Production of π0 and η mesons up to high transverse momentum in pp collisions at 2.76 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Production of π0 and η mesons up to high transverse momentum in pp collisions at 2.76 TeV ALICE Collaboration ALICE Collaboration. (2017). Production of π0 and η mesons up to high transverse momentum in pp collisions at 2.76 TeV. European Physical Journal C, 77(5), Article 339. https://doi.org/10.1140/epjc/s10052-017-4890-x 2017 Eur. Phys. J. C (2017) 77:339 DOI 10.1140/epjc/s10052-017-4890-x Regular Article - Experimental Physics Production of π0and ηmesons up to high transverse momentum in pp collisions at 2.76 TeV ALICE Collaboration CERN, 1211 Geneva 23, Switzerland Received: 1 March 2017 / Accepted: 5 May 2017 © CERN for the benefit of the CMS collaboration 2017. This article is an open access publication Abstract Theinvariantdifferentialcrosssectionsforinclusive π0and ηmesons at midrapidity were measured in pp collisions at √s=2.76 TeV for transverse momenta 0.4<pT<40 GeV/cand 0.6<pT<20 GeV/c, respectively, using the ALICE detector. This large range in pT was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter(EMCal).Inparticular,anewsingle-cluster,shower-shape based method was developed for the identification of highpTneutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. Above 4GeV/c, the measured cross sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π0, and between 30–50% for the ηmeson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10–30%, except at pT<1GeV/c. The new data can therefore be used to further improve the theoretical description of π0and ηmeson production. 1 Introduction Measurements of identified hadron spectra in proton–proton (pp) collisions are well suited to constrain predictions from Quantum Chromodynamics (QCD) [1]. Such predictions are typically calculated in the pertubative approximation of QCD (pQCD) based on the factorization of the elementary short-range scattering processes (such as quark–quark, quark–gluon and gluon–gluon scatterings) involving large momentum transfer (Q2) and long-range universal properties of QCD that need to be experimentally constrained. The universal properties are typically modeled by parton distribution functions (PDFs), which describe the kinematic distributions of quarks and gluons within the proton in the collinear approximation, and fragmentation functions (FFs), which describe the probability for a quark or gluon to fragment e-mail: [email protected] into hadrons of a certain type. The cross section for the production of a given hadron of type H can be written as a sum over parton types Ed3σH dp= a,b,c fa(x1,Q2)⊗fb(x2,Q2) ⊗DH c(zc,Q2)⊗dˆσab→cX(Q2,x1,x2), (1) where fi(x)denotes the proton PDF of parton icarrying a fraction xof the proton’s longitudinal momentum, DH i(zi) the FF of parton iinto hadron H carrying a fraction ziof the parton’s momentum, and dˆσij→kX the inclusive shortdistance scattering cross section of partons iand jinto k(see e.g. [2]). Measurements of hadron production provide constraints onthePDFsandFFs,whicharecrucialforpQCDpredictions, and at LHC energies probe rather low values of x∼0.001 and z∼0.1. The neutral pion (π0)is of special interest because as the lightest hadron it is abundantly produced, and atLHCcollisionenergiesbelowatransversemomentum(pT) of 20 GeV/cdominantly originates from gluon fragmentation. While the collision energy (√s) dependence of π0cross sections has been useful for guiding the parametrization of the FFs [3], experimental data for neutral pions [4,5]atthe LHC are not available above 20 GeV/c, where quark fragmentation starts to play a role. The new π0data presented in this paper extend our previous measurement [5] in pp collisions at √s=2.76 TeV to pTvalues of 40 GeV/callowing one to investigate the pTdependence of the π0cross section at high transverse momentum. In addition, we present the cross section of the ηmeson, which due to its strange quark content provides access to the study of possible differences of fragmentation functions with and without strange quarks [6]. Furthermore, the ηmeson constitutes the second most important source of decay photons and electrons after the π0. Hence, π0and ηmeson spectra over a large pTrange are needed for a precise characterization of the decay photon (electron) background for direct photon (semileptonic open charm and beauty) measurements. 123 339 Page 2 of 25 Eur. Phys. J. C (2017) 77:339 The new measurement of the π0cross section is a result of five analyses using data from various ALICE detector systems and different identification techniques. The decay photons are either measured directly in the Electromagnetic Calorimeter (EMCal), the Photon Spectrometer (PHOS) or via the photon conversion method (PCM). In the PCM measurement, the photons are reconstructed via their conversions into e+e−pairs within the detector material, where the e+e− pairsarereconstructedwiththecharged-particletrackingsystems. The π0is reconstructed statistically using the invariant mass technique. At high pT, where the decay photons are too close together to be resolved individually, the π0can still be measured via the characteristic shape of their energy deposition in the EMCal. We combine statistically independent analyses where (1) both photons are individually resolved in the EMCal (EMC), (2) one photon is identified in the EMCal and one is reconstructed via its conversion to e+e− (PCM–EMC), and (3) the photon pair’s energy is merged in the EMCal (mEMC). Finally, the previously published measurements based on methods where both photons are reconstructed with (4) PHOS or (5) PCM are included as well [5]. The addition of the EMCal based measurements extends the pTreach from 12 to 40 GeV/c, the highest pTfor identified hadrons achieved so far. The ηmeson cross section that was previously not available at √s=2.76 TeV is measured in the range from 0.6to20GeV/cusing the PCM, PCM-EMC and EMC methods. Consequently, the η/π0ratio is measured in the same pTrange. The article is organized as follows: Sect. 2briefly describesthe experimentalsetup.Section 3describesthe data samples and event selection. Section 4describes the neutral meson reconstruction techniques and corresponding corrections for the cross section measurements. Section 5discusses the systematic uncertainties of the various measurements. Section 6presents the data and comparison with calculations and Sect. 7provides a summary. 