Measurement of Z → τ +τ − production in proton-proton collisions at √s=8 TeV

JHEP09(2018)159 Published for SISSA by Springer Received:June 15, 2018 Revised:August 3, 2018 Accepted:September 20, 2018 Published:September 27, 2018 Measurement of Z →τ+τ−production in proton-proton collisions at √s=8 TeV The LHCb collaboration E-mail: [email protected] Abstract: A measurement of Z→τ+τ−production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb−1, from pp collisions at √s= 8 TeV collected by the LHCb experiment. The τ+τ−candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for Zbosons with invariant mass between 60 and 120 GeV/c2, which decay to tau leptons with transverse momenta greater than 20 GeV/c and pseudorapidities between 2.0 and 4.5. The crosssection is determined to be σpp→Z→τ+τ−= 95.8±2.1±4.6±0.2±1.1 pb, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for Z→τ+τ−to Z→µ+µ−(Z→e+e−), determined to be 1.01 ±0.05 (1.02 ±0.06), is consistent with the lepton-universality hypothesis in Zdecays. Keywords: Electroweak interaction, Forward physics, Hadron-Hadron scattering (experiments), Lepton production, Tau Physics ArXiv ePrint: 1806.05008 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. https://doi.org/10.1007/JHEP09(2018)159 JHEP09(2018)159 Contents 1 Introduction 1 2 Detector and datasets 2 3 Event selection 2 4 Signal and background estimation 5 5 Cross-section measurement 8 6 Conclusion 9 The LHCb collaboration 14 1 Introduction The measurement of the production cross-section for a Zboson1using different decay modes in proton-proton (pp) collisions, σpp→Z→f¯ f, is an important verification of Standard Model (SM) predictions. The ratio of the Z→τ+τ−production cross-sections to other leptonic decay modes provides a test of lepton universality (LU). The LEP experiments have performed high accuracy tests of LU at the Zpole, with a precision better than 1% [1]. Consequently, the observation in proton-proton collisions of any apparent deviation from LU in Zdecays would be an evidence of new phenomena producing final-state leptons, like in the theoretical context of mSUGRA [2], constrained NMSSM [3], Randall-Sundrum models [4,5], or lepton-violating decays of Higgs-like bosons [6–10]. This analysis extends the LHCb results obtained with pp collisions at √s= 7 TeV [11] to √s= 8 TeV. The cross-section is measured for leptons from the Zdecay with transverse momentum (pT) above 20 GeV/c and a Zinvariant mass between 60 and 120 GeV/c2, as for the previously published Z→µ+µ−and Z→e+e−cross-sections [12,13]. The cross-section measurements in the pseudorapidity range 2.0< η < 4.5 covered by the LHCb experiment are complementary to those with the central detectors ATLAS [14] and CMS [15]. In the present analysis, the reconstruction of the tau-pair candidates is performed in both leptonic and hadronic decay modes of the tau, requiring at least one leptonic mode for the tau-pair candidate. The reconstruction of high-pTtau leptons in the 3-prong decay mode is performed for the first time in LHCb. 1Zrefers to Z/γ?, i.e. includes contributions from the virtual photon production and interference. – 1 – JHEP09(2018)159 2 Detector and datasets The LHCb detector [16,17] is a single-arm forward spectrometer designed for the study of particles containing bor cquarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a largearea silicon-strip detector located upstream of a dipole magnet with a bending power of 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The tracking system provides a measurement of momentum of charged particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimum distance of a track to a primary vertex (PV), the impact parameter (IP), is measured with a resolution of (15 + 29/pT)µm, where pTis the component of the momentum transverse to the beam, in GeV/c. Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating-pad (SPD) and preshower detectors (PS), an electromagnetic calorimeter (ECAL) and a hadronic calorimeter (HCAL). Muons are identified by a system composed of five stations of alternating layers of iron and multiwire proportional chambers. The online event selection is performed by a trigger, which consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction. The hardware trigger imposes a global event cut (GEC) requiring the hit multiplicity in the SPD to be less than 600, to prevent events with high occupancy from dominating the processing time in the software trigger. This analysis uses pp collisions at √s= 8 TeV corresponding to a total integrated luminosity of L= (1976 ±23) pb−1[18]. Simulated data samples are used to study the event selection, determine efficiencies, and estimate systematic uncertainties. In the simulation, pp collisions are generated using Pythia 8 [19,20] with a specific LHCb configuration [21], and parton density functions taken from CTEQ6L [22]. Decays of hadronic particles are described by EvtGen [23], in which final-state radiation is generated using Photos [24]. The interaction of the generated particles with the detector, and its response, are implemented using the Geant4 toolkit [25,26] as described in ref. [27]. 3 Event selection The Zboson is reconstructed from τparticles decaying into leptonic (muons or electrons) or hadronic (one or three charged hadrons) final states. Charged tracks are reconstructed by the tracking system and matched with clusters of ECAL/HCAL cells and hits in the muon detector. Muon candidates are identified by matching tracks to hits in the muon stations downstream of the calorimeters. They are required to leave hits in at least three muon stations, or four muon stations if they have pT>10 GeV/c. Electron candidates must fail the muon identification criteria and fall within the acceptance of the PS, ECAL, and HCAL sub-detectors. On average, 30% of a material radiation length is crossed by a particle before the bending magnet, causing a considerable energy loss by bremsstrahlung for electrons and positrons. Hence, the electron or positron candidate momentum is corrected using a bremsstrahlung photon recovery technique [28]. However, since the ECAL – 2 – JHEP09(2018)159 is designed to register particles from heavy-flavour hadron decays, calorimeter cells with transverse energy above about 10 GeV saturate the electronics, and lead to incomplete electron bremsstrahlung recovery. A large energy deposit in the PS, ECAL, but not in HCAL is required, satisfying EPS >50 MeV, EECAL/p > 0.1, and EHCAL/p < 0.05, where pis the reconstructed momentum of the electron candidate, after applying the bremsstrahlung photon recovery. Charged hadrons are required to be within the HCAL acceptance, deposit an energy of EHCAL/p > 0.05, and must fail the muon identification criteria. The pion mass is assigned to all charged hadrons. The analysis is divided into seven “streams”, labelled as τµτµ,τµτe,τµτh1,τµτh3, τeτe,τeτh1, and τeτh3, where the subscript denotes the final state reconstructed. Chargeconjugate processes are implied throughout. The streams are chosen such that at least one τ lepton decays leptonically. The tau-pair candidates are selected by triggers requiring muons or electrons with a minimum transverse momentum of 15 GeV/c. The trigger efficiency is between 70% and 85%, depending on the number of leptons in the stream. The final states presented in this analysis account for 58% of all Z→τ+τ−decays. In the following, aτcandidate corresponds to a single particle for the τe,τµ, and τh1decay channels, or a combination of the three hadrons in the case of τh3. A pair of τcandidates must be associated to the same PV. In case where multiple PVs are presented in the event, the associated PV is defined as that with a smallest change in vertex-fit χ2when it is reconstructed with and without the τcandidate. The dominating backgrounds are of QCD origin with one or several jets (call “QCD events” in the following), as well as electroweak processes, mainly W/Z+jets (“Vj”). The following requirements on the transverse momentum of τdecay products are used to reduce these backgrounds. For all the streams the triggering lepton