Differential branching fraction and angular analysis of Λ 0 b →Λ μ + μ − decays
LHCb collaboration, R. Aaij, B. Adeva, M. Adinolfi, A. Affolder, Z. Ajaltouni, S. Akar, J. Albrecht, F. Alessio, M. Alexander, S. Ali, G. Alkhazov, P. Alvarez Cartelle, A.A. Alves Jr, S. Amato, S. Amerio, Y. Amhis, L. An, L. Anderlini, J. Anderson, M. Andreotti, J.E. Andrews, R.B. Appleby, O. Aquines Gutierrez, F. Archilli, A. Artamonov, M. Artuso, E. Aslanides, G. Auriemma, M. Baalouch, S. Bachmann, J.J. Back, A. Badalov, C. Baesso, W. Baldini, R.J. Barlow, C. Barschel, S. Barsuk, W. Barter, V. Batozskaya, V. Battista, A. Bay, L. Beaucourt, J. Beddow, F. Bedeschi, I. Bediaga, L.J. Bel, I. Belyaev, E. Ben-Haim, G. Bencivenni, S. Benson, J. Benton, A. Berezhnoy, R. Bernet, A. Bertolin, M.-O. Bettler, M. van Beuzekom, A. Bien, S. Bifani, T. Bird, A. Bizzeti, T. Blake, F. Blanc, J. Blouw, S. Blusk, V. Bocci, A. Bondar, N. Bondar, W. Bonivento, S. Borghi, M. Borsato, T.J.V. Bowcock, E. Bowen, C. Bozzi, S. Braun, D. Brett, M. Britsch, T. Britton, J. Brodzicka, N.H. Brook, A. Bursche, J. Buytaert, S. Cadeddu, R. Calabrese, M. Calvi, M. Calvo Gomez, P. Campana, D. Campora Perez, L. Capriotti, A. Carbone, G. Carboni, R. Cardinale, A. Cardini, P. Carniti, L. Carson, K. Carvalho Akiba, R. Casanova Mohr, G. Casse, L. Cassina, L. Castillo Garcia, et al. (604 additional authors not shown)
EEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP-2015-078LHCb-PAPER-2015-00924 March 2015
Differential branching fraction andangular analysis of Λ b → Λµ + µ − decays The LHCb collaboration † Abstract
The differential branching fraction of the rare decay Λ b → Λµ + µ − is measured as afunction of q , the square of the dimuon invariant mass. The analysis is performedusing proton-proton collision data, corresponding to an integrated luminosity of3.0 fb − , collected by the LHCb experiment. Evidence of signal is observed in the q region below the square of the J/ψ mass. Integrating over 15 < q <
20 GeV /c the branching fraction is measured asd B ( Λ b → Λµ + µ − ) / d q = (1 . + 0 . − . ± . ± . × − ( GeV /c ) − , where the uncertainties are statistical, systematic and due to the normalisationmode, Λ b → J/ψ Λ , respectively. In the q intervals where the signal is observed,angular distributions are studied and the forward-backward asymmetries in thedimuon ( A (cid:96) FB ) and hadron ( A h FB ) systems are measured for the first time. In therange 15 < q <
20 GeV /c they are found to be ‡ A (cid:96) FB = − . ± .
09 (stat) ± .
03 (syst) and A h FB = − . ± .
07 (stat) ± .
03 (syst) . Published in JHEP 06 (2015) 115, JHEP 09 (2018) 145. c (cid:13) CERN on behalf of the LHCb collaboration, licence CC-BY-4.0. † Authors are listed at the end of this paper. ‡ Please see erratum in appendix B a r X i v : . [ h e p - e x ] O c t i Introduction
The decay Λ b → Λµ + µ − is a rare ( b → s ) flavour-changing neutral current process that,in the Standard Model (SM), proceeds through electroweak loop (penguin and W ± box)diagrams. As non-SM particles may also contribute to the decay amplitudes, measurementsof this and similar decays can be used to search for physics beyond the SM. To date,emphasis has been placed on the study of rare decays of mesons rather than baryons, inpart due to the theoretical complexity of the latter [1]. In the particular system studied inthis analysis, the decay products include only a single long-lived hadron, simplifying thetheoretical modelling of hadronic physics in the final state.The study of Λ b baryon decays is of considerable interest for several reasons. Firstly, asthe Λ b baryon has non-zero spin, there is the potential to improve the limited understandingof the helicity structure of the underlying Hamiltonian, which cannot be extracted frommeson decays [1, 2]. Secondly, as the Λ b baryon may be considered as consisting of a heavyquark combined with a light diquark system, the hadronic physics differs significantly fromthat of the B meson decay. A further motivation specific to the Λ b → Λµ + µ − channel isthat the polarisation of the Λ baryon is preserved in the Λ → pπ − decay , giving access tocomplementary information to that available from meson decays [3].Theoretical aspects of the Λ b → Λµ + µ − decay have been considered both in the SMand in some of its extensions [3–16]. Although based on the same effective Hamiltonianas that for the corresponding mesonic transitions, the hadronic form factors for the Λ b baryon case are less well-known due to the less stringent experimental constraints. Thisleads to a large spread in the predicted branching fractions. The decay has a non-trivialangular structure which, in the case of unpolarised Λ b production, is described by thehelicity angles of the muon and proton, the angle between the planes defined by the Λ decay products and the two muons, and the square of the dimuon invariant mass, q .In theoretical investigations, the differential branching fraction, and forward-backwardasymmetries for both the dilepton and the hadron systems of the decay, have receivedparticular attention [3, 11, 15–17]. Different treatments of form factors are used dependingon the q region and can be tested by comparing predictions with data as a function of q .In previous observations of the decay Λ b → Λµ + µ − [18,19], evidence for signal had beenlimited to q values above the square of the mass of the ψ (2 S ) resonance. This region willbe referred to as “high- q ”, while that below the ψ (2 S ) will be referred to as “low- q ”. Inthis paper an updated measurement by LHCb of the differential branching fraction for therare decay Λ b → Λµ + µ − , and the first angular analysis of this decay mode, are reported.Non-overlapping q intervals in the range 0.1–20.0 GeV /c , and theoretically motivatedranges 1.1–6.0 and 15.0–20.0 GeV /c [3, 20, 21], are used. The rates are normalised withrespect to the tree-level b → ccs decay Λ b → J/ψ Λ , where
J/ψ → µ + µ − . This analysis uses pp collision data, corresponding to an integrated luminosity of 3.0 fb − , collected during2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, respectively. The inclusion of charge-conjugate modes is implicit throughout. Detector and simulation
The LHCb detector [22,23] is a single-arm forward spectrometer covering the pseudorapidityrange 2 < η <
5, designed for the study of particles containing b or c quarks. The detectorincludes a high-precision tracking system (VELO) consisting of a silicon-strip vertexdetector surrounding the pp interaction region [24], a large-area silicon-strip detectorlocated upstream of a dipole magnet with a bending power of about 4 Tm, and threestations of silicon-strip detectors and straw drift tubes [25] placed downstream of themagnet. The tracking system provides a measurement of momentum, p , with a relativeuncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV /c . The minimumdistance of a track to a primary vertex, the impact parameter, is measured with aresolution of (15 + 29 /p T ) µ m, where p T is the component of the momentum transverse tothe beam, in GeV /c . Different types of charged hadrons are distinguished using informationfrom two ring-imaging Cherenkov (RICH) detectors [26]. Photon, electron and hadroncandidates are identified using a calorimeter system that consists of scintillating-pad andpreshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muonsare identified by a system composed of alternating layers of iron and multiwire proportionalchambers [27].The trigger [28] consists of a hardware stage, based on information from the calorimeterand muon systems, followed by a software stage in which a full event reconstruction iscarried out. Candidate events are first required to pass a hardware trigger, which selectsmuons with a transverse momentum p T > .
