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This content was downloaded from IP address 158.37.84.122 on 30/07/2018 at 09:57 JCAP01(2016)032 hysics P le ic t ar eV and that the frequency of 16 doi:10.1088/1475-7516/2016/01/032 strop A . Similar events have been studied in previous under- 2 − 9 m . 5 > µ osmology and and osmology ρ C 1507.07577 cosmic ray experiments, cosmic rays detectors

ALICE is one of four large experiments at the CERN Large Hadron Collider near 2016 CERN for the benefit of ALICE Collaboration. Content rnal of rnal c from this work may be used under the terms of the Creative

ou An IOP and SISSA journal An IOP and E-mail: ALICE-publications@.ch these events can be successfullycosmic described rays in by this assuming energy ausing range. heavy the mass The latest composition development version of ofsignificant of the primary constraints QGSJET resulting on air to showers alternative, model was more hadronic simulated exotic, interactions. production This mechanismsKeywords: observation for places these events. ArXiv ePrint: Commons Attribution 3.0must maintain License. attribution tocitation the and Any author(s) DOI. and further the distribution title of of the this work, journal work ground experiments such as ALEPH andto DELPHI reproduce at LEP. the While these measuredlow experiments muon and were multiplicity able intermediate distribution multiplicities, withhighest their multiplicity Monte simulations events. Carlo failed In simulations this toALICE work at describe stem we show the from that primary frequency the cosmic high of rays multiplicity the with events observed energies in above 10

The ALICE collaboration Received August 7, 2015 Accepted December 17, 2015 Published January 19, 2016 Abstract. Geneva, specially designed tosions. study Located particle 52 productionused meters in to underground ultra-relativistic detect with heavy-ion 28 colli- paper, produced meters we by present of cosmic the overburden rayson multiplicity rock, interactions with distribution in it Monte of Carlo the has these simulations. upper alsocapability atmospheric This atmosphere. of been muons analysis the In and exploits ALICE the its this Timeof large compari- Projection high size Chamber. multiplicity and events excellent A containing tracking specialto more emphasis than a is 100 muon given reconstructed areal to muons density the and study corresponding

J Study of cosmic raymuon events multiplicity with using high thedetector ALICE at the CERNCollider Large Hadron JCAP01(2016)032 13 eV and ex- 9 eV. The detection of EAS 14 10 E > – 1 – eV), has been performed by several large-area arrays 15 10 × 3 ∼ k eV. In this study, we find that events containing more than four E 20 , stem from primaries with energy Cosmic ray muons are created in Extensive Air Showers (EAS) following the interaction The muon multiplicity distribution (MMD) was measured at LEP with the ALEPH de- 4.1 The4.2 muon multiplicity9 distribution High muon multiplicity9 events of cosmic ray primariesPrimary (protons cosmic and rays heavier span nuclei) a with broad nuclei energy in range, the starting upper at atmosphere. approximately 10 tector [11]. This study concludedstandard that hadronic the production bulk mechanisms, of the buting data that can around the be highest 75-150 successfully multiplicity describedabove muons, events, using expectation, contain- occur even with whenof a assuming iron frequency that nuclei. the which A primary is similar cosmic study almost rays was an carried are order out purely with of composed the magnitude DELPHI detector, which also found 1 Introduction ALICE (A Large Ion Colliderformation Experiment) in [1] ultra-relativistic designed heavy-ion to collisions study athas -Gluon the Plasma CERN also Large (QGP) Hadron been Collider (LHC), use used to of perform high-energyDELPHI studies physics [3] that detectors and are for L3extension of [4] cosmic of during relevance ray these the to physics earlier Large astro-particle studiesunder was - is stable physics. (LEP) pioneered now conditions collider possible by The era for attaking ALEPH at many the between CERN. years. [2], LHC, 2010 An ALICE where and experiments undertookcirculating 2013 can a in during operate programme the pauses of LHC. in cosmic collider ray operations data when there was no beam tending to more thanreconstructed 10 muons in themulti-muon ALICE events (TPC), which we refer to as at ground level (e.g.high [5–7]), energy while muonic deep component underground ofthe EAS. detectors mass The (e.g. composition main [8–10]) and aims have energy of studied spectrum these the of experiments were primary to cosmic explore rays. 5 Results Contents 11 Introduction 2 The ALICE2 experiment 3 Event reconstruction and data44 selection Analysis of the data and6 simulation 6 Summary The ALICE collaboration originating from interactions abovein this the energy, primary in spectrum particular ( around19 the energy of the knee 14 JCAP01(2016)032 . 2 − 9 m . 5 . However, > 2 µ ρ . The apparent area of the detector 2 – 2 – , at the surface, larger than 16 GeV reach the detectors [18]. The geometry of E In this paper, we exploit the large size and excellent tracking capability of the ALICE Details of the environment of ALICE and the detectors used for this analysis are de- The ALICE TPC is the largest detector of its type ever built. It was used to reconstruct Three detector subsystems were used to provide dedicatedACORDE is triggers an array for of 60 this scintillator study: modules located on the three upper faces of the TPC [15] tomultiplicity study distribution with the particular emphasis muonic onmore high component muon than multiplicity of events 100 containing EAS. muons in We a describe single the event and analysis corresponding of to the an muon areal density that Monte Carlo simulationstiplicity were events unable [12]. to accountexplain Several for this proposals the discrepancy. abundance have been ofpercentage Some high put of authors muon forward very suggest mul- in energeticof that the cosmic high hypothetical scientific rays muon strangelets literature [13], multiplicity formmass to events while primary a by others cosmic small the have rays creation tried (iron of nuclei) to the with explain nuclei QGP the in in excess the interactions atmosphere involving [14]. high scribed in the following section,reconstruct atmospheric while muons the are selection discussed of inand section the the 3. data study and The of the muonpresented multiplicity algorithm high in distribution adopted section muon to 5 multiplicity and, events finally, are in described section in 6 section we2 make 4. some concluding The remarks. The results are ALICE experiment ALICE is located at Pointlevel 2 in of the a LHC cavern acceleratorthe 52 complex, electromagnetic m approximately and 450 underground hadronic m components withan above of 28 energy sea the m observed EAS, of so overburden that rock. only muons The with rock absorbs all of the trajectory of cosmiccomprises ray a muons cylindrical passing gas through volume thehas divided an active into inner volume two radius of halves of the byLHC 80 a detector, beam cm, central which direction. an membrane. outer At The radiuschambers each TPC of end with 280 of pad cm the and readout. cylindricalof a volume total For the there length the are detector of multi-wire purpose due 500 proportional of to cm along its detecting the horizontal cosmic cylindrical ray geometry muons, is the approximately total 26 area m ACORDE (Alice COsmic Ray(Time DEtector) Of Flight [19], detector) SPD [21]. (Silicon Pixel Detector)octagonal [20] yoke of and the TOF solenoid,coincidence covering of 10% signals of in itscan two surface different also area. modules be A (a trigger configured two-foldmodules was coincidence), fire. to formed although select by the the events trigger when a single module fires or when more than two We employ a description ofhadronic the interaction shower model based commonly upon used the in latest EAS version simulations. of QGSJET [16, 17], a ALICE is typical ofthat a houses collider several experiment. detectors,on A including one large a end, solenoidal large, there magnetA is cylindrical forms a complete TPC. a single-arm, description Outside central forward of the barrel spectrometer, the solenoid, which apparatus and was is not given used in in [1]. this analysis. after placing a cutmaximum effective on area the reduces minimumalso to length varies approximately with 17 required the m todetail zenith reconstruct in angle section3. a of the cosmic An incident example ray muons. of track Track a the selection single is atmospheric discussed muon in event more is shown in1. figure JCAP01(2016)032 track), which are then , in the upper part of 2 down 1 coincidence. ± – 3 – track) and the other in the lower half ( up . A single atmospheric muon event. The thin outer cylinder is the Time Of Flight detector The SPD is part of the Inner Tracking System located inside the inner field cage of the The TOF is a cylindrical array of multi-gap resistive-plate chambers that completely Cosmic ray data were acquired with a combination (logical OR) of at least two out of TPC. It is composed of76 two mm layers of from silicon the pixelcentred modules LHC upon located beam the at axis, a nominal respectively.into distance interaction of the The point 39 trigger layers of mm have by and thethe an requiring LHC outermost active a layer. beams. length coincidence of The between 28.3 signals SPD cm, in was incorporated thesurrounds top the and outer bottom radius halvessponding of of to the a TPC. cluster The of TOF readout channels trigger covering requires an a area signal of in 500 a cm pad, corre- the three trigger conditionstrigger (ACORDE, efficiency SPD was and studied TOF)section4. with depending Most a events on detailed were the classified Monte as run Carlo either period. simulation, single muon which The events is or multi-muon discussed events, in with joined to create a single muon track. the detector and anothercoincidence signal with in respect a toinvolved pad the in in central the the axis opposite triggerflexibility of lower can has the part been be forming detector. exploited changedplus a The the to via back-to-back two configuration require software. adjacent of a pads In the signal forming pads some a in back-to-back periods an upper of data pad and taking, in this the opposing pad Figure 1 (1). The largecentre inner is the cylinder silicon isthe Inner upper the Tracking half Time System of (3). Projection the detector Muons Chamber ( are (2) reconstructed and as the two smaller TPC tracks, cylinder one at in the JCAP01(2016)032 , r ~ up t tracks. 3 cm in < down xz d and up with a reference track a ~ t – 4 – 990 to accept the analysed track. The reference track was track to reconstruct the full trajectory of the muons and . 0 > down . Finally, each up track was matched to the nearest down track if c = cos(∆Ψ) . r ◦ ~ t · 50 a ~ t to eliminate all possible background from and . In multi-muon < θ < c ◦ 3 cm. This value was chosen to be large enough to maximise the matching efficiency in To quantify the performance of the tracking and matching algorithms, we studied the A muon reconstructed with two TPC tracks (up and down) is called a “matched muon”. Each TPC track can be reconstructed with up to 159 individual space points. In order to As a consequence of reconstructing tracks from the outer radius of the TPC inwards, < xz multiplicity dependence of the reconstructionWe efficiency generated using 1000 Monte events Carlo1 for simulated and events. 20 300, discrete which were values then of reconstructed the using muon the multiplicity, same varying algorithms between applied to real events. the opposite side ofa the “single-track TPC, this muon”. trackwhere is Most part still single-track of accepted muons the as are muon a found trajectory muon candidate to falls but outside cross flagged the the as detector. TPC near its ends requiring that When a TPCmomentum track and fulfils parallelism, but all does the not criteria have a to corresponding be track within a muon track: number of space points, events, accepted tracks were requiredcoming from to the be same EAS approximately arriveforming parallel almost the parallel since at scalar atmospheric ground product muons level. of The the parallelism direction cut involves of thechosen analysed to track give theintroduces largest an number additional of momentum tracksfield. cut satisfying due the The to parallelism momentum the cut.between cut bending 1 This of is requirement and muon a tracks 2the function GeV/ in distance of the of the magnetic closest azimuthd approach angle between of them the athigh muon multiplicity the Monte track horizontal Carlo and events, mid while varies plane keeping combinatorial of background to the a TPC minimum. was maximise the detector acceptanceof for 50 this space analysis, points tracks and,0.5 were GeV/ in required events to where have the a magnetic minimum field was on, a momentum greater than a small percentage of “interaction”the events where iron highly yoke energetic of muons the have interacted magnet with producing a shower of3 particles that pass Event through the reconstruction TPC. andThe data TPC selection tracking algorithm [22]tion was region designed of to the reconstructof tracks two the produced LHC detector in beams. where, the during It interac- ysis collider finds operation, used tracks the the by track same working densitythrough tracking is inwards a lowest. algorithm from central but The the interaction present removed outer point. anal- for any radius However, requirement very the that inclined tracking tracks algorithm (quasiassociated should has pass horizontal) with not the tracks. been most optimised range inclined Therefore, 0 showers, to we avoid restricted reconstruction the zenith inaccuracies angle ofcosmic all ray events muons to are the halves typically reconstructed of as the two TPCFollowing separate this as tracks first in shown pass the of intrack upper the with figure1. reconstruction and its a lower We corresponding newto refer algorithm eliminate to was double applied counting. these towhere Starting tracks match the with as each matching single of muon tracks eventsevents (producing is containing straightforward, two hundreds the TPC of reconstruction tracks), muons.to has been optimise High tuned the multiplicity to matching Monte handle performance. Carlo events have been used JCAP01(2016)032 50, (3.1) ≈ tracks) µ N , down or ) gen up ) N gen gen N )/ N )/ rec rec N • N • gen gen N (# generated muons) N / MEAN (( RMS (( 300). ≈ µ N – 5 – ALICE, MC simulation # reconstructed muons) − as those events with more than four reconstructed muons in 0 50 100 150 200 250 300 0 0.1

