Light meson nuclear modification factor in p-Pb collisions over an unprecedented p T range with ALICE
LLight meson nuclear modification factor in p-Pbcollisions over an unprecedented ppp T range withALICE Nicolas Schmidt * , on behalf of the ALICE collaboration Oak Ridge National Laboratory & IKF FrankfurtE-mail: [email protected]
Light neutral meson differential invariant cross section and nuclear modification factor measure-ments have been carried out with the ALICE detector at the CERN LHC in pp collisions at √ s = √ s NN = .
16 TeV. The analysis combines results from severalpartially independent reconstruction techniques where the π and η meson decay photons weredetected with the electromagnetic calorimeter, EMCal, the photon spectrometer, PHOS, or viareconstruction of e + e − pairs from conversions in the ALICE detector material using the centraltracking system. The neutral pion measurement reaching a p T of 200 GeV/ c poses as the highestmeasured identified particle spectrum to date while the η meson is measured to an unprecedented p T of 50 GeV/ c . The spectra are found to be generally overestimated by NLO pQCD calculations.The nuclear modification factors of both mesons exhibit a suppression for p T <
10 GeV/ c whichis stronger compared to previous measurements at √ s NN = .
02 TeV and consistent with CGC andcold nuclear matter energy loss calculations. For p T >
10 GeV/ c , R pPb is consistent with unityand theory predictions. HardProbes20201-6 June 2020Austin, Texas * Speaker. © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p and η R pPb to high p T with ALICE Nicolas Schmidt
1. Introduction
Light neutral meson measurements in small systems like proton-proton and proton-nucleuscollisions at LHC energies over a large transverse momentum range provide important insightsinto particle production mechanisms and their modifications at small parton fractional momentum x and high parton densities. The nuclear modification factor R pPb presents these modifications bydeviating from unity and shows distinct features depending on p T as seen in previous measurementsat RHIC and LHC energies. The low p T region has been covered by several measurements andis well understood for various particle species where nuclear shadowing and saturation effectsare dominant and can be described using nuclear parton distribution functions (nPDFs) [1] andthe Color-Glass-Condensate (CGC) framework [2], respectively. The high transverse momentumregion of identified particle measurements is so far very little explored and the effects of energyloss in cold nuclear matter or anti-shadowing are not yet fully constrained. New measurementscovering large p T ranges can therefore provide additional constraints for theory calculations andallow one to disentangle initial and final state effects in the modification of particle production.
2. Detector description and datasets
The central barrel of ALICE [3] provides multiple detector systems for particle tracking, iden-tification and reconstruction. The presented neutral meson measurements require the reconstructionof decay photons for which ALICE provides two calorimeter-based methods using the Electromag-netic Calorimeter (EMCal) and the Photon Spectrometer (PHOS) as well as the reconstruction ofphotons from the e + e − pairs produced by conversions in the detector material using the InnerTracking System (ITS) and the Time Projection Chamber (TPC). The data used in the presentedmeasurements is from pp ( √ s = √ s NN = .
16 TeV) collisions recorded in 2012and 2016, respectively. Both systems have slightly different detector configurations due to upgradesinstalled during the LHC long shutdown 1 which are mentioned in the following. The EMCal isa lead-scintillator sampling calorimeter that covered ∆ ϕ = ◦ for | η | < . ∆ ϕ = ◦ for 0 . < | η | < . ∆ η × ∆ ϕ = . × . in size whichprovide an energy resolution of σ E / E = . / E ⊕ . / √ E ⊕ .
7% with E in units of GeV. ThePHOS is a lead tungstate crystal calorimeter that covered in its 2012 configuration ∆ ϕ = ◦ for | η | < .
13 and is made of cells with 22 ×
22 mm which are close to the Molière radius of 2 cmthat provide an excellent resolution of σ E / E = . / E ⊕ . / √ E ⊕ . | η | < . E /d x down to p T ≈
100 and 50 MeV/ c for primary and secondary tracks, respectively. The V0 detectors are twoscintillator arrays covering the full azimuth and the forward pseudorapidities 2 . < η < . − . < η < − .