48 GeV /c in the 7 TeV data or p T > .
76 GeV /c in the 8 TeV data. In the subsequent software trigger, at least one of the final-statecharged particles is required to have both p T > . /c and impact parameter greaterthan 100 µ m with respect to all of the primary pp interaction vertices (PVs) in the event.Finally, the tracks of two or more of the final-state particles are required to form a vertexthat is significantly displaced from the PVs.Simulated samples of pp collisions are generated using Pythia [29] with a specificLHCb configuration [30]. Decays of hadronic particles are described by
EvtGen [31],in which final-state radiation is generated using
Photos [32]. The interaction of thegenerated particles with the detector, and its response, are implemented using the
Geant4 toolkit [33] as described in Ref. [34]. The model used in the simulation of Λ b → Λµ + µ − decays includes q and angular dependence as described in Ref. [16], together with Wilsoncoefficients based on Refs. [35, 36]. Interference effects from J/ψ and ψ (2 S ) contributionsare not included. For the Λ b → J/ψ Λ decay the simulation model is based on the angulardistributions observed in Ref. [37].
Candidate Λ b → Λµ + µ − (signal mode) and Λ b → J/ψ Λ (normalisation mode) decays arereconstructed from a Λ baryon candidate and either a dimuon or a J/ψ meson candidate,respectively. The Λ b → J/ψ Λ mode, with the
J/ψ meson reconstructed via its dimuondecay, is a convenient normalisation process because it has the same final-state particles2s the signal mode. Signal and normalisation channels are distinguished by the q intervalin which they fall.The dimuon candidates are formed from two well-reconstructed oppositely chargedparticles that are significantly displaced from any PV, identified as muons and consistentwith originating from a common vertex.Candidate Λ decays are reconstructed in the Λ → pπ − mode from two oppositelycharged tracks that either both include information from the VELO ( long candidates),or both do not include information from the VELO ( downstream candidates). The Λ candidates must also have a vertex fit with a good χ , a decay time of at least 2 ps and aninvariant mass within 30 MeV /c of the known Λ mass [38]. For long candidates, chargedparticles must have p T > .
25 GeV /c and a further requirement is imposed on the particleidentification (PID) of the proton using a likelihood variable that combines informationfrom the RICH detectors and the calorimeters.Candidate Λ b decays are formed from Λ and dimuon candidates that have a combinedinvariant mass in the interval 5.3–7.0 GeV /c and form a good-quality vertex that is well-separated from any PV. Candidates pointing to the PV with which they are associatedare selected by requiring that the angle between the Λ b momentum vector and the vectorbetween the PV and the Λ b decay vertex, θ D , is less than 14 mrad. After the Λ b candidateis built, a kinematic fit [39] of the complete decay chain is performed in which the protonand pion are constrained such that the pπ − invariant mass corresponds to the known Λ baryon mass, and the Λ and dimuon systems are constrained to originate from theirrespective vertices. Furthermore, candidates falling in the 8 −
11 and 12 . −
15 GeV /c q intervals are excluded from the rare sample as they are dominated by decays via J/ψ and ψ (2 S ) resonances.The final selection is based on a neural network classifier [40, 41], exploiting 15 variablescarrying kinematic, candidate quality and particle identification information. Both thetrack parameter resolutions and kinematic properties are different for downstream andlong Λ decays and therefore a separate training is performed for each category. Thesignal sample used to train the neural network consists of simulated Λ b → Λµ + µ − events,while the background is taken from data in the upper sideband of the Λ b candidate massspectrum, between 6.0 and 7.0 GeV /c . Candidates with a dimuon mass in either the J/ψ or ψ (2 S ) regions ( ±
100 MeV /c intervals around their known masses) are excluded fromthe training samples. The variable that provides the greatest discrimination in the case oflong candidates is the χ from the kinematic fit. For downstream candidates, the p T of the Λ candidate is the most powerful variable. Other variables that contribute significantlyare: the PID information for muons; the separation of the muons, the pion and the Λ b candidate from the PV; the distance between the Λ and Λ b decay vertices; and the pointingangle, θ D .The requirement on the response of the neural network classifier is chosen separatelyfor low- and high- q candidates using two different figures of merit. In the low- q region,where the signal has not been previously established, the figure of merit ε/ ( √ N B + a/
2) [42]is used, where ε and N B are the signal efficiency and the expected number of backgrounddecays and a is the target significance; a value of a = 3 is used. In contrast, for the3igh- q region the figure of merit N S / √ N S + N B is maximised, where N S is the expectednumber of signal candidates. To ensure an appropriate normalisation of N S , the numberof Λ b → J/ψ Λ candidates that satisfy the preselection is scaled by the measured ratioof branching fractions of Λ b → Λµ + µ − to Λ b → J/ψ ( → µ + µ − ) Λ decays [19], and the J/ψ → µ + µ − branching fraction [38]. The value of N B is determined by extrapolatingthe number of candidate decays found in the background training sample into the signalregion. Relative to the preselected event sample, the neural network retains approximately96 % (66 %) of downstream candidates and 97 % (82 %) of long candidates for the selectionat high (low) q . In addition to combinatorial background formed from the random combination of particles,backgrounds due to specific decays are studied using fully reconstructed samples ofsimulated b hadron decays in which the final state includes two muons. For the Λ b → J/ψ Λ channel, the only significant contribution is from B → J/ψ K S decays, with K S → π + π − where one of the pions is misidentified as a proton. This decay contains a long-lived K S meson and therefore has the same topology as the Λ b → J/ψ Λ mode. This contributionleads to a broad shape that peaks below the Λ b mass region, which is taken into accountin the mass fit.For the Λ b → Λµ + µ − channel two sources of peaking background are identified. Thefirst of these is Λ b → J/ψ Λ decays in which an energetic photon is radiated from either ofthe muons; this constitutes a background in the q region just below the square of the J/ψ mass and in a mass region significantly below the Λ b mass. These events do not contributesignificantly in the q intervals chosen for the