2 ALICE detector A detailed description of the ALICE detector systems and their performance can be found in Refs. [7,8]. The new measurements primarily use the Electromagnetic Calorimeter (EMCal), the Inner Tracking System (ITS), and the Time Projection Chamber (TPC) at mid-rapidity, which are positioned within a 0.5 T solenoidal magnetic field. Two forward scintillator arrays (V0A and V0C) subtending a pseudorapidity (η) range of 2.8<η<5.1 and −3.7<η<−1.7, respectively, provided the minimum bias trigger, which will be further discussed in the next section. The ITS [7] consists of two layers of Silicon Pixel Detectors (SPD) positioned at a radial distance of 3.9 and 7.6cm, two layers of Silicon Drift Detectors (SDD) at 15.0 and 23.9cm, and two layers of Silicon Strip Detectors (SSD) at 38.0 and 43.0cm from the beamline. The two SPD layers cover a pseudorapidity range of |η|<2 and |η|<1.4, respectively. The SDD and the SSD subtend |η|<0.9 and |η|<1.0, respectively. The primary vertex can be reconstructed with a precision of σz(xy)=A/(dNch/dη)β⊕B, where A≈600 (300) µm, for the longitudinal (z) and transverse (xy) directions, respectively, B≈40 µm and β≈1.4. The TPC [9]isalarge(90m 3) cylindrical drift detector filled with a Ne/CO2gas mixture. It covers a pseudorapidity range of |η|<0.9 over the full azimuthal angle for the maximum track length of 159 reconstructed space points. The ITS and the TPC were aligned with respect to each other to a precision better than 100µm using tracks from cosmic rays and proton–proton collisions [10]. The combined information of the ITS and TPC allows one to determine the momenta of charged particles in the range of 0.05–100GeV/cwith a resolution between 1% at low pTand 10% at high pT. In addition, the TPC provides particle identification via the measurement of the specific energy loss (dE/dx) with a resolution of ≈5%. The tracking detectors are complemented by the Transition Radiation Detector (TRD) and a large time-of-flight (TOF) detector. These detectors were used to estimate the systematic uncertainty resulting from the non-perfect knowledge of the material in front of the EMCal. The EMCal [11] is a layered lead-scintillator sampling calorimeter with wavelength shifting fibers for light collection. The overall EMCal covers 107◦in azimuth and −0.7≤η≤0.7 in pseudorapidity. The detector consists of 12,288 cells (also called towers) with a size of η ×ϕ =0.0143 ×0.0143 corresponding to about twice the effective Molière radius; the cells are read out individually. With a depth of 24.6cm,or≈20 radiation lengths, 2×2 cells comprise a physical module. The 3072 modules are arranged in 10 full-sized and 2 one-third-sized supermodules, consisting of 12 ×24 and 4 ×24 modules, respectively, of which only the full-sized modules, corresponding to an azimuthal coverage of 100◦, were readout for the data recorded in 2011–2013.1The modules are installed with a radial distance to the nominal collision vertex of 4.28 m at the closest point, and assembled to be approximately projective in η. The scintillation light from each cell is collected with wavelength shifting fibers that are connected to a5×5mm 2active-area avalanche photodiode. The relative energy and position resolutions improve with rising incident energy of the particle [12]. The energy resolution can be described by a constant and two energy dependent terms parametrized as σE E=A2⊕B2 E⊕C2 E2% with A=1.7±0.3, B=11.3±0.5, C=4.8±0.8 and Ein GeV. The position 1Thedetector wasinstalledin itscomplete configuration byearly2012, while 4 and 10 full-sized supermodules were present in 2010 and 2011, respectively. 123 Eur. Phys. J. C (2017) 77:339 Page 3 of 25 339 Table 1 Approximate trigger threshold and corresponding trigger rejection factor for EMCal triggers, as well as integrated luminosity for minimum bias and various EMCal triggers Year Trigger Trigger name Approx. threshold Trigger rejection factor (RTrig)Lint (nb−1) 2011 MBOR INT1 0 1 0.524 ±0.010 EMCal L0 EMC1 3.4 GeV 1217 ±67 13.8±0.806 2013 MBAND INT7 0 1 0.335 ±0.013 EMCal L0 EMC7 2.0 GeV 126.0±4.31.19 ±0.062 EMCal L1 (G2) EG2 3.5 GeV 1959 ±131 6.98 ±0.542 EMCal L1 (G1) EG1 5.5 GeV 7743 ±685 47.1±4.57 resolution is linear as a function of 1/√Eand parametrized as 1.5mm+5.3mm √Ewith Ein GeV. Starting with the highest cell Eseed >0.5 GeV, the energy depositions from directly adjacent EMCal cells with Ecell >0.1 GeV are combined to form clusters representing the total energy and physical position of incident particles [8]. The clustering algorithm allows only one local energy maximum in a cluster; if a second is found a new cluster is initiated. Each cell is restricted to only be part of one cluster. Individual cells were calibrated using the π0mass peak position evaluated cell-by-cell, achieving a relative variation of below 1%. 3 Data samples and event selection The data presented in this paper were recorded during the 2011 and 2013 periods with pp collisions at √s=2.76 TeV. VariousEMCal triggerswereemployedand,whilethemajority of the minimum bias data were recorded in 2011, the 2013 running period took advantage of higher threshold EMCal triggers to collect a notable high-pTdata sample. For the pp data collected in 2011, the minimum bias trigger (MBOR) required a hit in either V0 detector or a hit in the SPD, while it required hits in both V0 detectors for the data collected in 2013 (MBAND). The respective cross sections were determined based on van-der-Meer scans, and found to be σMBAND =47.7±0.9 mb with σMBAND/σMBOR = 0.8613 ±0.0006 and σMBAND /σinel =0.760+0.052 −0.028 [13]. For the normalisation of the 2013 data, for which there was no vdM scan, the uncertainty σMBAND was conservatively increased to 4%, to account for possible variations of the MBAND trigger efficiency between 2011 and 2013. The resulting uncertainty due to the luminosity determination is 2.5% for both datasets together. The EMCal issues triggers at two different levels, Level 0 (L0) and Level 1 (L1). The events accepted at L0 are further processed at L1. The L0 decision, issued latest 1.2µsafter the collision, is based on the analog charge sum of 2×2 adjacent cells evaluated with a sliding window algorithm within each physical Trigger Region Unit (TRU) spanning 4 ×24 cells in coincidence with a minimum bias trigger. The L1 trigger decision, which must be taken within 6.2µsafterthe collision, can incorporate additional information from different TRUs, as well as other triggers or detectors. The data presented in this paper used the photon (EG) trigger at L1, which extends the 2×2 sliding window search across neighboring TRUs, resulting in a ≈30% larger trigger area than the L0 trigger. In 2011, only the L0 trigger was used with one threshold (EMC1), while in 2013, one L0 (EMC7) and two L1 triggers (EG1, EG2) with different thresholds were used, as summarized in Table 1. The lower L1 trigger threshold in 2013 was set to approximately match the L0 threshold in 2011 for consistency. In case an event was associated with several triggers, the trigger with the lowest threshold was retained. However, the thresholds are configured in the hardware via analog values, not actual units of energy. Their transformation into energy values directly depends on the energy calibration of the detector. For a reliable normalization of each trigger, theTriggerRejection Factor(RTrig) isused. The RTrig takes into account a combination of the efficiency, acceptance