7. They are used to define the minimum bias trigger which requires a coincidencesignal in both arrays and for determining the event multiplicity. Further event triggers are providedby both calorimeters based on large energy deposits in small arrays of cells.1 and η R pPb to high p T with ALICE Nicolas Schmidt
3. Photon and meson reconstruction in ALICE
The decay photons of the π and η mesons are reconstructed with three reconstruction tech-niques. The photon conversion method (PCM) uses the ITS and TPC to reconstruct photons frome + e − pairs produced in photon conversions that occur with an approximate 8.9% probability inthe inner detector material of ALICE. The identification criteria are similar to those given in Refs.[4, 5] with additional improvements to increase the reconstruction efficiency at high p T by loos-ening the constraints on the variable q T which is the projection of the electron momentum to thephoton candidate momentum. The photon reconstruction with PHOS and EMCal is based on theclusterization of energy deposits in the calorimeter cells from electromagnetic showers induced byphotons or electrons. The selection criteria for the calorimetric analyses are adapted from Refs.[4, 5] for the EMCal while the PHOS has only been used in the pp reference measurement. ForEMCal additional improvements were made regarding the nonlinearity of the calorimeter responseat high energies. The corresponding studies resulted in two nonlinearity corrections which accountfor a hardware nonlinearity of up to 14% at cluster energies of 200 GeV and a residual energy andposition correction which is applied in simulations to match the π invariant mass peak positionsin data resulting in a per-mille level of agreement.The neutral mesons are reconstructed using an invariant mass technique where photons recon-structed with the same technique are combined, called PCM, EMC or PHOS, or following a hybridapproach where a PCM photon is paired with an EMCal photon called PCM-EMC. The latter prof-its from the high resolution of the conversion photon and the high efficiency of the EMC methodand is able to cover a large transverse momentum range while the pure PCM method is strongest atlow p T and the pure EMC method suffers, despite its high efficiency, from cluster merging startingfrom 16 GeV/ c for the π . Combinatorial background in this method is removed using an eventmixing technique and raw yields are obtained by integration of wide ranges around the peak po-sition which is obtained from a combined gaussian and one-sided exponential fit on the invariantmass distributions.In addition to the techniques based on the invariant mass, a particle identification method basedon merged photon clusters from π decays is employed, called mEMC [7]. This method exploitsthat the opening angle of high momentum ( p T >
16 GeV/ c ) π mesons are too small to inducetwo separate showers in the EMCal. Instead the showers overlap and thus produce a single showerof “elliptical” shape which can be distinguished from single photon clusters via the shower shapeparameter σ which can be interpreted as the long axis of the shower ellipse. Photon clustersare located at small values of σ thus the application of a selection with σ > . p T . As thismethod relies on a good description of σ in simulation compared to data, an additional detectoreffect, called cross-talk, had to be emulated within the same readout card as described in [6].
4. Spectra and nuclear modification measurements
The π and η meson differential invariant cross sections have been measured in pp collisionsat √ s = √ s NN = .
16 TeV with several partially independentreconstruction techniques. The results of the individual measurements were combined using the2 and η R pPb to high p T with ALICE Nicolas Schmidt − −
10 1 ) c - ( pb G e V p d σ d E ALICE Preliminary π NLO p-Pb, PDF:CT10 - FF:DSS14pp, PDF:MSTW08 - FF:DSS14 a) T C M f i t Th e o r y , D a t a p-Pbb) ) c (GeV/ T p T C M f i t Th e o r y , D a t a ppc) ) c - ( pb G e V p d σ d E η = 8.16 TeV NN s p-Pb, norm. unc. 1.9% = 8 TeV s pp, norm. unc. 2.6% TCM fit PYTHIA 8
PDF:nCTEQ15PDF:nCTEQ15
NLO
PDF:CTEQ6M5 - FF:AESSS d) T C M f i t Th e o r y , D a t a p-Pbe) ) c (GeV/ T p π / η : PDF:CTEQ6M5 - FF:DSS07 π pPb NLO, : PDF:CTEQ6M5 - FF:AESSS η f) ALI−PREL−349045
Figure 1:
Top: Differential invariant cross section spectra of π and η mesons in pp collisions at √ s = √ s NN = .
16 TeV together with two-component model fits and Pythia 8 calculationswith different nPDFs [1] as well as NLO calculations using CT10, MSTW08 and CTEQ6M5 PDFs withDSS14 and AESSS fragmentation functions. Lower panels: Ratio of the p–Pb √ s NN = .
16 TeV π and η meson spectra and their corresponding theory calculations to their respective two-component model fit.Statistical uncertainties are given by the vertical lines while systematic uncertainties are shown as boxes.Bottom right: η / π ratio in both collision systems together with NLO calculations. Best Linear Unbiased Estimates (BLUE) method accounting for correlations of uncertainties. Thep–Pb measurement is therefore a combination of the PCM, EMC, PCM-EMC and mEMC methodswhile for the pp measurement also PHOS was used to reconstruct the π . The spectra togetherwith two-component model (TCM) fits and NLO [1] as well as Pythia 8 calculations are shownin Figure 1. In addition, the ratios of the spectra and theory predictions to the TCM fits are givenwhich show an overestimation of the spectra by NLO calculations especially for the η meson. The η / π ratio is presented in the same figure (bottom right) and found to be consistent between bothcollision systems with a constant value of about 0 .