4). In total, we collected a sample of 7487 multi-muon events.

0.08 0.06 0.04 0.02 gen rec gen

)/ • ( N N N > µ multi-muon events N (# generated muons . Root-mean-square and mean values of the relative difference between the number of To illustrate the similarity of the data and the Monte Carlo simulation, figure3 shows Data were recorded between 2010 and 2013 during pauses in collider operations when increasing to 5% at high muon multiplicities ( the TPC ( the ratio of the number of muons reconstructed as single tracks (either no beam was circulating inresulting the in LHC. The approximately total 22.6 accumulatedtrack million run or events time matched) with amounted to in atWe 30.8 the least days, define one TPC. reconstructed Only muon multi-muon (single- events are discussed further in this paper. Figure 2 generated and reconstructed muons for events simulated with different muon multiplicities. In each event, muonsTPC were volume. generated parallel2 Figure toshowsrelative the each difference mean other between values like (MEAN) the in and number EAS root-mean-square of and (RMS) generated cross of and the the reconstructed whole muons, as a function ofolution the on number the of numberhighest generated of muons. multiplicities reconstructed it The muons root-mean-square is and represents around is the typically 2%. res- less than The 4%, mean while value for is the lessto than the 1% total up numbermultiplicities. of to The reconstructed ratio muons obtainedsimulated (both from samples single the of and data purerange matched is of proton tracks) compared intermediate primary for with muon cosmic different good the multiplicities agreement ratios rays shown, between obtained and data the from and puresimulated ratio simulations. proton iron varies There and between primaries. is iron 0.2 no samples. significant Over and difference the 0.4 between the with JCAP01(2016)032 15 (ACORDE). > µ N ALICE Number of muons 10 (TOF) and > µ N – 6 – Data Monte Carlo: proton Monte Carlo: Fe 70). The efficiency of the TOF trigger as a function of the muon ≤ 6 8 10 12 14 16 µ N 1 0

0.8 0.6 0.4 0.2

≤

single tracks single muons

/ = R N N 100 high muon multiplicity (HMM) events. Given the nature and topology of > . The ratio of muons reconstructed as single tracks to the total number of reconstructed µ N The MMD obtained from the whole data sample and corrected for trigger efficiency is multiplicity is shown inback-to-back figure4. coincidence The requirement efficiency is oftrigger lower has the at a TOF similar, low trigger. increasing muonthe trend multiplicity two with due The the triggers to efficiency muon reach the multiplicity. of full The the (100%) multiplicities at ACORDE efficiency which are Figure 3 muons (both single and matched tracks) in the data and simulations with4 proton and iron primaries. Analysis of the dataTo and obtain simulation the MMDtrigger. we The have correction was correctedin calculated the this from section. measured a Monte distribution Giventrigger Carlo the for was simulation complementary mainly that the coverage is responsible of efficiencymultiplicities described for the of (7 later selecting TOF the events barrel in to the the low-to-intermediate TPC, range the of TOF muon high multiplicity events, all triggerefficiency. conditions contributed The to aim thismultiplicities of sample and with the to close explore following to the analysis 100% origin is of to the model HMM events. the MMD at low-to-intermediate shown in figure5. Values forof the multiplicity systematic have been uncertainty in estimated the bymatching number varying algorithms. the of parameters events as of We a the70 find function track reconstruction and a and smooth thenwith distribution 5 up events to with a a muon muon multiplicity of multiplicity around greater than 100. We define the events Given the much smallerSPD area of trigger the is SPDcontribution significantly in to lower comparison the than MMD with in the both the TPC, ACORDE low-to-intermediate the range and efficiency of TOF. of muon the It multiplicities. makes only a minor JCAP01(2016)032 ALICE ALICE Number of muons Number of muons Data Systematic uncertainty – 7 – . TOF trigger efficiency as a function of muon multiplicity. 0 2 4 6 8 10 12 14 16 18 20 1