must have pT>20 GeV/c. For the processes τµτµ,τeτe, and τµτethe second lepton pTthreshold is 5 GeV/c. The hadron of the τh1candidates is required to have pT>10 GeV/c. For the τh3decay channel, each of the three charged hadrons are selected with pT>1 GeV/c, and at least one must be above 6 GeV/c. In addition, the τh3candidates must have a total pTin excess of 12 GeV/c, and an invariant mass in the range 0.7 to 1.5 GeV/c2. This leads to the τh3identification efficiency of about 30%, comparable with the value of 35% found in the context of the B0→D∗−τ+ντ analysis [29]. For all streams, the reconstructed direction of the τcandidate must be in the fiducial geometrical acceptance 2.0< η < 4.5. Additional selection criteria are needed to suppress background processes due to semileptonic cor b-hadron decays, misidentification of hadrons as leptons, or, especially in the τh3stream, combinations of unrelated particles. Signal candidates tend to have back-to-back tracks in the plane transverse to the beam axis, and a higher invariant mass than the background. Hence, the tau-pair is required to have an invariant mass above 20 GeV/c2, or 30 GeV/c2for the stream containing τh1,τh3candidates. Additionally, for the dilepton streams τµτµ,τeτe, the selected mass range is below 60 GeV/c2, to avoid the on-shell Z→µ+µ−and Z→e+e−regions. The absolute difference in azimuthal angle of the two τcandidates is required to be greater than 2.7 radians. The above selections are found to be 70 to 80% efficient, depending on the analysis stream. – 3 – JHEP09(2018)159 Charged particles in QCD events tend to be associated with jet activity, in contrast to signal candidates where they are isolated. An isolation variable, ˆ IpT, is defined as the pTof the candidate divided by the transverse component of the vectorial sum of all track momenta in a cone surrounding the candidate of radius Rηφ = 0.5, defined in the pseudorapidity-azimuthal angle (η−φ) space. A fully isolated candidate has ˆ IpT= 1, while lower values indicate the presence of jet activity. The selection ˆ IpT>0.9 is applied to all τ candidates, with an efficiency of more than 64% for the tau-pair signal and rejecting about 98% of QCD events. The lifetime of the τlepton is used to separate the signal from prompt background. For the τdecay channels with a single charged particle, it is not possible to reconstruct a secondary vertex and a selection on the particle IP to the associated PV is applied. The efficiency on the signal from these criteria is in the range 71 to 79%. In the τh3case, a vertex reconstruction is possible: the maximum distance between the three tracks in the η−φspace is required to be less than 0.005 ·pTwhere pTis the transverse momentum of τh3in GeV/c. The proper decay time is subsequently estimated from the distance of the reconstructed vertex to the associated PV, and the momentum of the candidate, taken as an approximation of the τmomentum. A minimum of 60 fs is imposed for this variable, efficiently discarding the prompt background whilst keeping about 77% of the signal. For the τh3decay, a correction to the mass is also possible by exploiting the direction of flight, recovering part of the momentum lost due to undetected particles. The corrected mass is defined as mcorr ≡qm2+p2sin2θ+psin θ , (3.1) where mand pare the invariant mass and momentum computed from the three tracks and θ is the angle between the momentum and flight direction of the candidate. The requirement mcorr <3 GeV/c2reduces the QCD background by about 50% and the Vj background by about 60%, retaining 80% of the signal. Figure 1shows the mass distributions of τh3 candidates before and after correction for data, compared to the distributions of Z→τ+τ− decays and of the Vj background from simulation. In the τeτeand τµτµstreams an additional background component arises from Z→l+l− decays. This process produces two muons or two electrons with similar pTvalues, in contrast to signal which tends to have unbalanced pTdue to the missing momentum from unreconstructed neutrinos and neutral