erokhin , M.R. Ersdal , B. Espagnon ,G. Eulisse , J. Eum , D. Evans , S. Evdokimov , L. Fabbietti
103 ,116 , M. Faggin , J. Faivre ,A. Fantoni , M. Fasel , L. Feldkamp , A. Feliciello , G. Feofilov , A. Fern´andez T´ellez ,A. Ferrero , A. Ferretti , A. Festanti , V.J.G. Feuillard , J. Figiel , S. Filchagin , D. Finogeev ,F.M. Fionda , G. Fiorenza , F. Flor , S. Foertsch , P. Foka , S. Fokin , E. Fragiacomo ,A. Francisco , U. Frankenfeld , G.G. Fronze , U. Fuchs , C. Furget , A. Furs , M. Fusco Girard ,J.J. Gaardhøje , M. Gagliardi , A.M. Gago , K. Gajdosova
88 ,37 , A. Gal , C.D. Galvan , P. Ganoti ,C. Garabatos , E. Garcia-Solis , K. Garg , C. Gargiulo , K. Garner , P. Gasik
103 ,116 , E.F. Gauger ,M.B. Gay Ducati , M. Germain , J. Ghosh , P. Ghosh , S.K. Ghosh , P. Gianotti , P. Giubellino
104 ,58 ,P. Giubilato , P. Gl¨assel , D.M. Gom´ez Coral , A. Gomez Ramirez , V. Gonzalez ,P. Gonz´alez-Zamora , S. Gorbunov , L. G¨orlich , S. Gotovac , V. Grabski , L.K. Graczykowski ,K.L. Graham , L. Greiner , A. Grelli , C. Grigoras , V. Grigoriev , A. Grigoryan , S. Grigoryan ,J.M. Gronefeld , F. Grosa , J.F. Grosse-Oetringhaus , R. Grosso , R. Guernane , B. Guerzoni ,M. Guittiere , K. Gulbrandsen , T. Gunji , A. Gupta , R. Gupta , I.B. Guzman , R. Haake
145 ,34 ,M.K. Habib , C. Hadjidakis , H. Hamagaki , G. Hamar , M. Hamid , J.C. Hamon , R. Hannigan ,M.R. Haque , A. Harlenderova , J.W. Harris , A. Harton , H. Hassan , D. Hatzifotiadou
53 ,10 ,P. Hauer , S. Hayashi , S.T. Heckel , E. Hellb¨ar , H. Helstrup , A. Herghelegiu , E.G. Hernandez ,G. Herrera Corral , F. Herrmann , K.F. Hetland , T.E. Hilden , H. Hillemanns , C. Hills ,B. Hippolyte , B. Hohlweger , D. Horak , S. Hornung , R. Hosokawa , J. Hota , P. Hristov , C. Huang , C. Hughes , P. Huhn , T.J. Humanic , H. Hushnud , L.A. Husova , N. Hussain ,S.A. Hussain , T. Hussain , D. Hutter , D.S. Hwang , J.P. Iddon , R. Ilkaev , M. Inaba ,M. Ippolitov , M.S. Islam , M. Ivanov , V. Ivanov , V. Izucheev , B. Jacak , N. Jacazio ,P.M. Jacobs , M.B. Jadhav , S. Jadlovska , J. Jadlovsky , S. Jaelani , C. Jahnke ,M.J. Jakubowska , M.A. Janik , M. Jercic , O. Jevons , R.T. Jimenez Bustamante , M. Jin ,P.G. Jones , A. Jusko , P. Kalinak , A. Kalweit , J.H. Kang , V. Kaplin , S. Kar , A. Karasu Uysal ,O. Karavichev , T. Karavicheva , P. Karczmarczyk , E. Karpechev , U. Kebschull , R. Keidel ,M. Keil , B. Ketzer , Z. Khabanova , A.M. Khan , S. Khan , S.A. Khan , A. Khanzadeev ,Y. Kharlov , A. Khatun , A. Khuntia , B. Kileng , B. Kim , B. Kim , D. Kim , D.J. Kim ,E.J. Kim , H. Kim , J.S. Kim , J. Kim , J. Kim , J. Kim , M. Kim
60 ,102 , S. Kim , T. Kim ,T. Kim , K. Kindra , S. Kirsch , I. Kisel , S. Kiselev , A. Kisiel , J.L. Klay , C. Klein , J. Klein ,S. Klein , C. Klein-B¨osing , S. Klewin , A. Kluge , M.L. Knichel , A.G. Knospe , C. Kobdaj ,M. Kofarago , M.K. K¨ohler , T. Kollegger , A. Kondratyev , N. Kondratyeva , E. Kondratyuk ,P.J. Konopka , M. Konyushikhin , L. Koska , O. Kovalenko , V. Kovalenko , M. Kowalski ,I. Kr´alik , A. Kravˇc´akov´a , L. Kreis , M. Krivda
65 ,108 , F. Krizek , M. Kr¨uger , E. Kryshen ,M. Krzewicki , A.M. Kubera , V. Kuˇcera