analysis. The second source of backgroundis due to B → K S µ + µ − decays, where K S → π + π − and one of the pions is misidentifiedas a proton. This contribution is estimated by scaling the number of B → J/ψ K S eventsfound in the Λ b → J/ψ Λ fit by the ratio of the world average branching fractions for thedecay processes B → K S µ + µ − and B → J/ψ ( → µ + µ − ) K S [38]. Integrated over q thisis estimated to yield fewer than ten events, which is small relative to the expected totalbackground level. The yields of signal and background events in the data are determined in the mass range5.35–6.00 GeV /c using unbinned extended maximum likelihood fits for the Λ b → Λµ + µ − and the Λ b → J/ψ Λ modes. The likelihood function has the form L = e − ( N S + N C + N P ) × N (cid:89) i =1 [ N S P S ( m i ) + N C P C ( m i ) + N P P P ( m i )] , (1)4here N S , N C and N P are the number of signal, combinatorial and peaking backgroundevents, respectively, P j ( m i ) are the corresponding probability density functions (PDFs)and m i is the mass of the Λ b candidate. The signal yield itself is parametrised in the fitusing the relative branching fraction of the signal and normalisation modes, N S ( Λµ + µ − ) k = (cid:20) d B ( Λµ + µ − ) / d q B ( J/ψ Λ ) (cid:21) · N S ( J/ψ Λ ) k · ε rel k · ∆ q B ( J/ψ → µ + µ − ) , (2)where k is the candidate category (long or downstream), ∆ q is the width of the q intervalconsidered and ε rel k is the relative efficiency, fixed to the values obtained as described inSec. 6. Fitting the ratio of the branching fractions of signal and normalisation modessimultaneously in both candidate categories makes better statistical use of the data.The signal shape, in both Λ b → Λµ + µ − and Λ b → J/ψ Λ modes, is described by the sumof two Crystal Ball functions [43] that share common means and tail parameters but haveindependent widths. The combinatorial background is parametrised by an exponentialfunction, independently in each q interval. The background due to B → J/ψ K S decaysis modelled by the sum of two Crystal Ball functions with opposite tails. All shapeparameters are independent for the downstream and long sample.For the Λ b → J/ψ Λ mode, the widths and common mean in the signal parametrisationare free parameters. The parameters describing the shape of the peaking background arefixed to those derived from simulated B → J/ψ K S decays, with only the normalisationallowed to vary to accomodate differences between data and simulation.For the Λ b → Λµ + µ − decay, the signal shape parameters are fixed according to the resultof the fit to Λ b → J/ψ Λ data and the widths are rescaled to allow for possible differencesin resolution as a function of q . The scaling factor is determined comparing Λ b → J/ψ Λ and Λ b → Λµ + µ − simulated events. The B → K S µ + µ − background component is alsomodelled using the sum of two Crystal Ball functions with opposite tails where both theyield and all shape parameters are constrained to those obtained from simulated events. The invariant mass distribution of the Λ b → J/ψ Λ candidates selected with the high- q requirements is shown in Fig. 1, combining both long and downstream candidates. Thenormalisation channel candidates are divided into four sub-samples: downstream andlong events are fitted separately and each sample is selected with both the low- q andhigh- q requirements to normalise the corresponding q regions in signal. The number of Λ b → J/ψ Λ decays found in each case is given in Table 1.The fraction of peaking background events is larger in the downstream sample amount-ing to 28 % of the Λ b → J/ψ Λ yield in the full fitted mass range, while in the sample oflong candidates it constitutes about 4 %.The invariant mass distributions for the Λ b → Λµ + µ − process, integrated over 15 . 2∆ ln L , where ∆ ln L able 1: Number of Λ b → J/ψ Λ decays in the long and downstream categories found using theselection for low- and high- q regions. Uncertainties shown are statistical only. Selection N S (long) N S (downstream)high- q ± 70 11 497 ± q ± 59 7225 ± ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb Figure 1: Invariant mass distribution of the Λ b → J/ψ Λ candidates selected with the neuralnetwork requirement used for the high- q region. The (black) points show data, combiningdownstream and long candidates, and the solid (blue) line represents the overall fit function.The dotted (red) line represents the combinatorial and the dash-dotted (brown) line the peakingbackground from B → J/ψ K S decays. is the change in the logarithm of the likelihood function when the signal component isexcluded from the fit, relative to the nominal fit in which it is present. The measurement of the differential branching fraction of Λ b → Λµ + µ − relative to Λ b → J/ψ Λ benefits from the cancellation of several potential sources of systematic uncertaintyin the ratio of efficiencies, ε rel = ε tot ( Λ b → Λµ + µ − ) /ε tot ( Λ b → J/ψ Λ ). Due to the longlifetime of Λ baryons, most of the candidates are reconstructed in the downstream category,with an overall efficiency of 0.20 %, while the typical efficiency is 0.05 % for long candidates.The efficiency of the PID is obtained from a data-driven method [26] and found to6 c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb Figure 2: Invariant mass distribution of the Λ b → Λµ + µ − candidates, integrated over the region15 . < q < . /c together with the fit function described in the text. The points showdata, the solid (blue) line is the overall fit function and the dotted (red) line represents thecombinatorial background. The background component from B → K S µ + µ − decays, (brown)dashed line, is barely visibile due to the very low yield.Table 2: Signal decay yields ( N S ) obtained from the mass fit to Λ b → Λµ + µ − candidates ineach q interval together with their statistical significances. The yields are the sum of long anddownstream categories with downstream decays comprising ∼ 80 % of the total yield. The 8 − . − 15 GeV /c q intervals are excluded from the study as they are dominated by decaysvia charmonium resonances. q interval [ GeV /c ] Total signal yield Significance0.1 – 2.0 16 . ± . . ± . . ± . . ± . ± 12 6.515.0 – 16.0 57 ± ± 13 1318.0 – 20.0 100 ± 11 141.1 – 6.0 9 . ± . ± 20 217 c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [0.1,2.0] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [2.0,4.0] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [4.0,6.0] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [6.0,8.0] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [11.0,12.5] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [15.0,16.0] GeV] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [16.0,18.0] GeV ] c ) [MeV/ mmL M( c C a nd i d t a t e s p e r M e V / LHCb c / [18.0,20.0] GeV Figure 3: Invariant mass distributions of Λ b → Λµ + µ − candidates, in eight q intervals, togetherwith the fit function described in the text. The points show data, the solid (blue) line is theoverall fit function and the dotted (red) line represents the combinatorial background component. 