and the downscaling of the respective triggers. It can be obtained from the ratio Rof the number of clusters reconstructed in EMCal triggered events to those in minimum bias events at high cluster energy Ewhere Rshould be approximately constant (plateau region), assuming the trigger does not affect the cluster reconstruction efficiency, but only the overall rate of clusters. To reduce the statistical uncertainties on the normalization for the higher threshold triggers, RTrig was always estimated with respect to the trigger with the next lower threshold in the EMCal or the respective minimum bias trigger if no lower EMCal trigger was available. By consecutively multiplying the individual rejection factors up to the minimum bias trigger, the final RTrig was obtained with respect to the minimum bias trigger. The energy dependenceoftheratiosbetweenclusterspectraoftherelevanttrigger combinations (EMC1/INT1, EMC7/INT7, EG2/EMC7 and EG1/EG2) are shown in Fig. 1.AtlowE, there is a minimum at roughly the threshold of the lower-level trigger for EG2/EMC7 and EG1/EG2, while at high Ethere is a pronounced plateau for every trigger combination. The averages above the threshold in the plateau region, which 123 339 Page 4 of 25 Eur. Phys. J. C (2017) 77:339 Fig. 1 Energy dependence of ratios between cluster spectra for EMC1/INT1, EMC7/INT7, EG2/EMC7 and EG1/EG2. The trigger names INT1 and INT7 denote the minimum bias triggers MBOR and MBAND respectively. The trigger names EMC1, EMC7, EG2 and EG1 denote the EMCal triggers at L0 in 2011 and 2013, and the EMCal triggers at L1 in 2013 with increasing threshold respectively. The individual trigger rejection factors and their respective fit ranges in the plateau region are indicated as well. The final rejection factors with respect to the minimum bias trigger are given in Table 1 represent RTrig for the respective trigger combinations, are indicated by a line whose width represents the respective statistical uncertainty. The corresponding systematic uncertainties were obtained by varying the range for the fit of the plateau region. Finally, the values for the average trigger rejection factors above the threshold with respect to the corresponding minimum bias triggers are given in Table 1. For the PCM–EMC and EMC analyses, all available triggers were used, while for mEMC only the EMC1, EG2 and EG1 triggers were included. The collected integrated luminosities for minimum bias and EMCal triggers Lint =Ntrig σMB Rtrig,(2) where σMB refers to σMBOR for 2011 and σMBAND for 2013, are summarized in Table 1. The statistical uncertainties on RTrig are treated as systematic uncertainties on the integrated luminosity. Monte Carlo (MC) samples were generated using PYTHIA8 [14] and PHOJET [15]. The correction factors obtained independently from the two MC samples were foundto be consistent,and hence combined.Formesons with pT>5GeV/c, as in the triggered or merged cluster analyses, PYTHIA6 [16] simulations enriched with jets generated in bins of the hard scattering (pT,hard) were used. All MC simulations were obtained for a full ALICE detector description using the GEANT3 [17] framework and reconstructed with the same algorithms as for the data processing. The different triggers of the EMCal affect the properties of the reconstructible mesons, like the energy asymmetry (α=E1−E2 E1+E2) of the decay photons, and hence significantly alter the reconstruction efficiency above the trigger threshold in the trigger turn-on region. The efficiency biases κTrig induced by the triggers were simulated using the approximate thresholds and their spread for different TRUs. The bias was defined as the ratio of the π0or ηreconstruction efficiency in triggered events over that in minimum bias events. Figure 2shows the pTdependence of κTrig for different triggers and reconstruction methods for the π0and η meson. While κTrig is unity for the mEMC analysis in the considered kinematic range, it is significantly below one for the PCM–EMC and EMC neutral meson reconstruction, and reaches ≈1 only at about twice the trigger threshold. The corresponding correction factors are found to be larger for the PCM–EMC compared to the EMC method, and larger for the ηthan the π0meson. This is a consequence of the much lower energy threshold imposed on the photons reconstructed with PCM, which leads to wider opening angle and asymmetry distributions of the reconstructible mesons. At low pT,κTrig also exhibits the effect of the trigger on subleading particles, for which the efficiency in triggered events is strongly reduced. However, the various triggers are only used if the meson momentum is at least 1.5 times the trigger threshold, thus the effect on the subleading particles is negligible. In the offline analysis, only events with a reconstructed vertex with |zvtx|<10cm with respect to the nominal interaction vertex position along the beam direction were used. The finite primary vertex reconstruction efficiency for the MBOR(MBAND) trigger of about 0.92 (0.98) is taken into account in the normalization of the respective minimum bias triggers. Furthermore, only events with exactly one reconstructed vertex were accepted to remove pileup from inand out-of-bunch collisions. While the in-bunch pileup is negligible after the vertex selection, the out-of-bunch pileup accumulating in the TPC due to its readout time of 90ms, needs to be subtracted statistically for the mesons measured with PCM, as described in Ref. [5]. For the π0(η) mesons reconstructed with PCM the out-of-bunch pileup correction ranges from 20% (9%) at low pTto about 3% above 4 GeV/c. Analyses involving the EMCal are not affected because contributions of clusters from different bunch crossings are suppressed by a suitable selection of clusters within a certain time window around the main bunch crossing. 4 Neutral meson reconstruction Neutral mesons decaying into two photons fulfill M=2E1E2(1−cos θ12)(3) 123 Eur. Phys. J. C (2017) 77:339 Page 5 of 25 339 Fig. 2 Efficiency bias κTrig induced by different triggers (EMC1, EMC7 and EG1) for neutral pions (left panel)andηmesons (right panel)for PCM–EMC (open symbols)andEMC(closed symbols) where Mis the reconstructed mass of the meson, E1and E2are the measured energies of two photons, and θ12 is the opening angle between the photons measured in the laboratory frame. Photon candidates are measured either by a calorimeter or by PCM. Neutral meson candidates are then obtained by correlating photon candidates measured either by EMC, PHOS or PCM exclusively, or by a combination of them (PCM–EMC). The corresponding π0and ηmeson measurements are described in Sect. 4.1. The typical opening angle θ12 decreases with increasing pTof the meson due to the larger Lorentz boost. For π0mesons with pTabove 5–6 GeV/c, the decay photons become close enough so that their electromagnetic showers overlap in neighboring calorimeter cells of the EMCal. At pTabove 15 GeV/c, the clustering algorithm