93 ,60 , C. Kuhn , P.G. Kuijer , L. Kumar , S. Kumar ,S. Kundu , P. Kurashvili , A. Kurepin , A.B. Kurepin , S. Kushpil , J. Kvapil , M.J. Kweon ,Y. Kwon , S.L. La Pointe , P. La Rocca , Y.S. Lai , R. Langoy , K. Lapidus
34 ,145 , A. Lardeux ,P. Larionov , E. Laudi , R. Lavicka , T. Lazareva , R. Lea , L. Leardini , S. Lee , F. Lehas ,S. Lehner , J. Lehrbach , R.C. Lemmon , I. Le´on Monz´on , P. L´evai , X. Li , X.L. Li , J. Lien ,R. Lietava , B. Lim , S. Lindal , V. Lindenstruth , S.W. Lindsay , C. Lippmann , M.A. Lisa ,V. Litichevskyi , A. Liu , H.M. Ljunggren , W.J. Llope , D.F. Lodato , V. Loginov , C. Loizides ,P. Loncar , X. Lopez , E. L´opez Torres , P. Luettig , J.R. Luhder , M. Lunardon , G. Luparello ,M. Lupi , A. Maevskaya , M. Mager , S.M. Mahmood , T. Mahmoud , A. Maire , R.D. Majka ,M. Malaev , Q.W. Malik , L. Malinina
75 ,iii , D. Mal’Kevich , P. Malzacher , A. Mamonov ,V. Manko , F. Manso , V. Manzari , Y. Mao , M. Marchisone , J. Mareˇs , G.V. Margagliotti ,A. Margotti , J. Margutti , A. Mar´ın , C. Markert , M. Marquard , N.A. Martin
104 ,102 ,P. Martinengo , J.L. Martinez , M.I. Mart´ınez , G. Mart´ınez Garc´ıa , M. Martinez Pedreira ,S. Masciocchi , M. Masera , A. Masoni , L. Massacrier , E. Masson , A. Mastroserio
52 ,137 ,A.M. Mathis
103 ,116 , P.F.T. Matuoka , A. Matyja
129 ,117 , C. Mayer , M. Mazzilli , M.A. Mazzoni ,F. Meddi , Y. Melikyan , A. Menchaca-Rocha , E. Meninno , M. Meres , S. Mhlanga , Y. Miake ,L. Micheletti , M.M. Mieskolainen , D.L. Mihaylov , K. Mikhaylov
75 ,64 , A. Mischke
63 ,i , A.N. Mishra ,D. Mi´skowiec , C.M. Mitu , N. Mohammadi , A.P. Mohanty , B. Mohanty , M. Mohisin Khan
17 ,iv ,M.M. Mondal , C. Mordasini , D.A. Moreira De Godoy , L.A.P. Moreno , S. Moretto , A. Morreale ,A. Morsch , T. Mrnjavac , V. Muccifora , E. Mudnic , D. M¨uhlheim , S. Muhuri , M. Mukherjee ,J.D. Mulligan , M.G. Munhoz , K. M¨unning , R.H. Munzer , H. Murakami , S. Murray , L. Musa ,J. Musinsky , C.J. Myers , J.W. Myrcha , B. Naik , R. Nair , B.K. Nandi , R. Nania
53 ,10 , E. Nappi ,M.U. Naru , A.F. Nassirpour , H. Natal da Luz , C. Nattrass , S.R. Navarro , K. Nayak , R. Nayak ,T.K. Nayak
140 ,85 , S. Nazarenko , R.A. Negrao De Oliveira , L. Nellen , S.V. Nesbo , G. Neskovic ,F. Ng , B.S. Nielsen , S. Nikolaev , S. Nikulin , V. Nikulin , F. Noferini
10 ,53 , P. Nomokonov ,G. Nooren , J.C.C. Noris , J. Norman , A. Nyanin , J. Nystrand , M. Ogino , A. Ohlson ,J. Oleniacz , A.C. Oliveira Da Silva , M.H. Oliver , J. Onderwaater , C. Oppedisano , R. Orava ,A. Ortiz Velasquez , A. Oskarsson , J. Otwinowski , K. Oyama , Y. Pachmayer , V. Pacik ,D. Pagano , G. Pai´c , P. Palni , J. Pan , A.K. Pandey , S. Panebianco , V. Papikyan , P. Pareek ,J. Park , J.E. Parkkila , S. Parmar , A. Passfeld , S.P. Pathak , R.N. Patra , B. Paul , H. Pei ,T. Peitzmann , X. Peng , L.G. Pereira , H. Pereira Da Costa , D. Peresunko , G.M. Perez , E. PerezLezama , V. Peskov , Y. Pestov , V. Petr´aˇcek , M. Petrovici , R.P. Pezzi , S. Piano , M. Pikna ,P. Pillot , L.O.D.L. Pimentel , O. Pinazza