8e 98 % while all other efficiencies are evaluated using simulated data. The models usedfor the simulation are summarised in Sec. 2. The trigger efficiency is calculated usingsimulated data and increases from approximately 56 % to 86 % between the lowest andhighest q regions. An independent cross-check of the trigger efficiency is performed using adata-driven method. This exploits the possibility of categorising a candidate Λ b → Λµ + µ − or Λ b → J/ψ Λ decay in two ways depending on which tracks are directly responsible for itsselection by the trigger: “trigger on signal” candidates, where the tracks responsible forthe hardware and software trigger decisions are associated with the signal; and “triggerindependent of signal” candidates, with a Λ b baryon reconstructed in either of thesechannels but where the trigger decision does not depend on any of their decay products.As these two categories of event are not mutually exclusive, their overlap may be used toestimate the efficiency of the trigger selection using data. Using Λ b → J/ψ Λ candidates andcalculating the ratio of yields that are classified as both trigger on signal and independentof signal, relative to those that are classified as trigger independent of signal, an efficiencyof (70 ± 5) % is obtained, which is consistent with that of (73 . ± . 02) % computed fromsimulation.The relative efficiency for the ratio of branching fractions in each q interval, calculatedfrom the absolute efficiencies described above, is shown in Fig. 4. The increase in efficiencyas a function of increasing q is dominated by two effects. Firstly, at low q the muons havelower momenta and therefore have a lower probability of satisfying the trigger requirements.Secondly, at low q the Λ baryon has a larger fraction of the Λ b momentum and is morelikely to decay outside of the acceptance of the detector. Separate selections are used forthe low- and high- q regions and, as can be seen in Fig. 4, the tighter neural networkrequirement used in the low- q region has a stronger effect on downstream candidates.The uncertainties combine both statistical and systematic contributions (with thelatter dominating) and include a small correlated uncertainty due to the use of a singlesimulated sample of Λ b → J/ψ Λ decays as the normalisation channel for all q intervals.Systematic uncertainties associated with the efficiency calculation are described in detailin Sec. 7. Three sources of systematic uncertainty on the measured yields are considered for both the Λ b → J/ψ Λ and the Λ b → Λµ + µ − decay modes: the shape of the signal PDF, the shape ofthe background PDF and the choice of the fixed parameters used in the fits to data.For both decays, the default signal PDF is replaced by the sum of two Gaussian functions.All parameters of the Gaussian functions are allowed to vary to take into account the effectof fixing parameters. The shape of the background function is changed by permitting the K S µ + µ − peaking background yield, which is fixed to the value obtained from simulationthe nominal fit, to vary. For the resonant channel, the J/ψ K S peaking background shapeis changed by fixing the global shift to zero. Finally, simulated experiments are performed9 c / [GeV q R e l a ti v e e ff i c i e n c y longdownstream LHCbsimulation Figure 4: Total relative efficiency, ε rel , between Λ b → Λµ + µ − and Λ b → J/ψ Λ decays. Theuncertainties are the combination of both statistical and systematic components, and aredominated by the latter. using the default model, separately for each q interval, generating the same number ofevents as observed in data. Each distribution is fitted with the default model and themodified PDFs. The average deviation over the ensemble of simulated experiments isassigned as the systematic uncertainty. The relative change in signal yield due to thechoice of signal PDF varies between 0.6 % and 4.6 % depending on q , while the change dueto the choice of background PDF is in the range between 1.1 % and 2.5 %. The q intervalsthat are most affected are those in which a smaller number of candidates is observed andtherefore there are fewer constraints to restrict potentially different PDFs. The systematicuncertainties on the yield in each q interval are summarised in Table 3, where the total isthe sum in quadrature of the individual components. The dominant systematic effect is that related to the current knowledge of the angularstructure and the q dependence of the decay channels. The uncertainty due to the finitesize of simulated samples is comparable to that from other sources. The total systematicuncertainties on the efficiencies, calculated as the sums in quadrature of the individualcomponents described below, are summarised in Table 3.10 .2.1 Decay structure and production polarisation The main factors that affect the detection efficiencies are the angular structure of thedecays and the production polarisation ( P b ). Although these arise from different parts ofthe process, the efficiencies are linked and are therefore treated together.For the Λ b → Λµ + µ − decay, the impact of the limited knowledge of the production po-larisation, P b , is estimated by comparing the default efficiency, obtained in the unpolarisedscenario, with those in which the polarisation is varied within its measured uncertainties,using the most recent LHCb measurement, P b = 0 . ± . 