hadrons. The pTasymmetry, ApT, is defined as the absolute pTdifference of the two candidates divided by their sum. For the two leptonic streams ApTis required to be greater than 0.1. A particular case is the τµτestream, where background from Vj processes arises, with one lepton coming from the jet causing a relatively large pTimbalance with respect to the lepton from the W/Z boson. A suppression by a factor of two of this source of background, with a loss of 10% of the signal is obtained imposing a maximal ApTvalue of 0.6. For τh1and τh3the ApTcriterion has been found inefficient for background rejection, hence no such a constraint is imposed to these two decay modes. – 4 – JHEP09(2018)159 Figure 1. Distributions of invariant (dashed line) and corrected (full line, shaded) mass of τh3 candidates from the τµτh3channel. The yields are normalised to the integrated luminosity of the data. The results from data are represented by the black points. The error bars represent the statistical uncertainty only. The distributions are compared to the signal distributions from simulated Z→τ+τ−(blue) events and the Vj (red) background. τµτµτµτh1τµτh3τeτeτeτh1τeτh3τµτe Z→l+l−249.7(88) 1.2(5) — 420.8(253) 16.1(22) — 25.3(54) QCD 50.9(102) 235.8(193) 21.2(53) 42.7(88) 330.8(228) 19.4(51) 160.0(169) Vj 12.7(74) 144.2(430) 5.1(34) 5.8(27) 68.3(197) 10.1(58) 65.3(257) V V 0.2(1) 1.2(2) 0.2(1) 0.2(1) 0.8(1) 0.2(1) 10.0(5) tt 1.0(2) 2.2(2) 0.6(1) 0.2() 0.7(1) 0.1() 5.5(2) Z→bb 0.8(4) 0.3(2) 0.1(1) 0.1(1) 0.3(2) 0.1(1) 0.3(2) Cross-feed 4.5(11) 22.2(25) 13.9(20) 13.0(39) 16.5(24) 7.3(17) 52.5(42) Total bkg. 319.9(127) 407.1(375) 41.1(53) 482.7(242) 433.5(220) 37.2(58) 318.9(236) Observed 696 1373 205 610 861 110 1322 Z→τ+τ−376.1(290) 965.9(521) 163.9(142) 127.3(329) 427.5(358) 72.8(111) 1003.1(418) Table 1. Expected backgrounds yields and total number of candidates observed. In the last row the uncertainties are the statistical and systematic contributions combined. 4 Signal and background estimation After the selections described in the previous section, a maximum of one Z→τ+τ−candidate per event is found. The number of signal candidates is determined from the number of observed candidates in data subtracted by the total number of estimated backgrounds. The results are summarized in table 1. The invariant-mass distributions for such candidates are shown in figure 2, for the seven analysis streams. A data-driven approach is used to estimate the amount of background from QCD and Vj processes. Same-sign (SS) tau-pair candidates are selected with identical criteria as the signal, but requiring the tau candidates to have identical electric charge. From simulation, the SS candidates yield is found to originate mainly from QCD and Vj processes, while – 5 – JHEP09(2018)159 (a) (b) (c) (d) (e) (f) (g) Figure 2. Invariant-mass distributions for (a) τµτµ, (b) τeτe, (c) τµτh1, (d) τeτh1, (e) τµτh3, (f) τeτh3, (g) τµτecandidates with the excluded mass ranges indicated by the gray areas. The Z→τ+τ−simulation (red) is normalised to the observed signal. The Z (blue), QCD (brown), and electroweak (magenta) backgrounds are estimated from data. The tt,V V backgrounds and cross-feed (green) are estimated from simulation (see text) and generally not visible. – 6 – JHEP09(2018)159 the mis-reconstructed Z→τ+τ−process contributes less than 1%: NSS =NSS QCD +NSS Vj + NSS Z→τ+τ−. The last term originates, for instance, from an electron either from a π0decay or pair-production, a single hadron from partially-reconstructed τh3, 3-prong from false combinatorics, or a muon from a misidentified hadron. The amount of QCD and Vj events in the SS dataset is determined by a fit to the pT(τ1)−pT(τ2) distribution, for each analysis stream [11]. In the fit, the QCD distribution templates are taken from an SS QCD-enriched dataset, obtained by the anti-isolation requirement ˆ IpT<0.6; the distributions templates for the two Vj processes (W+jet, Z+jet) are obtained from simulation and are found to be statistically consistent. Subsequently, the number of