53 ,34 , L. Pinsky , S. Pisano , D.B. Piyarathna ,M. Płosko´n , M. Planinic , F. Pliquett , J. Pluta , S. Pochybova , P.L.M. Podesta-Lerma ,M.G. Poghosyan , B. Polichtchouk , N. Poljak , W. Poonsawat , A. Pop , H. Poppenborg ,S. Porteboeuf-Houssais , V. Pozdniakov , S.K. Prasad , R. Preghenella , F. Prino , C.A. Pruneau ,I. Pshenichnov , M. Puccio , V. Punin , K. Puranapanda , J. Putschke , R.E. Quishpe , S. Ragoni ,S. Raha , S. Rajput , J. Rak , A. Rakotozafindrabe , L. Ramello , F. Rami , R. Raniwala ,S. Raniwala , S.S. R¨as¨anen , B.T. Rascanu , R. Rath , V. Ratza , I. Ravasenga , K.F. Read
129 ,94 ,K. Redlich
84 ,v , A. Rehman , P. Reichelt , F. Reidt , X. Ren , R. Renfordt , A. Reshetin , J.-P. Revol ,K. Reygers , V. Riabov , T. Richert
88 ,80 , M. Richter , P. Riedler , W. Riegler , F. Riggi , C. Ristea , S.P. Rode , M. Rodr´ıguez Cahuantzi , K. Røed , R. Rogalev , E. Rogochaya , D. Rohr , D. R¨ohrich ,P.S. Rokita , F. Ronchetti , E.D. Rosas , K. Roslon , P. Rosnet , A. Rossi
56 ,29 , A. Rotondi ,F. Roukoutakis , A. Roy , P. Roy , O.V. Rueda , R. Rui , B. Rumyantsev , A. Rustamov ,E. Ryabinkin , Y. Ryabov , A. Rybicki , S. Saarinen , S. Sadhu , S. Sadovsky , K. ˇSafaˇr´ık
34 ,37 ,S.K. Saha , B. Sahoo , P. Sahoo , R. Sahoo , S. Sahoo , P.K. Sahu , J. Saini , S. Sakai ,S. Sambyal , V. Samsonov
96 ,91 , A. Sandoval , A. Sarkar , D. Sarkar , N. Sarkar , P. Sarma ,V.M. Sarti , M.H.P. Sas , E. Scapparone , B. Schaefer , J. Schambach , H.S. Scheid , C. Schiaua ,R. Schicker , A. Schmah , C. Schmidt , H.R. Schmidt , M.O. Schmidt , M. Schmidt ,N.V. Schmidt
69 ,94 , A.R. Schmier , J. Schukraft
88 ,34 , Y. Schutz
135 ,34 , K. Schwarz , K. Schweda ,G. Scioli , E. Scomparin , M. ˇSefˇc´ık , J.E. Seger , Y. Sekiguchi , D. Sekihata , I. Selyuzhenkov
104 ,91 ,S. Senyukov , E. Serradilla , P. Sett , A. Sevcenco , A. Shabanov , A. Shabetai , R. Shahoyan ,W. Shaikh , A. Shangaraev , A. Sharma , A. Sharma , M. Sharma , N. Sharma , A.I. Sheikh ,K. Shigaki , M. Shimomura , S. Shirinkin , Q. Shou , Y. Sibiriak , S. Siddhanta , T. Siemiarczuk ,D. Silvermyr , G. Simatovic , G. Simonetti
103 ,34 , R. Singh , R. Singh , V.K. Singh , V. Singhal ,T. Sinha , B. Sitar , M. Sitta , T.B. Skaali , M. Slupecki , N. Smirnov , R.J.M. Snellings ,T.W. Snellman , J. Sochan , C. Soncco , J. Song , A. Songmoolnak , F. Soramel , S. Sorensen ,F. Sozzi , I. Sputowska , J. Stachel , I. Stan , P. Stankus , E. Stenlund , D. Stocco ,M.M. Storetvedt , P. Strmen , A.A.P. Suaide , T. Sugitate , C. Suire , M. Suleymanov , M. Suljic ,R. Sultanov , M. ˇSumbera , S. Sumowidagdo , K. Suzuki , S. Swain , A. Szabo , I. Szarka ,U. Tabassam , J. Takahashi , G.J. Tambave , N. Tanaka , S. Tang , M. Tarhini , M.G. Tarzila ,A. Tauro , G. Tejeda Mu˜noz , A. Telesca , C. Terrevoli
29 ,125 , D. Thakur , S. Thakur , D. Thomas ,F. Thoresen , R. Tieulent , A. Tikhonov , A.R. Timmins , A. Toia , N. Topilskaya , M. Toppi ,S.R. Torres , S. Tripathy , T. Tripathy , S. Trogolo , G. Trombetta , L. Tropp , V. Trubnikov ,W.H. Trzaska , T.P. Trzcinski , B.A. Trzeciak , T. Tsuji , A. Tumkin , R. Turrisi , T.S. Tveter ,K. Ullaland , E.N. Umaka , A. Uras , G.L. Usai , A. Utrobicic , M. Vala