09 [37]. The larger of thesedifferences is assigned as the systematic uncertainty from this source. This yields a ∼ . ∼ . q dependence is found.To assess the systematic uncertainty due to the limited knowledge of the decay structure,the efficiency corresponding to the default model [16, 35, 44] is compared to that of amodel containing an alternative set of form factors based on a lattice QCD calculation [15].The larger of the full difference or the statistical precision is assigned as the systematicuncertainty.For the Λ b → J/ψ Λ mode, the default angular distribution is based on that observedin Ref. [37]. The angular distribution is determined by the production polarisation andfour complex decay amplitudes. The central values from Ref. [37] are used for the nominalresult. To assess the sensitivity of the Λ b → J/ψ Λ mode to the choice of decay model, theproduction polarisation and decay amplitudes are varied within their uncertainties, takinginto account correlations.To assess the potential impact that physics beyond the SM might have on the detectionefficiency, the C and C Wilson coefficients are modified by adding a non-SM contribution( C i → C i + C (cid:48) i ). The C (cid:48) i added are inspired to maintain compatibility with the recentLHCb result for the P (cid:48) observable [45] and indicate a change at the level of ∼ q interval, and 2–3 % in other regions. No systematic is assigned as a result ofthis study. Λ baryon The Λ baryon is reconstructed from either long or downstream tracks, and their relativeproportions differ in data and simulation. This proportion does not depend significantlyon q and therefore possible effects cancel in the ratio with the normalisation channel.Furthermore, since the analysis is performed separately for long and downstream candidates,it is not necessary to assign a systematic uncertainty to account for a potential effect due tothe different fractions of candidates of the two categories observed in data and simulation.To allow for residual differences between data and simulation that do not cancel completelyin the ratio between signal and normalisation modes, systematic uncertainties of 0.8 %and 1.2 % are estimated for the low- q and high- q regions, respectively, using the samedata-driven method as in Ref. [46]. 11 able 3: Systematic uncertainties as a function of q , assigned for yields and efficiencies. Valuesreported are the sums in quadrature of all contributions evaluated within each category. q interval [ GeV /c ] Syst. on yields [%] Syst. on eff. [%]0.1 – 2.0 3.4 +2 . − . +2 . − . +17 . − . +2 . − . +3 . − . +3 . − . +3 . − . +3 . − . +2 . − . +2 . − . Λ b baryon In Λ b → J/ψ Λ decays a small difference is observed between data and simulation in themomentum and transverse momentum distributions of the Λ b baryon produced. Simulateddata are reweighted to reproduce these distributions in data and the relative efficienciesare compared to those obtained using events that are not reweighted. This effect is lessthan 0.1 %, which is negligible with respect to other sources.Finally, the Λ b baryon lifetime used throughout corresponds to the most recent LHCbmeasurement, 1 . ± . 019 ps [47]. The associated systematic uncertainty is estimated byvarying the lifetime value by one standard deviation and negligible differences are found. The values for the absolute branching fraction of the Λ b → Λµ + µ − decay, obtained bymultiplying the relative branching fraction by the absolute branching fraction of thenormalisation channel, B ( Λ b → J/ψ Λ ) = (6 . ± . × − [38], are given in Fig. 5 andsummarised in Table 4, where the SM predictions are obtained from Ref. [15]. The relativebranching fractions are given in the Appendix.Evidence for signal is found in the q region between the charmonium resonances andin the interval 0 . < q < . /c , where an increased yield is expected due to theproximity of the photon pole. The uncertainty on the branching fraction is dominated bythe precision of the branching fraction for the normalisation channel, while the uncertaintyon the relative branching fraction is dominated by the size of the data sample available.The data are consistent with the theoretical predictions in the high- q region but lie below12he predictions in the low- q region. Table 4: Measured differential branching fraction of Λ b → Λµ + µ − , where the uncertaintiesare statistical, systematic and due to the uncertainty on the normalisation mode, Λ b → J/ψ Λ ,respectively. q interval [ GeV /c ] d B ( Λ b → Λµ + µ − ) / d q · − [( GeV /c ) − ]0.1 – 2.0 0.36 + 0 . − . 11 + 0 . − . ± . + 0 . − . 09 + 0 . − . ± . + 0 . − . 00 + 0 . − . ± . + 0 . − . 11 + 0 . − . ± . + 0 . − . 14 + 0 . − . ± . + 0 . − . 18 + 0 . − . ± . + 0 . − . 14 + 0 . − . ± . + 0 . − . 14 + 0 . − . ± . + 0 . − . 05 + 0 . − . ± . + 0 . − . 09 + 0 . − . ± . ] c / [GeV q ] - ) c / ( G e V - [ q ) / d m m L fi b L ( B d LHCb SM predictionData Figure 5: Measured Λ b → Λµ + µ − branching fraction as a function of q with the predictions ofthe SM [15] superimposed. The inner error bars on data points represent the total uncertaintyon the relative branching fraction (statistical and systematic); the outer error bar also includesthe uncertainties from the branching fraction of the normalisation mode. Angular analysis The forward-backward asymmetries of both the dimuon system, A (cid:96) FB , and of the pπ system, A h FB , are defined as A i FB ( q ) = (cid:82) 10 d Γd q dcos θ i dcos θ i − (cid:82) − Γd q dcos θ i dcos θ i dΓ / d q , (3)where d Γ / d q dcos θ i is the two-dimensional differential rate and dΓ / d q is the rateintegrated over the corresponding angles. The observables are determined by a fit toone-dimensional angular distributions as a function of cos θ (cid:96) , the angle between the positive(negative) muon direction and the dimuon system direction in the Λ b ( Λ b ) rest frame,and cos θ h , which is defined as the angle between the proton and the Λ baryon directions,also in the Λ b rest frame. The differential rate as a function of cos θ (cid:96) is described by thefunctiond Γ(Λ b → Λ (cid:96) + (cid:96) − )d q dcos θ (cid:96) = dΓd q (cid:20) (cid:0) θ (cid:96) (cid:1) (1 − f L ) + A (cid:96) FB cos θ (cid:96) + 34 f L sin θ (cid:96) (cid:21) , (4)where f L is the fraction of longitudinally polarised dimuons. The rate as a function ofcos θ h has the formd Γ(Λ b → Λ( → pπ − ) (cid:96) + (cid:96) − )d q dcos θ h = B (Λ → pπ − ) dΓ(Λ b → Λ (cid:96) + (cid:96) − )d q (cid:16) A h FB cos θ h (cid:17) . (5)These expressions assume that Λ b baryons are produced unpolarised, which is in agreementwith the measured production polarisation at LHCb [37].The forward-backward asymmetries are measured in data using unbinned maximumlikelihood fits. The signal PDF consists of a theoretical shape, given by Eqs. 4 and 5,multiplied by an acceptance function. Selection requirements on the minimum momentumof the muons may distort the cos θ (cid:96) distribution by removing candidates with extreme valuesof cos θ (cid:96) . Similarly, the impact parameter requirements affect cos θ h as very forward hadronstend to have smaller impact parameter values. The angular efficiency is parametrised usinga second-order polynomial and determined separately for downstream and long candidatesby fitting simulated events, with an independent set of parameters obtained for each q interval. These parameters are fixed in the fits to data. The acceptances are shown inFig. 