QCD and Vj background candidates is computed as NQCD =rQCD ·NSS QCD, and NVj =rVj ·NSS Vj . The value of rVj is obtained from simulation, considering both Wand Zcontributions, and varies from 1.05±0.08 for the τeτe up to 2.37 ±0.30 for the τµτh1. The same-sign and opposite-sign QCD-enriched datasets provide the rQCD values, which are all close to unity, with the exception of 1.30 ±0.05, obtained for τµτµ. The Z→l+l−decays (l=e, µ) are a background for all the streams, except for τµτh3 and τeτh3. The number of Z→l+l−decays contaminating the τµτµstream is determined by applying all selection criteria except for the requirement on the dimuon mass: this produces a sample with a clear peak at the Zmass, as well as an off-shell contribution at lower mass, as shown in figure 2a. A template distribution obtained from simulation is normalised to the data in the 80–100 GeV/c2mass interval. The fraction of genuine Z→τ+τ−candidates in the normalisation region is found to be negligible from simulation. The contribution from Z→µ+µ−decays to the background in the signal region is inferred from the normalised distribution. A similar procedure is applied to estimate the τeτebackground from Z→e+e− decays, but with the normalisation performed in the 70–100 GeV/c2interval to account for the electron momentum resolution degraded by an incomplete electron bremsstrahlung recovery. For this process, 1% of non-Zbackground candidates are subtracted from the normalisation region, as estimated from SS dilepton events. The process Z→µ+µ−can be observed as a fake τµτh1candidate when one of the muons is misidentified as a charged hadron. This background is evaluated by applying the τµτh1selection but requiring a second identified muon rather than a hadron, and scaling by the probability for a muon to be misidentified as a hadron. The misidentification probability, obtained from simulation and cross-checked using a tag-and-probe method applied to Z→µ+µ−data (requiring an identified muon as a tag, and an oppositelycharged track as a probe), is of the order of 10−3for muons with pT<10 GeV/c, and 10−4–10−5at larger pTvalues. The uncertainty on the estimation of this background is obtained from the lepton misidentification probability uncertainty combined with the statistical uncertainty of the dimuon candidates sample. A similar procedure allows the estimation of Z→µ+µ−,Z→e+e−backgrounds in τµτe,τeτh1streams. Other background processes are due to diboson decays, ttevents, and Zdecays into b hadrons. Their contributions are relatively small and obtained from simulation. Some of the selected tau-pair candidates may not originate from the stream under study. For instance, a τh1candidate may be selected from a partially reconstructed τh3 candidate. The fraction of cross-feed candidates is obtained from the Z→τ+τ−simulated – 7 – JHEP09(2018)159 sample. The statistical uncertainty is 1 to 3%, to which a small contribution from the uncertainties on the branching fractions of the contaminating streams is added. 5 Cross-section measurement The production cross-section of Zboson to tau-pair is measured for each analysis stream using σpp→Z→τ+τ−=Nobs/εobs rec −PkNbkg,k/εbkg,k rec L B A εsel ,(5.1) where Nobs is the number of observed Zbosons and Nbkg,k is the estimated background from source k. The total integrated luminosity is denoted by L, and Bis the product of the branching fractions of the tau lepton pair to decay to the given final state, with values and uncertainties taken from the world averages [30]. The acceptance factor, A, is needed to normalise the results of each analysis stream to the kinematical region 60 < Mττ <120 GeV/c2, 2.0< ητ<4.5, and pτ T>20 GeV/c, which allows the comparison with the Z→µ+µ−, Z→e+e−decay measurements in LHCb [12,13]. This factor is the fraction of Z→τ+τ− events where the generated τsatisfy the chosen kinematical selections, which also fulfill the fiducial acceptance selection. The value of Afor each stream is obtained from simulation, using the POWHEG-BOX [31–34] at next-to-leading order with PDF