can no longer efficiently distinguish the individual showers in the EMCal, and π0mesons can be measured by inspecting the shower shape of single clusters, referred to as “merged” clusters and explained in Sect. 4.2. To be able to directly compare the reconstruction performances of the various measurement techniques and triggers, the invariant differential neutral meson cross sections were expressed as Ed3σ dp3=Nrec pTpTκTrig ε 1 Lint 1 BR (4) with the inverse of the normalized efficiency 1 ε=1 2πAy P εrec (5) and integrated luminosity (see Eq. 2). The measured cross sectionswereobtainedbycorrectingthereconstructedmeson yield Nrec for reconstruction efficiency εrec, purity Pand acceptance A, efficiency bias κTrig, integrated luminosity Lint,aswellasforthepTand yinterval ranges, pT and y, respectively, and the γγ decay branching ratio BR. For invariant mass methods, the effect of reconstructed photon impurities on the meson purity are significantly reduced due to the subtraction of the combinatorial background, and hence the resulting meson impurities were neglected. For the mEMC method, the π0purity correction was obtained from MCsimulations tuned todata. Inthe case ofneutral pions,the contribution from secondary π0swas subtracted from Nrec before applying the corrections. The contribution from weak decays was estimated for the different methods by simulating the decays of the K0 Sand using their measured spectra [18], taking into account the reconstruction efficiencies, as well as resolution and acceptance effects for the respective daughter particles The contribution from neutral pions produced by hadronic interactions in the detector material was estimated based on the full detector simulations using GEANT3. Finally, the results were not reported at the center of the pTintervals used for the measurements, but following the prescription in Ref. [19] at slightly lower pTvalues, in order to take into account the effect of the finite bin width pT. The correction was found to be less than 1% in every pTinterval for the π0, and between 1–4% for the ηmeson. 4.1 Invariant mass analyses Applying Eq. 3, the invariant mass distribution is obtained by correlating all pairs of photon candidates per event. The neutral meson yield is then statistically extracted using the distinct mass line shape for identification of the signal and a model of the background. In the following, only the new measurements are described. Details of the PCM and PHOS π0measurements can be found in Refs. [4,5]. 123 339 Page 6 of 25 Eur. Phys. J. C (2017) 77:339 Table 2 Criteria for photon candidate selection for PCM Track selection Track quality selection pT>0.05 GeV/c NTPC cluster/Nreconstructible clusters >0.6 |η|<0.9 Electron selection −4<nσe<5 Pion rejection nσπ<1for0.4<p<3.5GeV/c, nσπ<0.5forp>3.5GeV/c(PCM) nσπ<1forp>0.4GeV/c(PCM–EMC) Photon criteria Conversion point |ηV0|<0.9 5cm<Rconv <180 cm |Zconv|<240 cm 0≤|ϕconv|≤2π cos(θpoint)>0.85 Photon quality |ψpair|<ψ pair,max −ψpair,max χ2 red,max χ2 red, with ψpair,max =0.1andχ2 red,max =30 Armenteros-Podolanski qT<qT,max1−α2 α2 max , with qT,max =0.05 GeV/cand αmax =0.95 For the reconstruction of photons with PCM, only tracks from secondary vertices without kinks with a minimum momentum of 0.05 GeV/cwere taken into account. The tracks had to be reconstructed within the fiducial acceptance of the TPC and ITS and with at least 60% of the reconstructible track points in the TPC. The photon momentum resolution is better than 1.5% at low pT, resulting from the precise determination of the track momenta by the TPC. Furthermore, the associated energy loss measured in the TPC was required to be within −4<nσe<5 of the electron expectation, where nσX=(dE/dx−dE/dxX)/σX with dE/dxXand σXthe average energy loss and resolution for particle X, respectively. The contamination from charged pions was suppressed by excluding all track candidates within nσπ<1 of the pion expectation. The charged pion rejection was applied for track momenta between 0.4< p<3.5GeV/cforPCMand p>0.4GeV/cforPCM–EMC, while for PCM it was released to nσπ<0.5 above p= 3.5GeV/c. Only conversions which were pointing to the primary vertex and could be reconstructed with a conversion point with 5 <Rconv <180 cm within the acceptance of the ITS and TPC were considered. Compared to previous PCM standalone measurements [5], the photon candidate selection criteria were optimized in order to reduce the combinatorial background. In particular, a two dimensional selection on the reduced χ2of the photon conversion fit and the angle between the plane defined by the conversion pair and the magnetic field |ψpair|was introduced to suppress random e+e−pairs. Furthermore, the selection in the ArmenterosPodolanski variables [20] was tightened to reduce the contamination from K0 Sand decays. A summary of the conversion photon selection criteria is given in Table 2. Clusters in the EMCal were reconstructed by aggregating cells with Ecell >0.1 GeV to a leading cell energy with at least Eseed >0.5 GeV, and were required to have only one local maximum. Photon candidates were obtained from reconstructed clusters by requiring a cluster energy of 0.7 GeV to ensure acceptable timing and energy resolution and to remove contamination from minimumionizing (< ∼300 MeV) and low-energy hadrons. Furthermore, a cluster had to contain at least two cells to ensure a minimum cluster size and to remove single cell electronic noise fluctuations. Clusters which could be matched to a track propagated to the average shower depth in the EMCal (at 440 cm) within |η|and |ϕ|criteria that depend on track pTas given in Table 3, were rejected to further reduce contamination by charged particles. The track-to-cluster matching efficiency amounts to about 97% for primary charged hadrons at cluster energies of Eclus >0.7 GeV, decreasing slowly to 92% for clusters of 50 GeV. The removal of matched tracks is particularly important for the PCM–EMC method as otherwise a severe auto-correlation between the clusters originating from one of the conversion electrons and the conversion photon would be introduced. Such auto-correlated pairs strongly distort the shape of the invariant mass distribution between the π0and ηmass peak region. The standard track matching applied to each conversion leg allowed for the removal of these auto-correlation pairs with an efficiency of more than 99% since the corresponding track was already found. An additional distinction between clusters from mainly photons, 123 Eur. Phys. J. C (2017) 77:339 Page 7 of 25 339 Table 3 Criteria for photon candidate selection for EMCal-based methods Cluster reconstruction Minimum cell energy Ecell >0.1GeV Minimum leading cell energy Eseed >0.5GeV Cluster selection Selection in η|η|<0.67, 1.40 rad <ϕ<3.15 rad Minimum cluster energy Eclus >0.7GeV Minimum number of cells Ncells ≥2 Cluster-shape parameter 0.1<σ2 long <0.5 (PCM–EMC) 0.1<σ2 long <0.7(EMC) σ2 long >0.27 (mEMC) Cluster