6 as a function of cos θ h and cos θ (cid:96) in the 15 < q < 20 GeV /c interval for eachcandidate category.The background shape is parametrised by the product of a linear function and thesignal efficiency, with the value of the slope determined by fitting candidates in the uppermass sideband, m ( Λµ + µ − ) > /c . To limit systematic effects due to uncertaintiesin the background parametrisation, an invariant mass range that is dominated by signalevents is used: 5580 < m ( Λµ + µ − ) < /c . The ratio of signal to backgroundevents in this region is obtained by performing a fit to the invariant mass distribution in awider mass interval. 14 q cos -1 -0.5 0 0.5 1 T o t . e ff . ( A . U . ) LHCbsimulation h q cos -1 -0.5 0 0.5 1 T o t . e ff . ( A . U . ) LHCbsimulation l q cos -1 -0.5 0 0.5 1 T o t . e ff . ( A . U . ) LHCbsimulation h q cos -1 -0.5 0 0.5 1 T o t . e ff . ( A . U . ) LHCbsimulation Figure 6: Angular efficiencies as a function of (left) cos θ (cid:96) and (right) cos θ h for (upper) long and(lower) downstream candidates, in the interval 15 < q < 20 GeV /c , obtained using simulatedevents. The (blue) line shows the fit that is used to model the angular acceptance in the fit todata. The angular fit is performed simultaneously for the samples of downstream and longcandidates, using separate acceptance and background functions for the two categories l q cos -1 -0.5 0 0.5 1 C a nd i d a t e s p e r . LHCb h q cos -1 -0.5 0 0.5 1 C a nd i d a t e s p e r . LHCb Figure 7: Angular distributions as a function of (left) cos θ (cid:96) and (right) cos θ h , for candidates inthe integrated 15 < q < 20 GeV /c interval with the overall fit function overlaid (solid blue).The (red) dotted line represents the combinatorial background. 10 Systematic uncertainties on angular observables To derive Eqs. 4 and 5, a uniform efficiency is assumed. However, non-uniformity isobserved, especially as a function of cos θ h (see Fig. 6). Therefore, while integrating overthe full angular distribution, terms that would cancel with constant efficiency may remainand generate a bias in the measurement of these observables. To assess the impact of thispotential bias, simulated experiments are generated in a two-dimensional (cos θ (cid:96) ,cos θ h )space according to the theoretical distribution multiplied by a two-dimensional efficiencyhistogram. Projections are then made and are fitted with the default one-dimensionalefficiency functions. The average deviations from the generated parameters are assigned assystematic uncertainties. The magnitudes of these are found to be − . 032 for A (cid:96) FB , 0 . A h FB and 0 . 028 for f L , independently of q . In most q intervals this is the dominantsource of systematic uncertainty. Resolution effects may induce an asymmetric migration of events between bins and thereforegenerate a bias in the measured value of the forward-backward asymmetries. To studythis systematic effect, a map of the angular resolution function is created using simulatedevents by comparing reconstructed quantities with those in the absence of resolutioneffects. Simulated experiments are then generated according to the measured angulardistributions and smeared using the angular resolution maps. The simulated events, beforeand after smearing by the angular resolution function, are fitted with the default PDF.The average deviations from the default values are assigned as systematic uncertainties.These are larger for the A h FB observable because the resolution is poorer for cos θ h andthe distribution is more asymmetric, yielding a net migration effect. The uncertaintiesfrom this source are in the ranges [0 . , . A (cid:96) FB , [ − . , − . A h FB and[0 . , . f L , depending on q . An imprecise determination of the efficiency due to data-simulation discrepancies couldbias the A FB measurement. To estimate the potential impact arising from this source, thekinematic reweighting described in Sec. 7.2 is removed from the simulation. Simulatedsamples are fitted using the same theoretical PDF multiplied by the efficiency functionobtained with and without kinematical reweighting. The average biases evaluated fromsimulated experiments are assigned as systematic uncertainties. These are larger for sparsely16 able 5: Measured values of leptonic and hadronic angular observables, where the first uncertain-ties are statistical and the second systematic. q interval [ GeV /c ] A (cid:96) FB f L A h FB . + 0 . − . ± . 03 0 . + 0 . − . ± . − . + 0 . − . ± . . + 0 . − . ± . 06 0 . + 0 . − . ± . − . + 0 . − . ± . − . + 0 . − . ± . 03 0 . + 0 . − . ± . − . + 0 . − . ± . − . + 0 . − . ± . 04 0 . + 0 . − . ± . − . + 0 . − . ± . . + 0 . − . ± . 04 0 . + 0 . − . ± . − . + 0 . − . ± . − . + 0 . − . ± . 03 0 . + 0 . − . ± . − . + 0 . − . ± . q intervals and vary in the intervals [0 . , . A (cid:96) FB , [0 . , . A h FB and [0 . , . f L , depending on q .The effect of the limited knowledge of the Λ b polarisation is investigated by varyingthe polarisation within its measured uncertainties, in the same way as for the branchingfraction measurement. No significant effect is found and therefore no contribution isassigned. As there is ambiguity in the choice of parametrisation for the background model, inparticular for regions with low statistical significance in data, simulated experiments aregenerated from a PDF corresponding to the best fit to data, for each q interval. Eachsimulated sample is fitted with two models: the nominal fit model, consisting of theproduct of a linear function and the signal efficiency, and an alternative model formed froma constant function multiplied by the efficiency shape. The average deviations are takenas systematic uncertainties. These are in the ranges [0 . , . A (cid:96) FB , [0 . , . A h FB and [0 . , . f L , depending on q . 11 Results of the angular analysis The angular analysis is performed using the same q intervals as those used in the branchingfraction measurement. Results are reported for each q interval in which the statisticalsignificance of the signal is at least three standard deviations. This includes all of the q intervals above the J/ψ resonance and the lowest q bin.The measured values of the leptonic and hadronic forward-backward asymmetries, A (cid:96) FB2 and A h FB , and the f L observable are summarised in Table 5, with the asymmetries shownin Fig. 8. The statistical uncertainties are obtained using the likelihood-ratio ordering During preparation of update mistake in the analysis was identified, which changes the meaning ofthe measured quantity. Please see appendix B for details. c / [GeV q l F B A -1-0.8-0.6-0.4-0.200.20.40.60.81 LHCb SM predictionData ] c / [GeV q h F B A -0.5-0.4-0.3-0.2-0.100.10.20.30.40.5 LHCb SM predictionData Figure 8: Measured values of (left) the leptonic and (right) the hadronic forward-backwardasymmetries in bins of q . Data points are only shown for q intervals where a statisticallysignificant