1 −

0 50 100 150 200 250 300 Trigger efficiency Trigger 10 8 7 6 5 4 3 2 1

10 10 10 10 10 10 10 10 Number of events of Number Figure 4 . Muon multiplicity distribution of the whole sample of data (2010-2013) corresponding to The difficulty in describing EAS, and consequently the number of muons reaching In this analysis we have adopted the CORSIKA [23] event generator incorporating ground level, mainly arisesin from hadron-air uncertainties interactions. in These theMonte interactions properties Carlo are of event often generators. described multi-particlecross phenomenologically production Model within sections, parameters, inelastic such scattering asby slopes total measurements and and obtained diffractive inelastic from structure hadron-proton accelerator functions, experiments. are constrained QGSJET [16] for the hadronicof interaction model EAS. to CORSIKA simulate version theMMD generation 6990 distribution and and incorporating development HMM QGSJEThas events; been II-03 CORSIKA used has version to 7350 beenbetween check incorporating the used and QGSJET two to confirm II-04 versions the studyof of results QGSJET the QGSJET for II-04 HMM are and events. a the retuning The inclusion of significant of the differences Pomeron model loops parameters using in early the LHC formalism data for the first Figure 5 30.8 days of data taking. JCAP01(2016)032 is eV θ 14 , cor- 2 ), where θ cos( / 4) that are of interest 03 for energies below . 0 > µ ± eV and with zenith angles 16 GeV N 7 . 18 > µ 10 = 2 E . eV, while primaries in the energy 1 γ − 13 4). Therefore, neglecting these events 10 centred upon the nominal LHC beam < E < < 2 eV were considered in the full simulation. 14 µ 70). Samples of proton and iron primary 14 03 for energies above the knee. The total N . ≤ s sr TeV) 0 10 < E < µ 2 205 m ± N 12 – 8 – × 0 . eV. This revealed that most single muon events E > ≤ 12 005 (m = 3 . 10 0 k γ ± E > , used in the later versions. This results in a steeper lateral 225 . The composition of cosmic rays in this energy range is a . ◦ max 50 X eV) and eV produce muon multiplicities typically in the range from 1 to 4, 15 mesons resulting in an enhancement of the muon content of EAS by 14 10 0 10 < θ < × ρ (1 TeV) = 0 ◦ F , has been adopted with a spectral index = 3 γ − k < E < E E 13 When generating cosmic ray events, the core of each shower was scattered randomly In previous studies of cosmic ray muon events at LEP, QGSJET 01 was used to model To have a fast and flexible method of estimating several important parameters and ob- To understand the complete sample of the recorded data, including the origin of low The first step in the analysis was to attempt to reproduce the measured MMD in the Fe) primaries with energies 56 at ground level over an area covering 205 time [24]. Most relevantforward to neutral the hadron present production studythe in is the production that QGSJET of pion II-04, exchange which is has assumed been to shown to dominate enhance muon distribution and anshower, which associated can increase also of have an the impact muon on density the close observed to rate of the HMM core events. of the range 10 independent of the masstherefore of produce the a negligible primary contribution cosmicin to this rays. multi-muon events study. ( Primaries Consequently, only with energies energies below 10 about 20% [25]. hadronic interactions. Apart fromsignificant the difference way between in thisdeeper which earlier nonlinear shower version maximum, effects of are the modelled, model another and QGSJET II-03/04 is the crossing point.without This creating area any was biasoutside chosen on this to area, the minimise only finaland a the these results. events very number were small We always of of number found low events of that, multiplicity ( to events when gave be the rise generated core to was muons crossing located the TPC servables involved in the analysis,simulation we did started not with a explicitlythe simplified model Monte trajectories interactions Carlo in of simulation.lines the the This to rock muons the above arriving depth the at of experiment. ALICE the Instead, while surface imposing were an simply energy extrapolated cut as straight muon multiplicity events, we generated( events initiated by the interaction of proton and iron low-intermediate range of multiplicity (7 does not affect the results reported in this paper. the zenith angle of theresponding muon. to the All horizontal muons cross-sectional passing area this of cut the and TPC, crossing were considered an to area be of detected. 17 m stem from primaries in the energy range 10 cosmic rays were generatedin in the the energy interval rangemixture 0 10 of many speciesenergy. To of simplify nuclei the analysis incosmic and a ray interpretation of flux ratio the using that datanuclei, a we is and have pure not modelled a proton the well-known pure sample,relation primary and to iron representing the which a sample, MMD, varies composition representing thegiven with dominated a proton multiplicity, while sample by composition the provides light iron dominated aspectrum, sample lower by provides limit heavy an on upper nuclei. the limit. number In of A events typical for power a law energy the knee ( (all particle) flux of cosmicmain rays chemical has elements been at calculatedestimated 1 by to TeV summing be [26] the where individual measurements fluxes of are the most precise. The flux was JCAP01(2016)032 2 30, > µ N 100 (as can be seen in figure5) > µ N 30 does not allow for a precise, quan- 03), which results in an uncertainty of eV were subsequently considered for a > . 0 µ 14 N ± 10 7 . – 9 – 70) we have used the same simulation framework = 2 E > ≤ γ µ N ≤ At lower multiplicities, corresponding to lower primary energies, we find that the data The errors in figure6 are shown separately (statistical and systematic) for data, while All events generated with energies Following success in describing the magnitude and shape of the MMD over this inter- approach the proton curve,ray which flux, represents while a lightcomposition. higher ion multiplicity The composition data limited of statistics lie the in closer the primary to range cosmic the iron curve, representing a heavier for Monte Carlo they areThe systematic the errors quadrature in sum the ofrays simulations at the take 1 statistical TeV, into and the accountthe systematic slope uncertainties rock uncertainties. of in above the the the flux energy experiment of(detector spectrum live cosmic and below time). the and The uncertainty above largest in thein contribution the knee, to the the the the spectral systematic description number error of of index is days due below of to data the the uncertainty taking knee ( complete analysis usingin a matter detailed surrounding simulation thehorizontal taking experiment. mid-plane into of account Incentred the all each upon experiment possible event, the and interactions all TPCwere flagged muons with recorded if were no along they extrapolated restriction withto hit to on their the an the the position ALICE enlarged energy simulation andthe area of framework. momentum environment the of at above In and muons. 36 ground thisdescribed. around level m framework, All the and the flagged Flagged apparatus used ALICE muons as muonsAny experimental as well were muon hall input as that propagated and crossed all through the thethat detector this detectors produced apparatus environment pseudo-raw are was data, with treated accurately which bythat GEANT3 was a was then [27]. detector applied processed response to with simulation the realalgorithm same data, developed reconstruction including for code the this TPC analysis. tracking algorithm and4.1 the track matching The muonWe multiplicity generated distribution simulated events equivalentwith to the 30.8 days data live without timethe the to trigger need permit to corrected, direct apply measured comparison comparison, an MMD the arbitrary with points normalisation the factor. obtained simulationsto with is A obtain the shown comparison the simulations in of curves were figure6. for fitted For proton with ease and a of iron. power-law function titative study of thethe composition, average but mass the ofconsistent distribution the with below primary several this cosmic previous multiplicity ray experiments suggests flux [28–31]. that increases with increasing energy, a finding to study the frequencystatistics of sample HMM of events. simulated Since HMMcomparison. events these was are required particularly to rare permit events, a a meaningful very quantitative 4.2 high High muonTaking multiplicity the events dataset astions, a we whole, find corresponding 5 to HMM 30.8 events with days muon and multiplicities a mixture of running condi- mediate range of multiplicities (7 approximately 15% in the MMD. Thecorresponds error in to the description an of the uncertaintywhich rock in above results the the experiment in energygives a threshold a systematic contribution of error of the aroundstatistical of muons 2% uncertainties approximately reaching to are the 4%. the dominant. systematic detector, error. Each of For muon the multiplicities other uncertainties JCAP01(2016)032 . 2 eV − 18 10 eV contribute < E < . The estimated 16 2 16 10 Hz. 5 m 6 . − 0 Hz at the underground E > ± 10 6 − × 9 Number of muons 10 . 0 × ± 9 . , counting both matched and single-track µ N Data Systematic uncertainty cosmic ray Monte Carlo: proton as primary ray Monte Carlo: Fe as primary cosmic Monte Carlo: proton fit Monte Carlo: Fe fit – 10 – 100. HMM events are therefore events with a muon areal > and correspond to a rate of 1 Hz. Each of these events were examined closely to exclude the µ 2 N 6 − − ALICE 10 4 m 10 20 30 40 50 60 70 . × 0 9 3 2

1 .