time |tclus|≤50 ns (2011) −35 ns <tclus <30 ns (2013) Cluster–track matching |η|≤0.010 +(pT+4.07)−2.5 |ϕ|≤0.015 +(pT+3.65)−2 electrons and neutrons is based on their shower shape. The shower shape can be characterized by the larger eigenvalue squared of the cluster’s energy decomposition in the EMCal η–ϕplane. It is expressed as σ2 long =0.5σ2 ϕϕ +σ2 ηη +(σ2 ϕϕ −σ2 ηη)2+4σ4 ϕη(6) where σ2 xz =xz−xzand x= 1 wtot wixiare weighted over all cells associated with the cluster in the ϕ or ηdirection. The weights wilogarithmically depend on the ratio of the energy of a given cell to the cluster energy, as wi=max(0,4.5+log Ei/E), and wtot =wi[21]. Nuclear interactions, in particular for neutrons, create an abnormal signal when hitting the corresponding avalanche photodiodes for the readout of the scintillation light. Such a signal is mainly localized in one high-energy cell with a few surrounding low-energy cells, and can be removed by requiring σ2 long >0.1. While the showers from electrons and photons tend to be similar, they can be distinguished based on their elongation, as most of the low-pTelectrons will hit the EMCal surface at an angle due to the bending in the magnetic field. Most of the pure photons are reconstructed with aσ2 long ≈0.25; only late conversions elongate the showers beyond this. Thus, rejecting clusters with σ2 long >0.7(0.5) for EMC (PCM–EMC) rejects the contamination from late conversion electrons significantly. At very high transverse momenta (>10 GeV/c), it also rejects part of the contamination from neutral pions for which both photons have been reconstructed in a single cluster. Contributions of clusters from different bunch crossings were suppressed by a suitable selection of clusters within a certain time window around the main bunch crossing. A summary of the selection criteria for EMCal photon candidates is given in Table 3. The good momentum resolution for the PCM photon was exploited to derive an improved correction for the relative energy scale, as well as for the residual misalignment of the EMCal between data and simulation. The neutral pion mass was evaluated for the PCM–EMC method as a function of the EMCal photon energy for data and simulation. A correction for the cluster energy was deduced which for a given simulation adjusts the neutral pion mass peak position to the measured position in the data as a function of the cluster energy. Above 1 GeV, the corrections for the various MC datasets are typically about 3%. Exampleinvariantmassdistributionsobtainedbycorrelatingphotonsreconstructed withEMCalor byonephoton from PCM and one from EMCal are shown in Fig. 3for neutral pionsand Fig.4forηmesons. Thecombinatorial background was calculated using the mixed event technique [22]using event pools binned by primary vertex position, multiplicity and transverse momentum. The mixed-event background has beennormalizedtotherightside oftheπ0(η) peak.Additionally,aresidualcorrelatedbackgroundestimatedusingalinear fit was subtracted. Only pairs with a minimum opening angle of 0.02 (0.005) mrad for EMC (PCM and PCM–EMC) methods were considered for signal and background construction. Finally, pairs are restricted to rapidity of |y|<0.8. A Gaussian with an exponential tail on the left side was fitted to the subtracted invariant mass distributions, in order to determine the mass position and width of the peak. The results of the fits for the mass position and widths of neutral pions and ηmesons are shown in Fig. 5. The performance of PHOS from Ref. [5] in the case of π0is added for completeness. For all systems, the data for both π0and ηare reproduced by the MC simulations to a precision on average better than 0.3% for the mass position. For EMC, the pTdependence of the mass position is especially pronounced, due to non-linearity effects for low pTclusters, shower merg123 339 Page 8 of 25 Eur. Phys. J. C (2017) 77:339 Fig. 3 Invariant mass distributions in the π0peak region for INT1 (left panels)andEG1(right panels) triggers and EMC (top panels)and PCM–EMC (bottom panels) methods ing and shower overlaps, and decay asymmetry enhanced by the employed triggers at high pT. The widths of the meson peaks are similarly well described, with the expected ordering for the various methods. In particular, the peak widths of the PCM–EMC fits are between the standalone measurements of PCM and EMC and are comparable to the PHOS measurement above 7 GeV/c. This illustrates that the inclusion of one photon from PCM significantly improves the resolution of the neutral meson measurements. The neutral meson raw yield was extracted by integrating the background-subtracted invariant mass distributions aroundthe measured peakmass. The integration windows for the different reconstruction techniques were adjusted based on the average width of the meson peaks and their signal shape:(Mπ0−0.035, Mπ0+0.010),(Mη−0.047, Mη+0.023) for PCM, (Mπ0−0.032, M0 π+0.022), (Mη−0.060, Mη+0.055) for PCM–EMC, and (Mπ0−0.05, M0 π+0.04), (Mη−0.080, Mη+0.08) for EMC. For both mesons, an asymmetric range around the measured mass position was used to account for the low mass tail originating not only from the bremsstrahlung energy loss of conversion electrons and positrons, but also from additional missing energy in the EMCal due to the partial reconstruction of the photon. The corrections for the geometric acceptance and reconstruction efficiency for the different mesons were calculated using MC simulations as mentioned in Sect. 3. The acceptance for the EMCal reconstruction techniques was calculated as the fraction of π0(η), whose decay photons point to the EMCal surface (|η|<0.67,1.40 rad <ϕ<3.15 rad), compared to the π0(η) generated with |y|<0.8. In the case of PCM–EMC, only one photon was required to point to the EMCal surface, while the other was required to be within the acceptance of the TPC (|η|<0.9,0rad <ϕ<2πrad). The output from the full event MC simulations was recon123 Eur. Phys. J. C (2017) 77:339 Page 15 of 25 339 (K0 L) and spectra [18]. The corresponding uncertainty was obtained by varying the kaon and yield within their measured uncertainties. Since the correction due to the secondaries is only 1–2%, for all but the mEMC reconstruction technique, even a variation of 15% on the input yields leads to a negligible contribution compared to other uncertainties. For mEMC, where the correction is about 5%, an uncertainty of ≈0.5% was obtained. In addition, ≈1.5% were added to the uncertainty to account for the limited precision in the shape and size of the correction factors of the full simulations for the pions from K0 S,K 0 Land , which was estimated byvaryingtheparametrization underlyingthe efficienciesfor secondary π0. Inner material: The uncertainty related to the knowledge of the inner (radius <180 cm) material budget reflects the uncertainty of the conversion probability of photons, and hence dominantly affects the PCM measurements. It was estimated to be 