MSTW08NLO90cl [35], and Pythia 8.175 [19,20]. The uncertainty on Afrom the choice of PDF is estimated following the procedure explained in ref. [36]. The event reconstruction and selection efficiencies, εrec and εsel, as well as their uncertainties, are estimated from simulation and calibrated using a data-driven method (where applicable) derived from the method described in refs. [11–13]. The term εrec is the product of the GEC, trigger, tracking and particle identification efficiencies. The smallest value of εrec is found to be 9% in the τeτh3stream, while the largest value is 65% for τµτµ. The GEC efficiency is determined from Z→l+l−decays in data collected with a relaxed requirement. The muon and electron trigger efficiencies are evaluated as a function of ηand pTusing a tag-and-probe method applied on Z→l+l−decays. The tracking efficiency for muons uses a tag-and-probe method from Z→µ+µ−decays in data, whereas for electrons and charged hadrons simulated samples are used. The particle identification efficiency is also obtained by a tag-and-probe procedure. In order to cover the signal pTspectrum, different data samples are selected: Z→µ+µ−and J/ψ →µ+µ−decays for muons, Z→e+e−and B+→J/ψ(→e+e−)K+decays for electrons, and D∗+→D0(→K−π+)π+decays for charged hadrons. The efficiency of the selection ranges between 20% for τeτeand 50% for τµτe. The values are obtained from the simulation. Corrections at the level of 1% are inferred by the comparison of the selection-variable distributions for Z→µ+µ−decays in data and simulated samples, which are also added to the systematic uncertainty. A summary of uncertainties is given in table 2, with the statistical uncertainty from Nobs obtained assuming Poissonian statistics. The contribution of the LHC beam energy uncertainty [37] is of 0.2% as studied with the Dynnlo generator [38]. The integrated – 8 – JHEP09(2018)159 L. Giubega32, K. Gizdov52, V.V. Gligorov8, D. Golubkov34, A. Golutvin55,70, A. Gomes1,a, I.V. Gorelov35, C. Gotti20,i, E. Govorkova27, J.P. Grabowski12, R. Graciani Diaz40, L.A. Granado Cardoso42, E. Graug´es40, E. Graverini44, G. Graziani17, A. Grecu32, R. Greim27, P. Griffith22, L. Grillo56, L. Gruber42, B.R. Gruberg Cazon57, O. Gr¨unberg67, C. Gu3, E. Gushchin36, Yu. Guz39,42, T. Gys42, C. G¨obel62, T. Hadavizadeh57, C. Hadjivasiliou5, G. Haefeli43, C. Haen42, S.C. Haines49, B. Hamilton60, X. Han12, T.H. Hancock57, S. Hansmann-Menzemer12, N. Harnew57, S.T. Harnew48, T. Harrison54, C. Hasse42, M. Hatch42, J. He63, M. Hecker55, K. Heinicke10, A. Heister9, K. Hennessy54, L. Henry72, E. van Herwijnen42, M. Heß67, A. Hicheur2, D. Hill57, M. Hilton56, P.H. Hopchev43, W. Hu65, W. Huang63, Z.C. Huard59, W. Hulsbergen27, T. Humair55, M. Hushchyn37, D. Hutchcroft54, D. Hynds27, P. Ibis10, M. Idzik30, P. Ilten47, K. Ivshin33, R. Jacobsson42, J. Jalocha57, E. Jans27, A. Jawahery60, F. Jiang3, M. John57, D. Johnson42, C.R. Jones49, C. Joram42, B. Jost42, N. Jurik57, S. Kandybei45, M. Karacson42, J.M. Kariuki48, S. Karodia53, N. Kazeev37, M. Kecke12, F. Keizer49, M. 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Mackowiak10, S. Maddrell-Mander48, O. Maev33,42, K. Maguire56, D. Maisuzenko33, M.W. Majewski30, S. Malde57, B. Malecki29, A. Malinin69, T. Maltsev38,w, G. Manca22,f , G. Mancinelli6, D. Marangotto21,q, J. Maratas5,v, J.F. Marchand4, U. Marconi15, C. Marin Benito40, M. Marinangeli43, P. Marino43, J. Marks12, G. Martellotti26, M. Martin6, M. Martinelli42, D. Martinez Santos41, F. Martinez Vidal72, A. Massafferri1, R. Matev42, A. Mathad50, Z. Mathe42, C. Matteuzzi20, A. Mauri44, E. Maurice7,b, B. Maurin43, A. Mazurov47, M. McCann55,42, A. McNab56, R. McNulty13, J.V. Mead54, B. Meadows59, C. Meaux6, F. Meier10, N. Meinert67, D. Melnychuk31, M. Merk27, A. Merli21,q, E. Michielin23, D.A. Milanes66, E. Millard50, M.