38 ,115 , L. Valencia Palomo ,N. Valle , N. van der Kolk , L.V.R. van Doremalen , J.W. Van Hoorne , M. van Leeuwen , P. VandeVyvre , D. Varga , A. Vargas , M. Vargyas , R. Varma , M. Vasileiou , A. Vasiliev , O. V´azquezDoce
116 ,103 , V. Vechernin , A.M. Veen , E. Vercellin , S. Vergara Lim´on , L. Vermunt , R. Vernet ,R. V´ertesi , L. Vickovic , J. Viinikainen , Z. Vilakazi , O. Villalobos Baillie , A. Villatoro Tello ,G. Vino , A. Vinogradov , T. Virgili , V. Vislavicius , A. Vodopyanov , B. Volkel , M.A. V¨olkl ,K. Voloshin , S.A. Voloshin , G. Volpe , B. von Haller , I. Vorobyev
103 ,116 , D. Voscek , J. Vrl´akov´a ,B. Wagner , M. Wang , Y. Watanabe , M. Weber , S.G. Weber , A. Wegrzynek , D.F. Weiser ,S.C. Wenzel , J.P. Wessels , U. Westerhoff , A.M. Whitehead , E. Widmann , J. Wiechula ,J. Wikne , G. Wilk , J. Wilkinson , G.A. Willems
143 ,34 , E. Willsher , B. Windelband , W.E. Witt ,Y. Wu , R. Xu , S. Yalcin , K. Yamakawa , S. Yang , S. Yano , Z. Yin , H. Yokoyama , I.-K. Yoo ,J.H. Yoon , S. Yuan , V. Yurchenko , V. Zaccolo
58 ,25 , A. Zaman , C. Zampolli , H.J.C. Zanoli ,N. Zardoshti
34 ,108 , A. Zarochentsev , P. Z´avada , N. Zaviyalov , H. Zbroszczyk , M. Zhalov ,X. Zhang , Y. Zhang , Z. Zhang , C. Zhao , V. Zherebchevskii , N. Zhigareva , D. Zhou , Y. Zhou ,Z. Zhou , H. Zhu , J. Zhu , Y. Zhu , A. Zichichi
27 ,10 , M.B. Zimmermann , G. Zinovjev , N. Zurlo , Affiliation notes i Deceased ii Dipartimento DET del Politecnico di Torino, Turin, Italy iii
M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia iv Department of Applied Physics, Aligarh Muslim University, Aligarh, India v Institute of Theoretical Physics, University of Wroclaw, Poland
Collaboration Institutes A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS),Kolkata, India Budker Institute for Nuclear Physics, Novosibirsk, Russia California Polytechnic State University, San Luis Obispo, California, United States Central China Normal University, Wuhan, China Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi’, Rome, Italy Chicago State University, Chicago, Illinois, United States China Institute of Atomic Energy, Beijing, China Chonbuk National University, Jeonju, Republic of Korea Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan Creighton University, Omaha, Nebraska, United States Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics, University of California, Berkeley, California, United States Department of Physics, University of Oslo, Oslo, Norway Department of Physics and Technology, University of Bergen, Bergen, Norway Dipartimento di Fisica dell’Universit`a ’La Sapienza’ and Sezione INFN, Rome, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell’Universit`a and Sezione INFN, Turin, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Padova, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento di Scienze e Innovazione Tecnologica dell’Universit`a del Piemonte Orientale and INFNSezione di Torino, Alessandria, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy European Organization for Nuclear Research (CERN), Geneva, Switzerland Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split,Split, Croatia Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague,Czech Republic Faculty of Science, P.J. ˇSaf´arik University, Koˇsice, Slovakia Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt,Germany Gangneung-Wonju National University, Gangneung, Republic of Korea Gauhati University, Department of Physics, Guwahati, India Helmholtz-Institut f¨ur Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universit¨at Bonn, Bonn,Germany Helsinki Institute of Physics (HIP), Helsinki, Finland High Energy Physics Group, Universidad Aut´onoma de Puebla, Puebla, Mexico Hiroshima University, Hiroshima, Japan Hochschule Worms, Zentrum f¨ur Technologietransfer und Telekommunikation (ZTT), Worms, Germany Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Indore, Indore, India Indonesian Institute of Sciences, Jakarta, Indonesia INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Roma, Rome, Italy INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Trieste, Trieste, Italy Inha University, Incheon, Republic of Korea Institut de Physique Nucl´eaire d’Orsay (IPNO), Institut National de Physique Nucl´eaire et de Physique desParticules (IN2P3/CNRS), Universit´e de Paris-Sud, Universit´e Paris-Saclay, Orsay, France Institute for Nuclear Research, Academy of Sciences, Moscow, Russia Institute for Subatomic Physics, Utrecht University/Nikhef, Utrecht, Netherlands Institute for Theoretical and Experimental Physics, Moscow, Russia Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Space Science (ISS), Bucharest, Romania Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico Instituto de F´ısica, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico iThemba LABS, National Research Foundation, Somerset West, South Africa Johann-Wolfgang-Goethe Universit¨at Frankfurt Institut f¨ur Informatik, Fachbereich Informatik undMathematik, Frankfurt, Germany Joint Institute for Nuclear Research (JINR), Dubna, Russia Korea Institute of Science and Technology Information, Daejeon, Republic of Korea KTO Karatay University, Konya, Turkey Laboratoire de Physique Subatomique et de Cosmologie, Universit´e Grenoble-Alpes, CNRS-IN2P3,Grenoble, France Lawrence Berkeley National Laboratory, Berkeley, California, United States Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Nagasaki Institute of Applied Science, Nagasaki, Japan Nara Women’s University (NWU), Nara, Japan National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens,Greece National Centre for Nuclear Research, Warsaw, Poland National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Nuclear Research Center, Baku, Azerbaijan National Research Centre Kurchatov Institute, Moscow, Russia Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Nikhef, National institute for subatomic physics, Amsterdam, Netherlands NRC Kurchatov Institute IHEP, Protvino, Russia NRNU Moscow Engineering Physics Institute, Moscow, Russia Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Institute of the Czech Academy of Sciences, ˇReˇz u Prahy, Czech Republic Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States Ohio State University, Columbus, Ohio, United States Petersburg Nuclear Physics Institute, Gatchina, Russia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia Physics Department, Panjab University, Chandigarh, India Physics Department, University of Jammu, Jammu, India