49 for p T > c in the p–Pb measurement.The nuclear modification factors for both mesons were calculated using the measured spectrain pp and p–Pb with R pPb = σ π , η pPb / ( · σ π , η pp ) where the reference measurement was scaled tothe p–Pb collision energy as well as corrected for the rapidity shift using Pythia8 Monash2013.The resulting R pPb are shown in Fig. 2 (left) for both mesons together with NLO calculationsusing different nPDFs [1]. In addition, calculations from the CGC framework [2] and cold nuclearmatter energy loss (FCEL) [8] are able to describe the p T <
10 GeV/ c region. Furthermore, acomparison to a previous measurement in p–Pb √ s NN = .
02 TeV, given in Fig. 2 (right) shows a 7%stronger suppression for p T < c in the new data which is supported by the CGC calculationshinting at gluon saturation effects. The neutral meson R pPb are found to be consistent with unityfor p T >
10 GeV/ c while the moderate enhancement of the CMS charged hadron measurement3 and η R pPb to high p T with ALICE Nicolas Schmidt ) c (GeV/ T p p A R = 8.16 TeV NN s ALICE Preliminary, p-Pb, <0.3 y , -1.3< π <0.3 y , -1.3< η T p = µ PDF:EPPS16 - FF:KKP,
NLO, π T p = µ PDF:nCTEQ - FF:KKP,
NLO, π fac., FF:DSS LO T k CGC, η , π gg → = -0.5, gg y FCEL, π ALI−PREL−349193 ) c (GeV/ T p p A R π ALICE, = 5.02 TeV NN s , p-Pb, ± h (preliminary) = 8.16 TeV NN s p-Pb, (EPJC 78 (2018) 624) = 5.02 TeV NN s p-Pb, (preliminary) = 5.02 TeV NN s (JHEP 1811 (2018) 013) ALICE (JHEP 1704 (2017) 039)
CMS
ALI−PREL−349201
Figure 2:
Left: Nuclear modification factor of π and η mesons in p–Pb collisions at √ s NN = .
16 TeVtogether with two NLO calculations as well as calculations from the Color-Glass-Condensate (CGC) frame-work [2] and cold nuclear matter energy loss calculations (FCEL) [8]. Right: Nuclear modification fac-tors of neutral pions in p–Pb collisions at √ s NN = .
02 and 8.16 TeV and central Pb–Pb collisions at √ s NN = .
02 TeV together with charged hadron measurements from ALICE and CMS in p–Pb collisionsat √ s NN = .
02 TeV. Statistical uncertainties are given by the vertical lines while systematic uncertaintiesare shown as boxes. which was interpreted as an anti-shadowing effect shown in the same figure is not observed in thepresented measurement.
5. Conclusions
The differential invariant cross sections and nuclear modification factors of π and η mesonshave been measured in p–Pb collisions at √ s NN = .
16 TeV over unprecedented transverse mo-mentum ranges of 0 . < p T <
200 GeV/ c and 1 < p T <
50 GeV/ c , respectively. The reference π measurement has been improved and extended up to the same p T using the merged cluster analy-sis in order to calculate R pPb . The nuclear modification factors were found to be consistent withCGC and cold nuclear matter energy loss calculations at low p T with a stronger suppression com-pared to previous measurements at a lower center-of-mass energy. For high transverse momentaof p T >
10 GeV/ c , R pPb is found to be consistent with unity and theory predictions. The moderateenhancement observed of the CMS charged hadron measurement is not observed. References [1] Kovarik et al. , Phys. Rev. D , 085037 (2016)[2] Lappi and Mäntysaari, Phys. Rev. D , 114020 (2013)[3] Abelev et al. , ALICE
Collaboration,
Int. J. Mod. Phys.
A29 (2014) 1430044[4] Acharya et al. , ALICE
Collaboration, EPJC , 8 (2018) 624[5] Acharya et al. , ALICE
Collaboration, EPJC , 3 (2018) 263[6] Acharya et al. , ALICE
Collaboration, EPJC , 11 (2019) 896[7] Acharya et al. , ALICE
Collaboration, EPJC , 9 (2017) 586[8] Arleo et al. , arXiv:2003.06337 [hep-ph], arXiv:2003.06337 [hep-ph]