4.5% independent of pTbased on detailed comparison between simulation and data for pp collisions at √s=7TeV[4]. Thus, it affects the PCM meson measurementswith9%,whileitonlycontributes4.5%toPCM–EMC. In η/π0, the uncertainty cancels as both mesons are affected in the same way. Outer material: For the reconstructed photons in the EMCal, a possible mismatch between the material present in reality and assumed in the simulation in front of the EMCal may cause an error in the absorption rate or the production of secondary pions. In most cases, however, the photon simply converts and at least one of its daughter electrons can be reconstructedintheEMCalsothattheπ0likelywillbereconstructed as well, although with degraded pTresolution. The probability to still reconstruct the neutral meson increases with increasing conversion radius, i.e. the closer the conversion happens to the surface of the EMCal. Most of the material is located at most 1.5 m away from the EMCal, namely the TPC outer wall, the TRD and the Time-Of-Flight (TOF) detectorplustheirsupportstructures.TheTRDwasonlyfully installedintheLHCshutdownperiodafter2013.Forthe2011 and 2013 data there were regions in ϕwithout TRD modules in front of the EMCal. Hence, the net-effect of the material in front of the EMCal could be studied by comparing fully corrected π0yields for different ϕregions with or without the TRD in front of the EMCal. From the observed difference measured using the EMC and PCM–EMC measurements, an uncertainty on the neutral meson yields of 4.2% independent of pTwas derived, and assigned to all measurements involving the EMCal. For η/π0the uncertainty is assumed to cancel as both mesons should be affected in a similar way. Trigger normalization and pileup: The uncertainties for the trigger normalization were calculated by varying the range for the fit of the plateau region (see Fig. 1) for the different trigger combinations, leading to the respective rejection factors with their uncertainties given in Table 1. Since the final spectra for each measurement technique using the EMCal are composed of several triggers, the contributions of the respective trigger rejection uncertainties enter the final measurement with different magnitudes depending on pT. The uncertainties range between 0.5 and 8.8%. For η/π0the uncertainties cancel as the ratio was measured per trigger and reconstruction method and combined afterwards. For PCM only minimum bias triggers were used, and hence no uncertainty due to the trigger rejection was assigned. However, an uncertainty of 0.8–0.4% was taken into account for the out-of-bunch pileup subtraction described in [5]. The pileup uncertainty is about 1.8% for the ηmeson. It largely cancels in the η/π0ratio, however, and the remaining error can be neglected compared to other error sources. 6 Results Since the meson measurements with PHOS, PCM, EMC, PCM–EMC and mEMC have partly uncorrelated systematic uncertainties, their combination will increase the precision of the respective cross section measurements. The BLUE method [27–29] was used to calculate the combined spectra of the π0and ηmesons as well as the η/π0ratio. For the combination of the spectra, the full correlation matrix was taken into account by estimating the correlated and uncorrelated part of the systematics for all pairs of measurements versus pT. Correlations are most apparent between the three EMCrelated measurements(EMC,PCM–EMCand mEMC), as well as for the PCM–EMC and PCM results. At high pT, for instance, the uncertainties are dominated by the uncertainty on RTrig which is largely common between the EMCal triggered analyses. Uncertainties between PHOS, PCM, and EMC (mEMC) are uncorrelated. The combined spectra were fitted with a two-component model (TCM) Ed3σ dp3=Aeexp M−p2 T+M2 Te+A1+p2 T nbrT2−nbr (7) introduced by Bylinkin and Rostovtsev [25] and Bylinkin and Ryskin [26], which serves as convenient parametrization of the data without aiming for a physics interpretation. The parameters for the π0and ηfits are given in Table 7for χ2/ndof values of better than 0.5 taking statistical and systematic uncertainties in quadrature. Unlike for Tsallis [30] and power-law distributions, which at high and low pT, respectively, systematically deviate from the data, the TCM parameterization describes the data over the full measured range to better than 10%. 123 339 Page 16 of 25 Eur. Phys. J. C (2017) 77:339 Table 7 Parameters of the two-component model, Eq. 7[25,26], which are used to parametrize the neutral pion and ηmeson spectra, respectively, for the comparisons to models and among the different methods Meson Ae(pb GeV−2c3)Te(GeV/c)A(pb GeV−2c3)T(GeV/c)nbr π0(0.79 ±0.35)×1090.566 ±0.035 (74.3±12.9)×1090.441 ±0.021 3.083 ±0.027 η(18.5±22.1)×1090.149 ±0.070 (1.4±1.0)×1090.852 ±0.136 3.318 ±0.122 Fig. 8 Comparison of the individual measurements in their respective measured transverse momentum ranges relative to the two-component model fits [25,26] of the final spectra. The final spectra are obtained by combining the individual measurements in the overlapping pTregions with the highest granularity using the full correlation matrix as defined in the BLUE-algorithm [27–29] Table 8 Summary of the pTreach (in GeV/c) of the various reconstruction methods for π0,ηand η/π0 Method π0ηη/π 0 PCM 0.4–8.0 0.5–6.0 0.5–6.0 PHOS 0.8–12.0 n/a n/a EMC 1.4–20.0 2.0–20.0 2.0–20.0 PCM–EMC 0.8–20.0 1.0–16.0 1.0–16.0 mEMC 16.0–40.0 n/a n/a Figure 8shows a comparison of the individual measurements in their respective measured pTranges summarized in Table 8to the two-component model fits for the π0and η mesons. As already mentioned above, the π0spectrum in pp collisions at √s=2.76 TeV has been measured by ALICE using the PHOS and PCM [5]. The new results obtained with the different EMC measurements and with the hybrid PCM–EMC method are consistent with these earlier results, andthe combinationwiththe formermeasurementsimproves the precision of the data. The figure also demonstrates an approximately fourfold extension of the pTreach of the measurement by using the EMCal. The ηmeasurement, which is the first such measurement at √s=2.76 TeV, spans from 0.6to20GeV/c. There is good agreement within the statistical uncertainties among the different detection techniques. Above pT>4GeV/c, the result is dominated by the EMCal measurements. Figure 9shows the combined π0and ηcross sections in pp collisions at √s=2.76 TeV, and Fig. 10 the corresponding η/π0ratio. As mentioned earlier, the data were parameterized with a two-component model of Bylinkin and Ryskin[26](seeTable7)andcomparedtorecentNLOpQCD calculations[3,6],and PYTHIA8.2[31] generator-levelsimulations using the widely-used Monash 2013 tune [32]. A large fraction of hadrons at low pTis produced in pp collisions via soft parton interactions and from resonance decays, which cannot be well described within the framework of pQCD, but are taken into account in the event-generator approach. For the