signal yield is found, see text for details. The (red) triangle represents the values forthe 15 < q < 20 GeV /c interval. Standard Model predictions are obtained from Ref. [17]. l FB A -0.5 0 0.5 L f LHCb c / [15.0,20.0] GeV Figure 9: Two-dimensional 68 % CL region (black) as a function of A (cid:96) FB and f L . The shadedarea represents the region where the PDF is positive over the complete cos θ (cid:96) range. The best fitpoint is given by the (blue) star. method [48] where only one of the two observables at a time is treated as the parameterof interest. In this analysis nuisance parameters were accounted for using the plug-inmethod [49]. In Fig. 9 the statistical uncertainties on A (cid:96) FB and f L are also reported (for theinterval 15 < q < 20 GeV /c ) as a two-dimensional 68 % confidence level (CL) region,18here the likelihood-ratio ordering method is applied by varying both observables andtherefore taking correlations into account. Confidence regions for the other q intervalsare shown in Fig. 10, see Appendix. 12 Conclusions A measurement of the differential branching fraction of the Λ b → Λµ + µ − decay is performedusing data, corresponding to an integrated luminosity of 3.0 fb − , recorded by the LHCbdetector at centre-of-mass energies of 7 and 8 TeV. Signal is observed for the first timeat a significance of more than three standard deviations in two q intervals: 0 . < q < . /c , close to the photon pole, and between the charmonium resonances. Nosignificant signal is observed in the 1 . < q < . /c range. The uncertainties of themeasurements in the region 15 < q < 20 GeV /c are reduced by a factor of approximatelythree relative to previous LHCb measurements [19]. The improvements in the results, whichsupersede those of Ref. [19], are due to the larger data sample size and a better control ofsystematic uncertainties. The measurements are compatible with the predictions of theStandard Model in the high- q region and lie below the predictions in the low- q region.The first measurement of angular observables for the Λ b → Λµ + µ − decay is reported,in the form of two forward-backward asymmetries, in the dimuon and pπ systems and thefraction of longitudinally polarised dimuons. The measurements of the A h FB observableare in good agreement with the predictions of the SM, while for the A (cid:96) FB observablemeasurements are consistently above the prediction.19 Appendix The measured values of the branching fraction of the Λ b → Λµ + µ − decay normalisedto Λ b → J/ψ Λ decays are given in Table 6, where the statistical and total systematicuncertainties are shown separately. Table 6: Differential branching fraction of the Λ b → Λµ + µ − decay relative to Λ b → J/ψ Λ decays,where the uncertainties are statistical and systematic, respectively. q interval [ GeV /c ] d B ( Λ b → Λµ + µ − ) / d q B ( Λ b → J/ψ Λ ) · − [( GeV /c ) − ]0.1 – 2.0 0.56 +0 . − . 17 +0 . − . +0 . − . 15 +0 . − . +0 . − . 04 +0 . − . +0 . − . 17 +0 . − . +0 . − . 23 +0 . − . +0 . − . 28 +0 . − . +0 . − . 22 +0 . − . +0 . − . 22 +0 . − . +0 . − . 09 +0 . − . +0 . − . 14 +0 . − . FB A -0.5 0 0.5 L f LHCb c / [0.1,2.0] GeV l FB A -0.5 0 0.5 L f LHCb c / [11.0,12.5] GeV l FB A -0.5 0 0.5 L f LHCb c / [15.0,16.0] GeV l FB A -0.5 0 0.5 L f LHCb c / [16.0,18.0] GeV l FB A -0.5 0 0.5 L f LHCb c / [18.0,20.0] GeV Figure 10: Two-dimensional 68 % CL regions (black) as a function of A (cid:96) FB and f L . The shadedareas represent the regions in which the PDF is positive over the complete cos θ (cid:96) range. Thebest fit points are indicated by the (blue) stars. The two-dimensional 68 % CL regions for the observables A (cid:96) FB and f L are given inFig 10, for each q interval in which signal is observed.21 Erratum The angular distribution of the dimuon system of the decays Λ b → Λµ + µ − and Λ b → Λµ + µ − can be described bydΓd cos θ (cid:96) = 38 (1 + cos θ (cid:96) )(1 − f L ) + A (cid:96) FB cos θ (cid:96) + 34 f L sin θ (cid:96) , (6)where A (cid:96) FB is the forward-backward asymmetry of the dimuon system and f L is itslongitudinal polarisation fraction. For the Λ b decay, the angle θ (cid:96) is calculated as the anglebetween the direction of the µ + lepton, in the rest frame of the dimuon pair, and thedirection of the dimuon pair, in the rest frame of the Λ b decay. The forward-backwardasymmetry of the lepton pair, A (cid:96) FB , is “odd” under CP conjugation and changes in signbetween the Λ b and Λ b decays. To compensate for this sign, the angle θ (cid:96) is usuallycalculated from the µ − lepton rather than the µ + lepton such that A (cid:96) FB can be calculatedfrom the combined sample. This was the intended approach of this paper. Unfortunately, A (cid:96) FB was determined using the µ + lepton when determining θ (cid:96) for both the Λ b and the Λ b decays. Consequently, the value of A (cid:96) FB in this paper corresponds to a difference A ( A (cid:96) FB ) inasymmetries between the Λ b and Λ b decays rather than a proper average and is expectedto be zero if CP is conserved. The result quoted as A (cid:96) FB in this paper should therefore beinterpreted as A ( A (cid:96) FB ) = − . ± . 09 (stat) ± . 03 (syst) , (7)and is indeed consistent with the Standard Model expectation that CP violating effectsshould be small in the decay Λ b → Λµ + µ − . This is in itself a useful result. A measurementof A (cid:96) FB has since been presented in Ref. [50]. The results in Ref. [50] supersede thecorresponding results in this paper. Note, the mistake in the angular definition only affectsthe value of A (cid:96) FB presented in the paper. The values of A h FB and the differential branchingfraction are unchanged, due to the symmetry of the efficiency model in cos θ (cid:96) .22 cknowledgements We express our gratitude to our colleagues in the CERN accelerator departments forthe excellent performance of the LHC. We thank the technical and administrative staffat the LHCb institutes. We acknowledge support from CERN and from the nationalagencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3(France); BMBF, DFG, HGF and MPG (Germany); INFN (Italy); FOM and NWO (TheNetherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FANO(Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (UnitedKingdom); NSF (USA). The Tier1 computing centres are supported by IN2P3 (France),KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC(Spain), GridPP (United Kingdom). We are indebted to the communities behind themultiple open source software packages on which we depend. We are also thankful forthe computing resources and the access to software R&D tools provided by Yandex LLC(Russia). 