-N. Minard4, L. Minzoni16,g, D.S. Mitzel12, A. Mogini8, J. Molina Rodriguez1,z, T. Momb¨acher10, I.A. Monroy66, S. Monteil5, M. Morandin23, G. Morello18, M.J. Morello24,t, O. Morgunova69, J. Moron30, A.B. Morris6, R. Mountain61, F. Muheim52, M. Mulder27, C.H. Murphy57, D. Murray56, D. M¨uller42, J. M¨uller10, K. M¨uller44, V. M¨uller10, P. Naik48, T. Nakada43, R. Nandakumar51, A. Nandi57, T. Nanut43, I. Nasteva2, M. Needham52, N. Neri21, S. Neubert12, N. Neufeld42, M. Neuner12, T.D. Nguyen43, C. Nguyen-Mau43,n, S. Nieswand9, R. Niet10, N. Nikitin35, A. Nogay69, D.P. O’Hanlon15, A. Oblakowska-Mucha30, V. Obraztsov39, S. Ogilvy18, R. Oldeman22,f , C.J.G. Onderwater68, A. Ossowska29, J.M. Otalora Goicochea2, P. Owen44, A. Oyanguren72, P.R. Pais43, A. Palano14, M. Palutan18,42, G. Panshin71, A. Papanestis51, M. Pappagallo52, L.L. Pappalardo16,g, W. Parker60, C. Parkes56, G. Passaleva17,42, A. Pastore14, M. Patel55, C. Patrignani15,e, A. Pearce42, A. Pellegrino27, G. Penso26, M. Pepe Altarelli42, S. Perazzini42, D. Pereima34, P. Perret5, L. Pescatore43, K. Petridis48, A. Petrolini19,h, A. Petrov69, S. Petrucci52, M. Petruzzo21,q, B. Pietrzyk4, G. Pietrzyk43, M. Pikies29, M. Pili57, D. Pinci26, J. Pinzino42, F. Pisani42, A. Piucci12, V. Placinta32, S. Playfer52, J. Plews47, M. Plo Casasus41, F. Polci8, M. Poli Lener18, A. Poluektov50, N. Polukhina70,c, I. Polyakov61, E. Polycarpo2, G.J. Pomery48, – 15 – JHEP09(2018)159 S. Ponce42, A. Popov39, D. Popov47,11, S. Poslavskii39, C. Potterat2, E. Price48, J. Prisciandaro41, C. Prouve48, V. Pugatch46, A. Puig Navarro44, H. Pullen57, G. Punzi24,p, W. Qian63, J. Qin63, R. Quagliani8, B. Quintana5, B. Rachwal30, J.H. Rademacker48, M. Rama24, M. Ramos Pernas41, M.S. Rangel2, F. Ratnikov37,x, G. Raven28, M. Ravonel Salzgeber42, M. Reboud4, F. Redi43, S. Reichert10, A.C. dos Reis1, F. Reiss8, C. Remon Alepuz72, Z. Ren3, V. Renaudin7, S. Ricciardi51, S. Richards48, K. Rinnert54, P. Robbe7, A. Robert8, A.B. Rodrigues43, E. Rodrigues59, J.A. Rodriguez Lopez66, M. Roehrken42, A. Rogozhnikov37, S. Roiser42, A. Rollings57, V. Romanovskiy39, A. Romero Vidal41, M. Rotondo18, M.S. Rudolph61, T. Ruf42, J. Ruiz Vidal72, J.J. Saborido Silva41, N. Sagidova33, B. Saitta22,f , V. Salustino Guimaraes62, C. 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Zucchelli15 1Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China 4Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy, France 5Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6Aix Marseille Universit´e, CNRS/IN2P3, CPPM, Marseille, France – 16 – JHEP09(2018)159 7LAL, Universit´e Paris-Sud, CNRS/IN2P3, Universit´e Paris-Saclay, Orsay, France 8LPNHE, Sorbonne Universit´e, Paris Diderot Sorbonne Paris Cit´e, CNRS/IN2P3, Paris, France 9I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany 10 Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany 11 Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany 12 Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 13 School of Physics, University College Dublin, Dublin, Ireland 14 INFN Sezione di Bari, Bari, Italy 15 INFN Sezione di Bologna, Bologna, Italy 16 INFN Sezione di Ferrara, Ferrara, Italy 17 INFN Sezione di Firenze, Firenze, Italy 18 INFN Laboratori Nazionali di Frascati, Frascati, Italy 19 INFN Sezione di Genova, Genova, Italy 20 INFN Sezione di Milano-Bicocca, Milano, Italy 21 INFN Sezione di Milano, Milano, Italy 22 INFN Sezione di Cagliari, Monserrato, Italy 23 INFN Sezione di Padova, Padova, Italy 24 INFN Sezione di Pisa, Pisa, Italy 25 INFN Sezione di Roma Tor Vergata, Roma, Italy 26 INFN Sezione di Roma La Sapienza, Roma, Italy 27 Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands 28 Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, Netherlands 29 Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 30 AGH — University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ow, Poland 31 National Center for Nuclear Research (NCBJ), Warsaw, Poland 32 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 33 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 34 Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 35 Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia 36 Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS), Moscow, Russia 37 Yandex School of Data