π0, the pQCD calculation [3], which uses the DSS14 fragmentation functions seems to have a different shape than the data. It overpredicts the data by about 30% at intermediate pT(5 GeV/c<pT<16 GeV/c), while it agrees with the data at higher pT. The PYTHIA 8.2 calculation describes the data well, except below 1 GeV/c, where it overpredicts the data by up to 30%. For pTabove 15 GeV/cPYTHIA has a tendency to underpredict the data by about 10%; however this slight difference is covered by the uncertainties of the measurement. For the ηmeson, the data and the NLO pQCD calculation [6], which uses the AESSS fragmentation functions, agree within the uncertainties for μ=2pTfor factorization and fragmentation scale, while for μ=0.5pTthe calculation overpredicts the data by up to a factor of 2–3, leaving room for future improvements in the understanding of the strange versus non-strange quark fragmentation functions. The PYTHIA 8.2 simulation 123 Eur. Phys. J. C (2017) 77:339 Page 17 of 25 339 Fig. 9 Invariant differential cross section of the π0(left,top panel)and ηmeson (right,top panel) for pp collisions at √s=2.76 TeV. The data are compared to PYTHIA 8.2 [31] generator-level simulations using the Monash 2013 tune as well as recent NLO pQCD calculations [3,6]. The ratios of the data and the calculations to the respective two-component model fits [25,26] to the data are shown in the lower panels.Thehorizontal error bars denote statistical, the boxes systematic uncertainties with the Monash 2013 tune performs slightly worse for the ηthan for the π0, in particular for pT>3GeV/cwhere it underpredicts the data by about 20–30%. In the η/π0ratio, parts of the systematic uncertainties cancel not only for the data but also for the NLO pQCD calculation. Thus, even the predictions using older fragmentation functions for the π0[33] and the η[6], which can not reproduce the individual spectra [5], are in good agreement for the η/π0measurement. PYTHIA 8.2 using the Monash 2013 tune can reproduce the pTdependence of the ratio; however it underpredicts the ratio by about 20–30% above 3 GeV/c, albeit still in agreement with the data to within 1–2σ. The measured η/π0ratio is found to agree with previous measurements in pp collisions at √s=0.2TeV[34] and √s=7 TeV [4] suggesting that η/π0is collision-energy independent. Above 4 GeV/c, both mesons exhibit a similar powerlaw behavior with nπ0=6.29 ±0.02stat ±0.04sys and nη=6.38 ±0.09stat ±0.15sys with χ2/ndof of below 1.8. This is also reflected in the η/π0ratio, which above 4 GeV/c reaches a value of 0.48 ±0.02stat ±0.04sys. 7 Summary Theinvariant differential crosssections for inclusiveπ0andη production at midrapidity in pp collisions at √s=2.76 TeV were measured over a large range in transverse momentum of 0.4<pT<40 GeV/cand 0.6<pT<20 GeV/c, respectively. To achieve these measurements, for the π0(η) five (three) different reconstruction techniques and multiple higher-level triggers involving the EMCal in ALICE were exploited. In particular, a new single-cluster, shower-shape based method was developed to identify high-pTneutral pions whose decay photons overlap in the EMCal. Above 123 339 Page 18 of 25 Eur. Phys. J. C (2017) 77:339 Fig. 10 Measured η/π0ratio in pp collisions at √s=2.76 TeV compared to NLO pQCD calculations [6,33] and PYTHIA 8.2 [14] generator-level simulations using the Monash 2013 tune. The horizontal error bars denote statistical, the boxes systematic uncertainties. The data at √s=0.2TeV[34]and√s=7TeV[4] are shown with statistical and systematic uncertainties added in quadrature 4GeV/c, both the π0and ηcross sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. The data were compared to state-of-the-art NLO pQCD calculations which are found to reproduce the neutral pion cross section within 30%, while the deviations for the ηmeson are significantly larger. Calculations using PYTHIA 8.2 at generator-level with the Monash 2013 tune turn out to be consistent with the π0measurement, except below 1 GeV/c, where the calculation overpredicts the data by up to 50%. For the η, the agreement is slightly worse than for the π0, in particular for pT>3GeV/cwhere the calculation underpredicts the data by about 20–30%. The η/π0ratio, which was found to be described by the calculations to within 1– 2σ,is0.48 ±0.02stat ±0.04sys above 4 GeV/c, consistent with previous measurements. The new data provide significant constraints for future calculations of hadron spectra over a large range in pT. Acknowledgements We thank Werner Vogelsang and Marco Stratmannforprovidingus withtheircalculations.TheALICECollaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support 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 and running 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 and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Education and Sport and Croatian Science Foundation, Croatia; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research | Natural Sciences, the Carlsberg Foundation and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; Ministry of Education, Research and Religious Affairs, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; Indonesian Institute of Science, Indonesia; Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology , Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nationaal instituut voor subatomaire fysica (Nikhef), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Korea Institute 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 Romanian National Agency for Science, Technology and Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation and National Research Centre Kurchatov Institute, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba, Ministerio de Ciencia e Innovacion and Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Spain; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. 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Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2Benemérita Universidad Autónoma de Puebla, Puebla, Mexico 3Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine 4Department of Physics, Centre for Astroparticle Physics and Space Science (CAPSS), Bose Institute, Kolkata, India 5Budker Institute for Nuclear Physics, Novosibirsk, Russia 6California Polytechnic State University, San Luis Obispo, CA, USA 7Central China Normal University, Wuhan, China 8Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France 9Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 10 Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 11 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City, Mérida, Mexico 12 Centro Fermi-Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi’, Rome, Italy 13 Chicago State University, Chicago, IL, USA 14 China Institute of Atomic Energy, Beijing, China 15 COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 16 Departamento de Física de Partículas and IGFAE, Universidad de Santiago de Compostela, Santiago