± 10 Number of events of 10 Number 10 9 . 5 > µ . The measured muon multiplicity distribution compared with the values and fits obtained ρ The estimated maximum fiducial area of the TPC due to its horizontal cylindrical The rate of HMM events obtained with the Monte Carlo can be compared with the One of the aims with this study is to compare the rate of HMM events obtained from sim- error in themuons, number is of around reconstructed 5% muons, density for geometry and cut on the minimum number of TPC space points is 17 to these events. Therefore,have been only generated events to inprimaries. achieve the an range equivalent of of 365 primary days energy exposure 10 for both proton and iron location of ALICE. Basedstatistical upon uncertainty the is number of 45%, observed giving HMM an events, error the in estimated the relative rate of observed rate. Since we havetime, simulated samples the of statistical HMM uncertainty events in corresponding the to one simulated year rate live will be lower than that in the measured ulations to the measuredHMM rate. events, we To have limit simulated the aQGSJET live effect II-03 time of for equivalent to fluctuations the one hadronic instep year interaction the with model. of CORSIKA number 6990 the of The using simplified simulated analysis Monte demonstrated Carlo used that as only a first primaries with energy possibility of “interaction” events.was The found highest to multiplicity event containFor reconstructed 276 illustration, in muons, a the which TPC angular display corresponds distributions of to of this a thethe event muon muons spatial is areal from distribution shown the density ofin in same of matched figure9. HMM7. figure 18.1 and We event m single-track note are The muonsof that shown at the zenithal the in majority the TPC and figure8, of TPC whereeither azimuthal single-track while mid muons the muons plane are upper may is reconstructed or enter near shown lower or the halves leave ends of the the active detector. volume without producing a track Figure 6 from CORSIKA simulations withThe proton errors and are iron shownthe separately primary quadrature (statistical cosmic sum and rays systematic) of for for the 30.8 data, statistical days while and of for systematic data Monte uncertainties. taking. Carlo theygiving are a rate of 1 JCAP01(2016)032 Azimuth Angle (Deg) ALICE 170 180 190 200 210 220

0

80 60 40 20 200 Counts 180 160 140 120 100 – 11 – Zenith Angle (Deg) 10 20 30 40 50 ALICE

0

80 60 40 20 120 Counts 100 . Event display of a multi-muon event with 276 reconstructed muons crossing the TPC. . Zenithal and azimuthal distribution of the multi-muon event with 276 reconstructed muons. Figure 7 Figure 8 JCAP01(2016)032 Z [cm] Matched tracks Single tracks QGSJET II-04 CORSIKA 7350 . Given that the acceptance of the 2 4142 7248 8846 30 7836 32 84 52 29 83 71 35 62 31 61 58 Simple MC Full MC proton iron proton iron 205 m × ALICE – 12 – QGSJET II-03 CORSIKA 6990 4040 6131 6426 27 4333 24 52 51 25 64 42 20 31 22 34 53 Simple MC Full MC proton iron proton iron 0 •300 •200 •100 0 100 200 300

300 200 100 •100 •200 •300 X [cm] X 1 2 3 4 5 Run . Spatial distribution of the 276 recostructed muons indicating matched and single-track . Number of HMM events for each run obtained with the simplified Monte Carlo and the full TPC is almost 3000 timesA smaller, this summary ensures of that the theCORSIKA samples 6990 results are statistically with obtained independent. QGSJET for II-03 all and five CORSIKA simulations 7350 is with QGSJET presented II-04. in table1 for both Table 1 simulation. Each run isCORSIKA equivalent 6990 to with 365 QGSJET days of II-03 data and taking. CORSIKA 7350 The with events have QGSJET been II-04. rate. generated using Resultsthe obtained simplified for Monte Carlo thesimulations) and are number the shown in full of the simulation firstthe HMM row (the detailed of first modelling events table1. of of Comparison expected the five ofof underground the statistically in environment results HMM independent has demonstrates one about events. that a yearEAS 30% Due effect sample from to on to the the both number perform smallshower four over numbers additional the of simulations usual HMM by surface events randomly level we assigning area reused the of the core 205 same of simulated each Figure 9 muons. JCAP01(2016)032 49 6.2 1.9 Data 7%) . 22 28 1.0 1.9 11.6 6.0 QGSJET II-04 CORSIKA 7350 proton iron 7%) 3.5 (5 . QGSJET II-04 CORSIKA 7350 31.4 60.8 proton iron 1.1 (3 25 25 0.8 1.3 – 13 – 15.5 8.6 QGSJET II-03 CORSIKA 6990 proton iron 5%) 5.0 (12%) . QGSJET II-03 CORSIKA 6990 23.6 42.2 proton iron 1.3 (5 Hz] i σ 6 N − h 10 × Period [days per event] Rate [ Uncertainty (%) (syst + stat) HMM events . Mean value and statistical uncertainty in the number of HMM events for 365 days live time . Comparison of the HMM event rate obtained with the full simulation and from measurement. There are two major contributions to the systematic uncertainty on the number of Final values for the HMM event rate for proton and iron primaries were calculated by HMM events. Theestimate its first contribution contribution wedata stems taking, took from for the each the element firstof and muon simulated each HMM CORSIKA reconstruction sample, events code algorithm. corresponding version usingsecond and to different contribution redetermined To 365 the stems tunes number from days ofas the of the discussed uncertainties track in of selection the section of parameters and.4.1 HMM used matching events This in of algorithms. was the approximatelydays), estimated 20%. simulations, The the to Due systematic give to uncertainty anuncertainty the is is uncertainty large dominant. in dominant, sample the The while used systematicstatistical predicted in in uncertainties uncertainty the rate the have in simulations been data the (365 added final (30.8obtained in comparison from days) quadrature of the the to the Monte the statistical observed Carlo rate simulations. of HMM events with that 5 Results In table3 we presentHMM the events with results the ofCORSIKA measured this 7350 rate. analysis andthe where QGSJET We measured we II-04 note value. compare produces that Theis the a equivalent the lower, rate rate HMM pure although obtained of event iron with stillsimulations simulated rate sample CORSIKA consistent comes in 6990 simulated with primarily and close with from the QGSJETdifficult agreement the measured to II-03 with reconcile hadronic rate. the model measured The rate used of to difference HMM generate events between with the the the EAS. simulated two rate It obtained is using more taking the average value obtainedwas estimated from from the the five standard simulations, deviationthe of while mean the the number 5 statistical values of from uncertainty HMMthe the events mean. full expected2 simulation. Table summarises in one year for each primary ion calculated from Table 3 Table 2 calculated using the full simulation. JCAP01(2016)032 eV, corresponding 17 Z(m) 10 × 3 E > ALICE Simulation 0 20 40 60 80 100 – 14 – 20 − 40 − eV eV eV eV 17 17 18 16 10 10 × × • 10 • 10 60 16 17 − • 3 • 3 10 10 17 16 × × eV and corresponds to the equivalent of 5 years of data taking. 3 3 10 10 80 18 − 10 Energy range of primary cosmic ray Energy range − 100 0 − 16