Analysis, Moscow, Russia 38 Budker Institute of Nuclear Physics (SB RAS), Novosibirsk, Russia 39 Institute for High Energy Physics (IHEP), Protvino, Russia 40 ICCUB, Universitat de Barcelona, Barcelona, Spain 41 Instituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela, Santiago de Compostela, Spain 42 European Organization for Nuclear Research (CERN), Geneva, Switzerland 43 Institute of Physics, Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 44 Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland 45 NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 46 Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 47 University of Birmingham, Birmingham, United Kingdom 48 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 49 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 50 Department of Physics, University of Warwick, Coventry, United Kingdom 51 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 52 School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 53 School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 54 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 55 Imperial College London, London, United Kingdom – 17 – JHEP09(2018)159 56 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 57 Department of Physics, University of Oxford, Oxford, United Kingdom 58 Massachusetts Institute of Technology, Cambridge, MA, United States 59 University of Cincinnati, Cincinnati, OH, United States 60 University of Maryland, College Park, MD, United States 61 Syracuse University, Syracuse, NY, United States 62 Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2 63 University of Chinese Academy of Sciences, Beijing, China, associated to 3 64 School of Physics and Technology, Wuhan University, Wuhan, China, associated to 3 65 Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to 3 66 Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia, associated to 8 67 Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 12 68 Van Swinderen Institute, University of Groningen, Groningen, Netherlands, associated to 27 69 National Research Centre Kurchatov Institute, Moscow, Russia, associated to 34 70 National University of Science and Technology “MISIS”, Moscow, Russia, associated to 34 71 National Research Tomsk Polytechnic University, Tomsk, Russia, associated to 34 72 Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia — CSIC, Valencia, Spain, associated to 40 73 University of Michigan, Ann Arbor, United States, associated to 61 74 Los Alamos National Laboratory (LANL), Los Alamos, United States, associated to 61 aUniversidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil bLaboratoire Leprince-Ringuet, Palaiseau, France cP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia dUniversit`a di Bari, Bari, Italy eUniversit`a di Bologna, Bologna, Italy fUniversit`a di Cagliari, Cagliari, Italy gUniversit`a di Ferrara, Ferrara, Italy hUniversit`a di Genova, Genova, Italy iUniversit`a di Milano Bicocca, Milano, Italy jUniversit`a di Roma Tor Vergata, Roma, Italy kUniversit`a di Roma La Sapienza, Roma, Italy lAGH — University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ow, Poland mLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain nHanoi University of Science, Hanoi, Vietnam oUniversit`a di Padova, Padova, Italy pUniversit`a di Pisa, Pisa, Italy qUniversit`a degli Studi di Milano, Milano, Italy rUniversit`a di Urbino, Urbino, Italy sUniversit`a della Basilicata, Potenza, Italy tScuola Normale Superiore, Pisa, Italy uUniversit`a di Modena e Reggio Emilia, Modena, Italy vMSU — Iligan Institute of Technology (MSU-IIT), Iligan, Philippines wNovosibirsk State University, Novosibirsk, Russia xNational Research University Higher School of Economics, Moscow, Russia ySezione INFN di Trieste, Trieste, Italy zEscuela Agr´ıcola Panamericana, San Antonio de Oriente, Honduras aa School of Physics and Information Technology, Shaanxi Normal University (SNNU), Xi’an, China ab Physics and Micro Electronic College, Hunan University, Changsha City, China †Deceased – 18 –