Physics Department, University of Rajasthan, Jaipur, India
Physikalisches Institut, Eberhard-Karls-Universit¨at T¨ubingen, T¨ubingen, Germany
Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany
Physik Department, Technische Universit¨at M¨unchen, Munich, Germany
Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum f¨urSchwerionenforschung GmbH, Darmstadt, Germany
Rudjer Boˇskovi´c Institute, Zagreb, Croatia
Russian Federal Nuclear Center (VNIIEF), Sarov, Russia
Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India
School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru
Shanghai Institute of Applied Physics, Shanghai, China
St. Petersburg State University, St. Petersburg, Russia
Stefan Meyer Institut f¨ur Subatomare Physik (SMI), Vienna, Austria
SUBATECH, IMT Atlantique, Universit´e de Nantes, CNRS-IN2P3, Nantes, France
Suranaree University of Technology, Nakhon Ratchasima, Thailand
Technical University of Koˇsice, Koˇsice, Slovakia
Technische Universit¨at M¨unchen, Excellence Cluster ’Universe’, Munich, Germany
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland
The University of Texas at Austin, Austin, Texas, United States
Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico
Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil
Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil
Universidade Federal do ABC, Santo Andre, Brazil
University College of Southeast Norway, Tonsberg, Norway
University of Cape Town, Cape Town, South Africa
University of Houston, Houston, Texas, United States
University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland
University of Liverpool, Liverpool, United Kingdom
University of Science and Techonology of China, Hefei, China
University of Tennessee, Knoxville, Tennessee, United States
University of the Witwatersrand, Johannesburg, South Africa
University of Tokyo, Tokyo, Japan
University of Tsukuba, Tsukuba, Japan
Universit´e Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France
Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, Lyon, France
Universit´e de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France
Universit´e Paris-Saclay Centre d¿ ´Etudes de Saclay (CEA), IRFU, Department de Physique Nucl´eaire(DPhN), Saclay, France
Universit`a degli Studi di Foggia, Foggia, Italy
Universit`a degli Studi di Pavia, Pavia, Italy
Universit`a di Brescia, Brescia, Italy
Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India
Warsaw University of Technology, Warsaw, Poland
Wayne State University, Detroit, Michigan, United States
Westf¨alische Wilhelms-Universit¨at M¨unster, Institut f¨ur Kernphysik, M¨unster, Germany
Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary
Yale University, New Haven, Connecticut, United States