20 40 60 80 80 60 40 20

− − − − 100 X(m) centred upon ALICE, which is located at the origin in figure 10 . The average 2 . The surface level spatial distribution of the cores of simulated EAS giving rise to more eV, where recent measurements [32, 33] suggest that the composition of the primary 16 140 m Finally, we have investigated the distribution of simulated EAS core positions at the 10 × to the highest energyrise interval studied to in HMM this eventscase, analysis, when produce the the larger mean shower showers core ofRMS that falls value the may of farther give shower the from core distribution the distribution is location from 18 of m. the ALICE. centre In6 of this ALICE is Summary 37 mIn and the the period 2010 toapproximately 2013, 22.6 ALICE million events acquired containing 30.8 atthe days least measured of one muon dedicated reconstructed multiplicity cosmic muon. distribution ray with Comparison data an of recording equivalent sample of Monte Carlo events location of ALICE for each of7350 the and HMM events QGSJET simulated II-04 withshown iron in primaries in1, table using figure CORSIKA equivalent to10,primary 5 where cosmic years ray the so of as colour data toof of give taking. a the each visual The core point representation distribution of from indicates the is ray. the correlation the We between centre energy the note of distance associated that ALICE140 with the at shower the surface cores level ofdistance and all the of HMM energy the events of shower fallvalue the core of within primary the from an cosmic distribution area the is of centre 16 approximately of m. ALICE Primaries for with all an events energy is 19 m and the RMS cosmic ray spectrum is dominated by heavier elements. than 100 muons inthe the energy ALICE range Time 10 Projection Chamber. The simulation was forproton iron primaries, independent primaries of in thethe version measured rate of prevents the us model. fromalthough drawing However, a heavy the firm nuclei large conclusion about uncertainty appearof in the to origin HMM of be these events events, theEAS in most described terms likely of by candidates.observations. a conventional Therefore, hadronic This heavy an is primary mechanismsE explanation consistent cosmic appears > with ray to the composition be fact at that compatible high they with stem energy our from and primaries with energies Figure 10 JCAP01(2016)032 E > have been recorded. We have found that ◦ eV due to the light component followed by 15 10 × – 15 – The expected rate of HMM events is sensitive to assumptions made about the dominant Compared to the previous studies at LEP, there are two distinguishing aspects of this High muon multiplicity events were observed in the past by experiments at LEP but eV were found to give rise to HMM events. This observation is compatible with a knee in 16 hadronic production mechanisms indiffers air from shower earlier development. versions The ina latest its higher treatment version muon of of yield forward QGSJET production neutral and meson has in production been 7 resulting TeV retuned in events, proton-proton taking observed collisions. at into the account relatively This earlyusing shallow is LHC depth a of results conventional the ALICE, hadronic on first has modeltion hadron been time for that satisfactorily the that places reproduced description the significant of constraints rate extensive on air of alternative, showers; HMM more anwork observa- exotic, that production have mechanisms. ledthe to ability these to new generate large insights samplesestimate into of of the very the energetic origin cosmic expected of rays,been rate allowing HMM the for of events. a recent these more The advances reliable field. events. first in We has the The note been hadronic second, that descriptionversion and in of of a more CORSIKA EAS. preparatory important, (version study This 6031), aspect [18]simulated no is has for carried HMM a 30 out events continually were days by observed evolving of ALICEpresent in data in the work, taking 2004, MMD with table3 using distribution agives an pureby a older iron two quantitative primary more comparison cosmic ray of recenthadronic composition. the versions description In rate of of the of EAS CORSIKAhas HMM in and been events recent QGSJET, predicted a years. illustrating significantobserved in increase Only the this in in evolution study. the the of latest rate the version of of HMM the events model thatAcknowledgments better there approaches the rate The ALICE Collaboration wouldvaluable like contributions to to the thank construction of allfor the its the experiment engineers and outstanding the and performance CERNacknowledges technicians accelerator of the teams for resources the and their LHC support in- complex.Computing provided by The Grid all Grid ALICE (WLCG) centres Collaboration and collaboration.lowing gratefully the funding Worldwide agencies LHC The for ALICE theirCommittee support Collaboration of in Science, acknowledges building World the and Federationmenia; running fol- of Conselho the Nacional Scientists ALICE de (WFS) Financiadora Desenvolvimento detector: Tecnol´ogico(CNPq), Cient´ıficoe and State Swiss Fonds Kidagan, Ar- a spectral steepening, the onset of which depends on the atomic number (Z) of the primary. the cosmic ray energy distribution around 3 the observed rate ofusing HMM QGSJET events II-04 is toiron model consistent composition the with for development the the of primary rate10 the cosmic predicted resulting rays. by air Only shower, CORSIKA primary assuming cosmic 7350 a rays pure with an energy without satisfactory explanation. Similarstudy high with multiplicity events ALICE. have Over beenmore the observed than 30.8 100 in muons this days and of zenith angles data less taking than 50 reported in this paper, 5 events with suggests a mixed-ion primarywith energy. cosmic This ray observation compositionrange is with of in the an agreement knee. with averagelow-to-intermediate most mass Following range the experiments that of successful working increases muon description instudy multiplicities of the the we the energy frequency used magnitude of the of HMM same the events. simulation MMD framework in to the JCAP01(2016)032 ]. m.w.e. SPIRE IN 320 S08002 3 (2005) 295[ JINST B 138 , 2008 ]. SPIRE IN Detection of muon bundles from cosmic (2003) 190[ – 16 – Nucl. Phys. Proc. Suppl. , A 510 The ALICE experiment at the CERN LHC Measurements of the muon component of extensive air showers at Nucl. Instrum. Meth. , collaboration, J. Ridky and P. Travnicek, ]. collaboration, SPIRE IN ALICE [ underground DELPHI ray showers by the DELPHI experiment [1] [2] C. Grupen et al., [3] References de Estudos e