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Ciezarek , P.E.L. Clarke , M. Clemencic , H.V. Cliff , J. Closier , V. Coco , J. Cogan ,E. Cogneras , V. Cogoni ,e , L. Cojocariu , G. Collazuol , P. Collins ,A. Comerma-Montells , A. Contu , , A. Cook , M. Coombes , S. Coquereau , G. Corti ,M. Corvo ,f , I. Counts , B. Couturier , G.A. Cowan , D.C. Craik , A.C. Crocombe ,M. Cruz Torres , S. Cunliffe , R. Currie , C. D’Ambrosio , J. Dalseno , P.N.Y. David ,A. Davis , K. De Bruyn , S. De Capua , M. De Cian , J.M. De Miranda , L. De Paula ,W. De Silva , P. De Simone , C.-T. Dean , D. Decamp , M. Deckenhoff , L. Del Buono ,N. D´el´eage , D. Derkach , O. Deschamps , F. Dettori , B. Dey , A. Di Canto ,F. Di Ruscio , H. Dijkstra , S. Donleavy , F. Dordei , M. Dorigo , A. Dosil Su´arez ,D. Dossett , A. Dovbnya , K. Dreimanis , G. Dujany , F. Dupertuis , P. Durante ,R. Dzhelyadin , A. Dziurda , A. Dzyuba , S. Easo , , U. Egede , V. Egorychev ,S. Eidelman , S. Eisenhardt , U. Eitschberger , R. Ekelhof , L. Eklund , I. El Rifai ,Ch. 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Graziani ,A. Grecu , E. Greening , S. Gregson , P. Griffith , L. Grillo , O. Gr¨unberg , B. Gui ,E. Gushchin , Yu. Guz , , T. Gys , C. Hadjivasiliou , G. Haefeli , C. Haen , .C. Haines , S. Hall , B. Hamilton , T. Hampson , X. Han , S. Hansmann-Menzemer ,N. Harnew , S.T. Harnew , J. Harrison , J. He , T. Head , V. Heijne , K. Hennessy ,P. Henrard , L. Henry , J.A. Hernando Morata , E. van Herwijnen , M. Heß , A. Hicheur ,D. Hill , M. Hoballah , C. Hombach , W. Hulsbergen , T. Humair , N. Hussain ,D. Hutchcroft , D. Hynds , M. Idzik , P. Ilten , R. Jacobsson , A. Jaeger , J. Jalocha ,E. Jans , A. Jawahery , F. Jing , M. John , D. Johnson , C.R. Jones , C. Joram ,B. Jost , N. Jurik , S. Kandybei , W. Kanso , M. Karacson , T.M. Karbach , S. Karodia ,M. Kelsey , I.R. Kenyon , M. Kenzie , T. Ketel , B. Khanji , ,k , C. Khurewathanakul ,S. Klaver , K. Klimaszewski , O. Kochebina , M. Kolpin , I. Komarov , R.F. Koopman ,P. Koppenburg , , M. Korolev , L. Kravchuk , K. Kreplin , M. Kreps , G. 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Perret ,L. Pescatore , K. Petridis , A. Petrolini ,j , E. Picatoste Olloqui , B. Pietrzyk , T. Pilaˇr ,D. Pinci , A. Pistone , S. Playfer , M. Plo Casasus , T. Poikela , F. Polci ,A. Poluektov , , I. Polyakov , E. Polycarpo , A. Popov , D. Popov , B. Popovici ,C. Potterat , E. Price , J.D. Price , J. Prisciandaro , A. Pritchard , C. Prouve ,V. Pugatch , A. Puig Navarro , G. Punzi ,s , W. Qian , R. Quagliani , , B. Rachwal ,J.H. Rademacker , B. Rakotomiaramanana , M. Rama , M.S. Rangel , I. Raniuk ,N. Rauschmayr , G. Raven , F. Redi , S. Reichert , M.M. Reid , A.C. dos Reis ,S. Ricciardi , S. Richards , M. Rihl , K. Rinnert , V. Rives Molina , P. Robbe , ,A.B. Rodrigues , E. Rodrigues , J.A. Rodriguez Lopez , P. Rodriguez Perez , S. Roiser ,V. Romanovsky , A. Romero Vidal , M. Rotondo , J. Rouvinet , T. Ruf , H. Ruiz ,P. Ruiz Valls , J.J. Saborido Silva , N. Sagidova , P. Sail , B. Saitta ,e ,V. Salustino Guimaraes , C. Sanchez Mayordomo , B. Sanmartin Sedes , R. Santacesaria , . 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Szczypka , , T. Szumlak ,S. T’Jampens , M. Teklishyn , G. Tellarini ,f , F. Teubert , C. Thomas , E. Thomas ,J. van Tilburg , V. Tisserand , M. Tobin , J. Todd , S. Tolk , L. Tomassetti ,f ,D. Tonelli , S. Topp-Joergensen , N. Torr , E. Tournefier , S. Tourneur , K. Trabelsi ,M.T. Tran , M. Tresch , A. Trisovic , A. Tsaregorodtsev , P. Tsopelas , N. Tuning , ,A. Ukleja , A. Ustyuzhanin , U. Uwer , C. Vacca ,e , V. Vagnoni , G. Valenti , A. Vallier ,R. Vazquez Gomez , P. Vazquez Regueiro , C. V´azquez Sierra , S. Vecchi , J.J. Velthuis ,M. Veltri ,h , G. Veneziano , M. Vesterinen , J.V. Viana Barbosa , B. Viaud , D. Vieira ,M. Vieites Diaz , X. Vilasis-Cardona ,p , A. Vollhardt , D. Volyanskyy , D. Voong ,A. Vorobyev , V. Vorobyev , C. Voß , J.A. de Vries , R. Waldi , C. Wallace , R. Wallace ,J. Walsh , S. Wandernoth , J. Wang , D.R. Ward , N.K. Watson , D. Websdale ,A. Weiden , M. Whitehead , D. Wiedner , G. Wilkinson , , M. Wilkinson , M. Williams ,M.P. Williams , M. Williams , F.F. Wilson , J. Wimberley , J. Wishahi , W. Wislicki ,M. Witek , G. Wormser , S.A. Wotton , S. Wright , K. Wyllie , Y. Xie , Z. Xu , Z. Yang ,X. Yuan , O. Yushchenko , M. Zangoli , M. Zavertyaev ,b , L. Zhang , Y. Zhang ,A. Zhelezov , A. Zhokhov , L. Zhong . Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil Center for High Energy Physics, Tsinghua University, Beijing, China LAPP, Universit´e Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany School of Physics, University College Dublin, Dublin, Ireland Sezione INFN di Bari, Bari, Italy Sezione INFN di Bologna, Bologna, Italy Sezione INFN di Cagliari, Cagliari, Italy Sezione INFN di Ferrara, Ferrara, Italy Sezione INFN di Firenze, Firenze, Italy Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy Sezione INFN di Genova, Genova, Italy Sezione INFN di Milano Bicocca, Milano, Italy Sezione INFN di Milano, Milano, Italy Sezione INFN di Padova, Padova, Italy Sezione INFN di Pisa, Pisa, Italy Sezione INFN di Roma Tor Vergata, Roma, Italy Sezione INFN di Roma La Sapienza, Roma, Italy Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science,Krak´ow, Poland National Center for Nuclear Research (NCBJ), Warsaw, Poland Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia Institute for High Energy Physics (IHEP), Protvino, Russia Universitat de Barcelona, Barcelona, Spain Universidad de Santiago de Compostela, Santiago de Compostela, Spain European Organization for Nuclear Research (CERN), Geneva, Switzerland Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, TheNetherlands NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine University of Birmingham, Birmingham, United Kingdom H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom Department of Physics, University of Warwick, Coventry, United Kingdom STFC Rutherford Appleton Laboratory, Didcot, United Kingdom School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom Imperial College London, London, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom Department of Physics, University of Oxford, Oxford, United Kingdom Massachusetts Institute of Technology, Cambridge, MA, United States University of Cincinnati, Cincinnati, OH, United States University of Maryland, College Park, MD, United States Syracuse University, Syracuse, NY, United States Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia, associated to Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to National Research Centre Kurchatov Institute, Moscow, Russia, associated to Yandex School of Data Analysis, Moscow, Russia, associated to Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain, associated to