de Compostela, Spain 17 Department of Physics, Aligarh Muslim University, Aligarh, India 18 Department of Physics, Ohio State University, Columbus, OH, USA 19 Department of Physics, Sejong University, Seoul, South Korea 20 Department of Physics, University of Oslo, Oslo, Norway 21 Department of Physics and Technology, University of Bergen, Bergen, Norway 22 Dipartimento di Fisica dell’Università ‘La Sapienza’ and Sezione INFN, Rome, Italy 23 Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy 24 Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy 25 Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy 123 Eur. Phys. J. C (2017) 77:339 Page 23 of 25 339 26 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Bologna, Italy 27 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy 28 Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padua, Italy 29 Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo Collegato INFN, Salerno, Italy 30 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 31 Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 32 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 33 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden 34 European Organization for Nuclear Research (CERN), Geneva, Switzerland 35 Excellence Cluster Universe, Technische Universität München, Munich, Germany 36 Faculty of Engineering, Bergen University College, Bergen, Norway 37 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia 38 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 39 Faculty of Science, P.J. Šafárik University, Kosice, Slovakia 40 Faculty of Technology, Buskerud and Vestfold University College, Tonsberg, Norway 41 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 42 Gangneung-Wonju National University, Gangneung, South Korea 43 Department of Physics, Gauhati University, Guwahati, India 44 Helmholtz-Institut für Strahlenund Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 45 Helsinki Institute of Physics (HIP), Helsinki, Finland 46 Hiroshima University, Hiroshima, Japan 47 Indian Institute of Technology Bombay (IIT), Mumbai, India 48 Indian Institute of Technology Indore, Indore, India 49 Indonesian Institute of Sciences, Jakarta, Indonesia 50 Inha University, Incheon, South Korea 51 Institut de Physique Nucléaire d’Orsay (IPNO), Université Paris-Sud, CNRS-IN2P3, Orsay, France 52 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 53 Institute for Subatomic Physics of Utrecht University, Utrecht, The Netherlands 54 Institute for Theoretical and Experimental Physics, Moscow, Russia 55 Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, Slovakia 56 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 57 Institute of Physics, Bhubaneswar, India 58 Institute of Space Science (ISS), Bucharest, Romania 59 Institut für Informatik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 60 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 61 Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany 62 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 63 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 64 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 65 IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette France, Saclay, France 66 iThemba LABS, National Research Foundation, Somerset West, South Africa 67 Joint Institute for Nuclear Research (JINR), Dubna, Russia 68 Konkuk University, Seoul, South Korea 69 Korea Institute of Science and Technology Information, Taejeon, South Korea 70 KTO Karatay University, Konya, Turkey 71 Laboratoire de Physique Corpusculaire (LPC), Clermont Université, Université Blaise Pascal, CNRS-IN2P3, Clermont-Ferrand, France 72 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 73 Laboratori Nazionali di Frascati, INFN, Frascati, Italy 74 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy 75 Lawrence Berkeley National Laboratory, Berkeley, CA, USA 76 Moscow Engineering Physics Institute, Moscow, Russia 123 339 Page 24 of 25 Eur. Phys. J. C (2017) 77:339 77 Nagasaki Institute of Applied Science, Nagasaki, Japan 78 Physics Department, National and Kapodistrian University of Athens, Athens, Greece 79 National Centre for Nuclear Studies, Warsaw, Poland 80 National Institute for Physics and Nuclear Engineering, Bucharest, Romania 81 National Institute of Science Education and Research, Bhubaneswar, India 82 National Nuclear Research Center, Baku, Azerbaijan 83 National Research Centre Kurchatov Institute, Moscow, Russia 84 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 85 Nikhef, Nationaal instituut voor subatomaire fysica, Amsterdam, The Netherlands 86 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, UK 87 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, ˇ Rež u Prahy, Czech Republic 88 Oak Ridge National Laboratory, Oak Ridge, TN, USA 89 Petersburg Nuclear Physics Institute, Gatchina, Russia 90 Physics Department, Creighton University, Omaha, NE, USA 91 Physics Department, Panjab University, Chandigarh, India 92 Physics Department, University of Cape Town, Cape Town, South Africa 93 Physics Department, University of Jammu, Jammu, India 94 Physics Department, University of Rajasthan, Jaipur, India 95 Physikalisches Institut, Eberhard Karls Universit?t Tübingen, Tübingen, Germany 96 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 97 Physik Department, Technische Universität München, Munich, Germany 98 Purdue University, West Lafayette, IN, USA 99 Pusan National University, Pusan, South Korea 100 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 101 Rudjer Boškovi´c Institute, Zagreb, Croatia 102 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 103 Saha Institute of Nuclear Physics, Kolkata, India 104 School of Physics and Astronomy, University of Birmingham, Birmingham, UK 105 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 106 Sezione INFN, Bari, Italy 107 Sezione INFN, Bologna, Italy 108 Sezione INFN, Cagliari, Italy 109 Sezione INFN, Catania, Italy 110 Sezione INFN, Padua, Italy 111 Sezione INFN, Rome, Italy 112 Sezione INFN, Trieste, Italy 113 Sezione INFN, Turin, Italy 114 SSC IHEP of NRC Kurchatov institute, Protvino, Russia 115 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 116 SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France 117 Suranaree University of Technology, Nakhon Ratchasima, Thailand 118 Technical University of Košice, Kosice, Slovakia 119 Technical University of Split FESB, Split, Croatia 120 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Kraców, Poland 121 Physics Department, The University of Texas at Austin, Austin, TX, USA 122 Universidad Autónoma de Sinaloa, Culiacán, Mexico 123 Universidade de São Paulo (USP), São Paulo, Brazil 124 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 125 Universidade Federal do ABC, Santo Andre, Brazil 126 University of Houston, Houston, TX, USA 127 University of Jyväskylä, Jyväskylä, Finland 128 University of Liverpool, Liverpool, UK 123