Projetos(FAPESP); National (FINEP), Amparo Natural Funda¸c˜aode `aPesquisa Science do FoundationEducation of Estado (CMOE) China and de (NSFC), the S˜aoPaulo the Ministryof Chinese of Education Science Ministry and and of Technology Youththe of of Carlsberg China the (MSTC); Foundation and Ministry Czech thesearch Republic; Danish Council National Danish under Research Natural the Foundation;Institute The Science European of European Research Community’s Physics Re- Seventh Council, and Framework theLoire’, Programme; Academy Helsinki ‘Region of Alsace’, Finland; Frenchfur ‘Region CNRS-IN2P3, Bildung, Auvergne’ the Wissenschaft, ‘Region and Pays Forschungation; CEA, und de General France; Technologie (BMBF) Secretariat German and forNational Bundesministerium the Research, Research Helmholtz Development and Associ- and Technology,Atomic Ministry Innovation Energy Office of and (NKFIH), Development, Department Hungary;tituto Greece; of Nazionale Department Science di of and Fisica TechnologyCentro Nucleare of Studi (INFN) the e and Government Centro Ricerche of(JSPS) Fermi “Enrico KAKENHI India; - and Fermi”, Is- Museo MEXT, Italy; Japan; Storico JapanResearch Joint della Institute Society Foundation Fisica for Nuclear for of e Research, the Korea Dubna;CYT), Promotion National (NRF); Direccion of General Consejo Science de NacionalLatine Asuntos Formation de academique del Cienca - Personal European y Academico(DGAPA),gram Commission M´exico,Amerique Tecnologia (European (ALFA-EC) and (CONA- Particle theOnderzoek EPLANET Physics Pro- der Latin Materie American (FOM)derzoek and Network); the (NWO), Stichting Netherlands; Nederlandse voor Research Organisatie CouncilPoland; voor Fundamenteel of Ministry Wetenschappelijk On- Norway of (NFR); National Nationalof Education/Institute Science Scientific Centre, for Research Atomic in Physicsucation Higher and and Education National Science (CNCSI-UEFISCDI), Council ofAgency Romania; Russian Ministry of Federation, of Atomic Russian Ed- Russian Energy, Academy Foundation Russian of for Federal Sciences, BasicScience Agency Russian Research; and Federal for Technology, Ministry South Science of Africa;tales Centro Education and y de Tecnologicas of Innovations Investigaciones (CIEMAT), E-Infrastructure Energeticas, Slovakia; and(EELA), shared Medioambien- Department The between Ministerio of Europe and de Latincia Econom´ıay America (Conseller´ıade Competitividad Educaci´on),Centro (MINECO) de(CEADEN), of Cubaenerg´ıa,Cuba, and Aplicaciones Spain, IAEA Tecnol´ogicasy (International Xunta Desarrollo AtomicResearch Energy de Nuclear Council Agency); Gali- Swedish (VR)istry and of Knut Education & and(STFC); Alice Science; The United United Wallenberg States Kingdom Foundation Departmentdation, of Science (KAW); the Energy, and Ukraine the State Technology United of Min- Facilities States Texas,of Council National and Croatia Science the and Foun- State Unity of throughResearch Ohio; (CSIR), Knowledge Ministry Fund, New of Croatia; Delhi, Science, Council India; Education of Pontificia and Scientific Universidad Sports and Cat´olicadel Per´u. 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Armesto 2 , A. Furs 103 , , , A. Festanti 95 , C. Cicalo , 72 57,81 , A. Bogdanov , E. D´enes 3 96 , R. Averbeck 97 13 28 , J. Castillo 71 , Z. Buthelezi 27 27 110 , A. Feliciello 57 81 , P. Ghosh , A. Bercuci , L. Cunqueiro 77 , , B. Bathen 36 , A. Bhasin , M.A. Diaz 28 , A. Dash , T. Breitner 62 , M. Estienne 78 63 , P. Bartalini , S. Biswas 4 , M. Cherney , A.M. Barbano 93,74 , D. Finogeev , S. Cho , A. De Falco , R. Belmont 36,33,59 36 72 , G.E. Bruno , N. Bianchi 136 132 4 51 3 , T. Drozhzhova , C. Alves Garcia , D. Elia 70 120 , D. Aleksandrov 36,62 61 98 128,93 43 121 122 , L. Fabbietti , P. Foka 110 122 22 , R. Bailhache , M. Chartier , A. Alkin 131 136 , A. Dobrin 132 , S. Arcelli 40 , A. Borissov 65 , C. Furget , T. Antiˇci´c , M. Gallio 36 , I.G. Bearden , S. Das , A. Deloff 110 , A. Ferretti 18 , D. Felea , M. Agnello 53 , Z. Chunhui 15 , F. Bock 36 54 22 , L. Calero Diaz , S. Basu 102 , L. Betev 17 , C. Ceballos Sanchez 54 36 95 12,104 , E. Cuautle , E. Garcia-Solis 53 , D. Colella , I. Berceanu , R. Biswas 70 , V. Barret , O. Busch 36 96,93 , A. Augustinus 136 , P. Di Nezza , F. Carnesecchi 100,51 11 96 33 , P. Christakoglou , M. Gheata , R.C. Baral , E. Bruna 27 , M. Bregant , I. Cort´esMaldonado 80 , B. Espagnon , O. Dordic 36 , S. Filchagin , L. Bianchi , M.E. Connors 27,36 36 , V. Chelnokov 101 36 , R. Bellwied 113 36 36 46 , S.N. Alam 96 , R.J. Ehlers 27 132 53 2 51 , I. Altsybeev 129 , H. Borel , S. Foertsch , J. de Cuveland 128 , U. Fuchs 100 58 70 , D.D. Chinellato , A. Alici , G. Eyyubova , S. Bagnasco 18 , J. Anielski 53 36 , I. Das , S. Chapeland 96 103 , C. Blume , A.M. Gago , Ø. Djuvsland 36 64 120,124 , N. Bastid 44 , P.C. Batzing 93 , S. Beole 111 , S.U. Chung , H. Caines , L. Feldkamp 100 99 , E.G. Ferreiro 123 27 36 , A. Deisting 28 7 66 , H. Appelsh¨auser , P. Buncic 2 74 135 128 , C. Cavicchioli 31 , A. Bilandzic , A. Gheata , D. Berzano , W. Carena , C. Garabatos , F. Colamaria , J. Bhom , L.S. Barnby 113 , G. Aglieri Rinella , B. Audurier – 19 – , A. Di Mauro , J. Book , B. D¨onigus 7 , R. Cruz Albino , F. Erhardt 25 83 57 36 89 , E.J. Brucken 45 , M. Floris 113 27,36 87 , P. Dupieux 53 33 41 , S. Choudhury 135 , D. Das 70 53 120 40 , X. Cai , D. Blau 93 , A. Akindinov , R. Divi`a , S. Altinpinar 80 62 130 , B. Chang , Y.W. Baek 36 10 , M. Basile , M. Fasel , H. Bello Martinez , V. Anguelov 53 , G. de Cataldo , U. Frankenfeld 136 43 , T. Chujo , Y. Corrales Morales , C. Gao , P. Braun-Munzinger , S. Chattopadhyay , Z. Conesa del Valle , M. Gagliardi 53 115 , S. Evdokimov 106 72 , G. Bencedi 28 96 , B. Batyunya 34 84 , R. Alfaro Molina 52 71 80 , M.A.S. Figueredo 132 76 70 , F. Carena 101 , M. Broz 31,12 36,30 88,36 , S. Bufalino , D. Di Bari , S. De Pasquale 101 , J. Cleymans , V. Chibante Barroso 26 , I. Erdemir , J. Bielˇc´ıkov´a , P. Crochet , L. Aphecetche , A. Danu 117 , R.A. Bertens , M. Germain , T. Alt 94 , E.A.R. Casula , M. Bombara , M.G. Fleck 53 , S. Aiola 48 , M. Arslandok , F. Blanco 40 36 , A. Fern´andezT´ellez 38 110 104 , L. Ducroux 130 , D. Caffarri 104 107 135 118 , M.M. Aggarwal 33 68 , B. Bhattacharjee , F. Baltasar Dos Santos Pedrosa 22 125 135 , M. Chojnacki , G.G. Barnaf¨oldi 83 , F. Bellini , A. Badal`a , J. Cerkala 57 118 20 , E. Bartsch , A. Fantoni 113,36 83 87 , P. Dillenseger 15 95 33 , S. B¨ottger 55 , D. Evans 43 , A. Andronic 71 110 , V. Belyaev 117 89 , J. Figiel , P. Ganoti 27 95 78 , A. De Caro , T.M. Cormier 64 , P. Christiansen , J. Alme , F. Costa , J.J. Gaardhøje , D. Domenicis Gimenez 74 , P. Camerini , J. Bielˇc´ık , A. Francescon , D. Alexandre 80 120 31 , F. Cindolo , S.U. Ahn 70,15 119 , B. Cheynis , D. Berenyi , A. Dainese , P. Dhankher 120 , A. Dubla 77,i , S. Chattopadhyay , H. Buesching , E.M. Fiore 102 , P. Antonioli 11,40 , T.A. Browning , K. Choi , J.T. Blair 109 , N. De Marco , A. Batista Camejo , J. Ferencei , E.F. Gauger , I.C. Arsene 110 , A.J. Castro , M. Bach , F. Barile , G. Conesa Balbastre , L. Boldizs´ar 85 33 132 , I. Belikov , J. Faivre 15 98 130 25 , J. Eum , B. Erazmus 53 36 57 , J.T. Buxton 132 , A.K. Bhati 134,57 , J. Bartke , T. Dietel , P. Cerello 15 , E. Botta 107 , D. Adamov´a 19 29 113 131 28 12,28 80 110 , S. De , A. Baldisseri 92,37 16 , C. Andrei 90 110 107 36 10 128 40 65 136 12,31 40 37,92 48 90 120 S. Esumi D. Fabris E. Epple Corchero T. Dobrowolski A.K. Dubey G. Batigne C. Bedda J.B. Butt E. Calvo Villar Gruttola G. D’Erasmo R. Bala K. Barth E. Belmont-Moreno Y. Berdnikov D. Budnikov Castellanos J. Cepila J.L. Charvet C. Cheshkov P. Chochula S. Dash M.D. Azmi R. Barbera I.R. Bhat C. Bianchin S. Bjelogrlic H. Bøggild F. Boss´u T.A. Broker C.H. Christensen L. Cifarelli M. Colocci J.G. Contreras M.R. Cosentino T. Dahms F. Antinori R. Arnaldi The ALICE collaboration J. Adam Z. Ahammed B. Alessandro J.R.M. Almaraz Prado M. Fusco Girard F.M. Fionda E. Fragiacomo D.R. Gangadharan P. Gasik V.J.G. Feuillard G. 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Mischke 117 36 85 135 , S. Kumar , A. Masoni 43 , T.K. Nayak 120 62 48 85 96 115 , J. Lien 27 79 , V. Lenti , M. Keil , M.G. Munhoz , T. Gunji , B.A. Hess , L. Karayan , K.L. Graham , V. Kondratiev , R. Lea , P. Hristov , A. Maevskaya , J.H. Kang 1 14 , D.A. Moreira De Godoy , G.V. Margagliotti 82 , S. Hayashi 57 , A. Mamonov , E. Gladysz-Dziadus 11 56 23,103 79,87 , A. Menchaca-Rocha , C. Mayer 36 136 , S. Kushpil 133 , M. Kim , Y. Kharlov 53 131 , H.J. Jang 128 , A.G. Knospe 120 30 60 96 , M. Malaev 30 , M.M. Mieskolainen , H.M. Ljunggren , S. Grigoryan , G. Kiss 104 76 , V. Muccifora 98 44 85 , V. Kuˇcera 117 , B. Naik 93 1 , R. Grosso , M. Kretz 20 120 , M. Marquard , N. Mohammadi , D.S. Hwang 36 136 , V. Nikulin , X. Li 128 20 133 , V. Ivanov 59 , P. Gonz´alez-Zamora , M. Haiduc , M. Kowalski 41 , K. Nayak 62 43 130 , E. Torres L´opez 99 , M. Kopcik , M. Nicassio 118 , M. Masera 96 , G. Mart´ınezGarc´ıa 52 51 131 7,70 , E. Laudi 70 56 , L. Kumar 2 125 96 , J. Kamin , J. Mareˇs 122 27 48 , J.S. Kim , H. Gulkanyan , R.T. Jimenez Bustamante 36 , R.C. Lemmon , A. Kolojvari , A. Matyja , J.D. Mulligan , M. Morando , P. Giubilato , R. Hosokawa , Y. Miake , M.A. Lisa , L.M. Minervini , A. Kisiel , T. Karavicheva , P. Malzacher , C. Jahnke 80 , Y. Melikyan , S. Li , A. Kuryakin 137 , A. Morsch 81 , C. Lagana Fernandes 27,89 122 , D.L.D. Keijdener – 20 – 10 55 , R.D. Majka 39 , M.L. Knichel 96 , P.H.F.N.D. Luz 22 58 56 128 , A. Grigoryan , G. Herrera Corral 58 , D. Hutter 132 24 , F. Ng 56 , S. Nikulin , C.M. Mitu 115 29 , A. Khanzadeev 135 , J. Musinsky , M. Ivanov , A.M. Kubera 113 96,43 , L.K. Graczykowski , D. Hatzifotiadou , X. Lopez , C. Markert 55 36 26 138 76 78 19 63 99 36,110 36 19 43 132 , A. Kravˇc´akov´a 132 64 13 74 96 , C. Nattrass , J. Kumar , C. Hadjidakis , J.-Y. Grossiord , V. Kovalenko , A. Lattuca , H. Kim 59 , S. Masciocchi , S. Jena , M.I. Mart´ınez , A. Kalweit 16 , A. Konevskikh 90 18 36 , F. Lehas 15 36 59 123 , H. Masui , P. L´evai , M. Meres , F. Meddi 120 , E. Montes , A. Gomez Ramirez 36 3,77 , S. Kiselev 111 , A. Maire , A. Kluge 33 27 79,30 , J. Milosevic 93 , L. Nellen , R. Keidel , L. Musa 11 64 , B. Hippolyte 43 , M. Irfan , J. Mitra 122 , C. Lippmann , P. La Rocca , S. Jadlovska , M. Marchisone , K. Gulbrandsen , T. Hussain , A. Morreale 22 54 , A. Mar´ın , A.B. Kurepin , M. Mukherjee , T. Kollegger , O. Karavichev 36 58 52 , S. Nikolaev , D. Mal’Kevich 65 , A. Harton 36 , S.A. Khan , V. Grigoriev , G. Luparello 96 , V. Grabski , I. Kr´alik 43 48 , C. Loizides 28 45 30 57 56 , P. Giubellino 110 36 69 80 , A. Herghelegiu , M. Krzewicki 132 36 30 , A. Mas , A. Kumar 136 76,99 76 100 , M.U. Naru 116 123 72 ,ii,66 , D.J. Kim , Ø. Haaland 3 38 103,36 36 , A. Lardeux , P. Kalinak 81 , C. Jena , I. Kisel , I. Legrand 103 44 54 52 43 101 , P. Martinengo 122 , O. Kovalenko 137 , L. Milano , M. Leoncino , S. Murray , E. Kondratyuk 36 , D. Mcdonald , J. Kral 113 , A. Mastroserio , P. Khan , S. Moretto , N. Hussain , U. Kebschull 76 , S. Muhuri 119 , A. Nedosekin , A. Kurepin , J. Margutti , B. Guerzoni , D. Mi´skowiec 48 2 , M.B. Jadhav , H. Hillemanns , C. Grigoras , J. Mercado P´erez , B.S. Nielsen 19 108 , S.M. Mahmood , M. Kofarago , V. Loginov 20 , C. Klein-B¨osing , S. Gotovac 66,58 56 113 , J.W. Harris 54 98 , P. Gianotti 77 37,92 , C. Lara 71 , M. Ippolitov , M. Lunardon , L. Monta˜noZetina , E. Nappi , S. Lee 49 , S.L. La Pointe , L. Malinina , D.M. Gom´ezCoral , J.F. Grosse-Oetringhaus 74 , R. Haake , V. Manzari , E. Kryshen 104 46 , H. Helstrup 36 31 36 , V. Lindenstruth 90 4 18 , P.G. Kuijer , Y. Martynov , S. Kirsch , D.W. Kim 114 , A. Jusko 42 117 53 55 135 , A. Karasu Uysal 63 93 90 70 83 128 54 22 104 101 137 109 55 36 44 36,93 137 137 132 I. Le´onMonz´on Cervantes F. Manso A. Margotti J. Martin Blanco Pedreira L. Massacrier M.A. Mazzoni E. Meninno K. Mikhaylov R.H. Munzer R. Nania S. Nazarenko L. G¨orlich R. Guernane T.E. Hilden M. Inaba P.M. Jacobs P.H.S.Y. Jayarathna H. Jung Meethaleveedu F. Krizek Y. Kwon S. Lindal P.I. Loenne P. Luettig S. Mahajan A.N. Mishra L. Molnar D. M¨uhlheim J. Niedziela A. Grigoras N. Grion R. Gupta G. Hamar M. Heide T.J. Humanic S. Kar E. Karpechev Mohisin Khan B. Kim C. Kouzinopoulos C. Kuhn P. Kurashvili R. Langoy G.R. Lee L.A.P. Moreno S.K. Ghosh P. Gl¨assel T. Kim C. Kobdaj N. Kondratyeva J. Klein JCAP01(2016)032 , , , , , , , , 35 , 61 , , , , 122 136 , 4 , , , 7 90 , , , 110 96 29 100 , , 134 62 , , 111 , 30 132 , , , , 57 53 , , , 125 , 134,36 , 99 , 36 49 117 19 18 , , , 81 , 83 , 104 109 99,76 , 135 , , 107 , , 137 , L. Valencia 93 , 71 , X. Ren , 77 127 120 , P. Roy , J. Pan , P.K. Sahu , L. Pinsky , 48 57 , 72,36 , , , F. Riggi 101 90 83 100 55 , N. Smirnov 110 55 61 , A. Toia , , S.K. Prasad , A. Sharma , H.R. Schmidt 93 36 132 117 , V. Petrov , K. Schwarz 128 129 , V. Singhal 93,36 83 123 ˇ Sumbera , A. Sevcenco 96 , M. Tariq 122 , C. Terrevoli , K.F. Read 135 , P. Pareek 40 111 , L. 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Ortiz Velasquez 93 , P. Vande Vyvre 22 , K. Redlich 47 116 90 133 130 , S. Sakai 132 , E. Rogochaya 96 110 85 , S. Vallero 25 , S. Rajput 18 62 , F. Soramel 46 83 71 22 4 7 132 70 L. Ronflette A.J. Rubio Montero K. Røed S. Sadovsky J. Saini G. Simatovic B.C. Sinha R. Renfordt V. Riabov C. Ristea V. Samsonov F. Scarlassara R.J.M. Snellings Z. Song M. Spyropoulou-Stassinaki R. Preghenella G. Puddu J.S. Real A. Shabanov M. Sharma S. Siddhanta M. Petrovici D.B. Piyarathna W. Poonsawat S. Raha R. Raniwala S. Schuchmann K. Schweda D. Sekihata W.J. Park T. Peitzmann C.E. P´erezLara P.L.M. Podesta-Lerma P. Nomokonov H. Oeschler M. Ozdemir A.K. Pandey J. Oleniacz R. Orava E. Stenlund T.J.M. Symons M. Szymanski M.A. Tangaro T. Sugitate M.G. Tarzila B. Teyssier S. Trogolo T.S. Tveter Palomo T. Vanat M. Vasileiou L. Vickovic A. Velure JCAP01(2016)032 , , 7 , 120 57 , , 3 , 43 31 , Z. Zhang , , , , 7 , H. Yang , 57 77 98 7,113 , 96 , H.J.C. Zanoli 134 96,36 58 , M. Zyzak 104 , T. Virgili 3 , J. 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Yang JCAP01(2016)032 ˇ Reˇzu Prahy, Czech Republic – 23 – ˇ a´rknvriy osc,Slovakia Saf´arikUniversity, Koˇsice, Faculty of Technology, Buskerud andFrankfurt Institute Vestfold for University Advanced College, Studies,Germany Vestfold, Johann Norway Wolfgang Goethe-Universit¨atFrankfurt, Frankfurt, Gangneung-Wonju National University, Gangneung,Gauhati South University, Korea Department ofHelsinki Physics, Institute Guwahati, of India PhysicsHiroshima (HIP), University, Helsinki, Hiroshima, Finland Japan Indian Institute of TechnologyIndian Bombay Institute (IIT), of Mumbai, India TechnologyInha Indore, University, Indore (IITI), Incheon, South India Institut Korea de Physique d’Orsay Nucl´eaire Institut (IPNO), f¨urInformatik, Universit´eParis-Sud, Johann CNRS-IN2P3, Wolfgang Orsay,Institut Goethe-Universit¨atFrankfurt, Frankfurt, France Germany f¨urKernphysik, Johann WolfgangInstitut Goethe-Universit¨atFrankfurt, M¨unster,Germany Frankfurt, Germany f¨urKernphysik, Westf¨alischeWilhelms-Universit¨atM¨unster, Institut Pluridisciplinaire Hubert CurienFrance (IPHC), Universit´ede Strasbourg, CNRS-IN2P3,Institute Strasbourg, for Nuclear Research,Institute Academy of for Sciences, Subatomic Moscow, PhysicsInstitute Russia of for Utrecht Theoretical University, and Utrecht,Institute Experimental Netherlands of Physics, Experimental Moscow, Physics, Russia Institute Slovak of Academy Physics, of Academy Sciences,Institute Slovakia of Koˇsice, of Sciences Physics, of Bhubaneswar, theInstitute India Czech of Republic, Space Prague, ScienceInstituto Czech (ISS), de Republic Bucharest, Ciencias Romania Nucleares,Instituto Universidad de Nacional Universidad F´ısica, Mexico Aut´onomade Nacional M´exico, City,iThemba Mexico Aut´onomade M´exico, Mexico LABS, City, National Mexico ResearchJoint Foundation, Somerset Institute West, for South NuclearKonkuk Research Africa University, (JINR), Seoul, Dubna, South Russia Korea Korea Institute of ScienceKTO and Karatay Technology Information, University, Daejeon, Konya,Laboratoire South de Turkey Korea Physique Corpusculaire (LPC),CNRS–IN2P3, Clermont Clermont-Ferrand, France Universit´e,Universit´eBlaise Pascal, Laboratoire de Physique Subatomique etGrenoble, France de Cosmologie, Universit´eGrenoble-Alpes, CNRS-IN2P3, Laboratori Nazionali di Frascati, INFN,Laboratori Nazionali Frascati, Italy di Legnaro, INFN,Lawrence Berkeley Legnaro, Italy National Laboratory, Berkeley,Lawrence Livermore California, National United Laboratory, States Livermore,Moscow California, Engineering United Physics States Institute,National Moscow, Centre Russia for NuclearNational Studies, Institute Warsaw, for Poland PhysicsNational and Institute Nuclear of Engineering, ScienceNiels Bucharest, Education Romania Bohr and Institute, Research, Bhubaneswar, UniversityNikhef, India of Nationaal Copenhagen, instituut Copenhagen, voorNuclear Denmark subatomaire Physics fysica, Group, Amsterdam, STFC Netherlands Nuclear Daresbury Physics Laboratory, Daresbury, Institute, United Academy Kingdom of Sciences of the Czech Republic, Dipartimento Interateneo di FisicaDivision ‘M. of Merlin’ and Experimental Sezione HighEberhard INFN, Energy Karls Physics, Bari, Universit¨atT¨ubingen,T¨ubingen,Germany University Italy European of Organization Lund, for Lund, Nuclear Sweden ResearchExcellence (CERN), Cluster Geneva, Universe, Switzerland TechnischeFaculty Universit¨atM¨unchen,Munich, Germany of Engineering, BergenFaculty University of College, Mathematics, Bergen, Physics Norway Faculty and of Informatics, Nuclear Comenius SciencesCzech University, and Republic Bratislava, Physical Slovakia Engineering, CzechFaculty Technical of University Science, in P.J. Prague, Prague, Oak Ridge National Laboratory,Petersburg Oak Nuclear Ridge, Physics Tennessee, Institute, United States Gatchina, Russia 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 42 33 34 35 36 37 38 39 40 41 84 85 JCAP01(2016)032 – 24 – Physics Department, Creighton University,Physics Omaha, Department, Nebraska, Panjab United University, States Physics Chandigarh, Department, India University ofPhysics Athens, Department, Athens, University Greece ofPhysics Cape Department, Town, University Cape of Town,Physics South Jammu, Department, Africa Jammu, University India ofPhysik Rajasthan, Department, Jaipur, Technische India Universit¨atM¨unchen,Munich, Germany Physikalisches Institut, Heidelberg, Ruprecht-Karls-Universit¨atHeidelberg, Germany Purdue University, West Lafayette,Pusan Indiana, National United University, States Pusan,Research Division South and Korea ExtreMeSchwerionenforschung, Matter Darmstadt, Institute Germany EMMI, GSIRudjer Zagreb, Helmholtzzentrum Croatia Boˇskovi´cInstitute, f¨ur Russian Federal Nuclear Center (VNIIEF),Russian Research Sarov, Centre Russia Kurchatov Institute,Saha Moscow, Institute Russia of NuclearSchool Physics, of Kolkata, Physics Departamento India and deSecci´onF´ısica, Astronomy, Ciencias, University Pontificia of Universidad delSezione Birmingham, Cat´olica Per´u,Lima, INFN, Birmingham, Peru Bari, United Kingdom Italy Sezione INFN, Bologna, Italy Sezione INFN, Cagliari, Italy Sezione INFN, Catania, Italy Sezione INFN, Padova, Italy Sezione INFN, Rome, Italy Sezione INFN, Trieste, Italy Sezione INFN, Turin, Italy SSC IHEP of NRCStefan Kurchatov Meyer institute, Institut Protvino, f¨urSubatomare Physik Russia SUBATECH, Ecole (SMI), des Vienna, Mines Austria Suranaree de University Nantes, of Universit´ede Technology, Nantes, NakhonTechnical University CNRS-IN2P3, Ratchasima, Slovakia of Nantes, Thailand Koˇsice, Koˇsice, France Technical University of SplitThe FESB, Henryk Split, Niewodniczanski Croatia InstitutePoland of Nuclear Physics,The Polish University Academy of of Sciences, TexasUniversidad Cracow, at Aut´onomade Austin, Sinaloa, Culiac´an,Mexico PhysicsUniversidade Department, Austin, de Texas, S˜aoPaulo (USP), U.S.A. 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