Observation of the decay Λ 0b → ψ(2S)pπ−

JHEP08(2018)131 Published for SISSA by Springer Received:June 22, 2018 Accepted:August 9, 2018 Published:August 21, 2018 Observation of the decay Λ0 b→ψ(2S)pπ− The LHCb collaboration E-mail: [email protected] Abstract: The Cabibbo-suppressed decay Λ0 b→ψ(2S)pπ−is observed for the first time using a data sample collected by the LHCb experiment in proton-proton collisions corresponding to 1.0, 2.0 and 1.9 fb−1of integrated luminosity at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The ψ(2S) mesons are reconstructed in the µ+µ−final state. The branching fraction with respect to that of the Λ0 b→ψ(2S)pK−decay mode is measured to be BΛ0 b→ψ(2S)pπ− BΛ0 b→ψ(2S)pK−= (11.4±1.3±0.2)% , where the first uncertainty is statistical and the second is systematic. The ψ(2S)p and ψ(2S)π−mass spectra are investigated and no evidence for exotic resonances is found. Keywords: B physics, Flavor physics, Hadron-Hadron scattering (experiments), Spectroscopy ArXiv ePrint: 1806.08084 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. https://doi.org/10.1007/JHEP08(2018)131 JHEP08(2018)131 Contents 1 Introduction 1 2 Detector and simulation 2 3 Event selection 3 4 Signal yields and efficiencies 4 5 Systematic uncertainties 6 6 Results and summary 7 The LHCb collaboration 12 1 Introduction The Λ0 bbaryon is the isospin-singlet ground state of a bound system of a beauty quark and two light quarks. The high production rate of b quarks at the Large Hadron Collider (LHC) [1–5], along with the excellent mass resolution and hadron-identification capabilities of the LHCb detector, give access to a variety of decay channels of the Λ0 bbaryon, including multibody, rare, charmless and semileptonic decays [6–25]. The high signal yield of the Λ0 b→J/ψpK−decay [15] facilitated a precise measurement of the Λ0 blifetime [26], while the relatively low energy released in the Λ0 b→ψ(2S)pK−and Λ0 b→χcpK−decays allowed for precise measurements of the Λ0 bmass [16,22]. A six-dimensional amplitude analysis of the Λ0 b→J/ψpK−decay resulted in the observation of the Pc(4380)+and Pc(4450)+pentaquark states decaying into the J/ψp final state [27]. Later, these states were confirmed using a model-independent technique [28]. Subsequently, an analysis of Cabibbo-suppressed Λ0 b→J/ψpπ−decays found evidence for contributions from the Pc(4380)+and Pc(4450)+pentaquarks and from the Zc(4200)−tetraquark [29]. The first observation of Λ0 bdecays to the excited charmonium state ψ(2S) was made in the Λ0 b→ψ(2S)Λ decay mode by the ATLAS collaboration [30]. Later, the decay Λ0 b→ψ(2S)pK−was observed by the LHCb collaboration [16]. The Cabibbo-suppressed analogue of the latter decay, Λ0 b→ψ(2S)pπ−, is of particular interest because of possible contributions from exotic states in both the ψ(2S)p system, similar to the Pc(4380)+and Pc(4450)+pentaquark states, and in the ψ(2S)π−system, analogous to the charged charmonium-like state Zc(4430)−studied in detail by the Belle and LHCb collaborations in B→ψ(2S)π−K decays [31–35]. Depending on the nature of a proposed exotic state, its coupling with the ψ(2S) meson can be larger than with the J/ψmeson. For example, – 1 – JHEP08(2018)131 the decay rate of the X(3872) particle to the ψ(2S)γfinal state was found to exceed the corresponding decay rate to the J/ψγ final state [36,37]. This paper reports the first observation of the decay Λ0 b→ψ(2S)pπ−using a data sample collected by the LHCb experiment in proton-proton collisions corresponding to 1.0, 2.0 and 1.9 fb−1of integrated luminosity at centre-of-mass energies of 7, 8 and 13 TeV, respectively. A measurement is made of the Λ0 b→ψ(2S)pπ−branching fraction relative to that of the Cabibbo-favoured decay Λ0 b→ψ(2S)pK−, Rπ/K≡BΛ0 b→ψ(2S)pπ− BΛ0 b→ψ(2S)pK−,(1.1) where the ψ(2S) mesons are reconstructed in the µ+µ−final state. Throughout this paper the inclusion of charge-conjugated processes is implied. 2 Detector and simulation The LHCb detector [38,39] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 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 the 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. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors (RICH). Photons, electrons and hadrons are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers. The online event selection is performed by a trigger [40], 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 selects muon candidates with high transverse momentum or dimuon candidates with high value of the product of the pTof each muon. The subsequent software trigger is composed of two stages, the first of which performs a partial event reconstruction, while full event reconstruction is done at the second stage. In the software trigger, each pair of oppositely charged muons forming a good-quality two-track vertex is required to be significantly displaced from all PVs and the mass of the pair is required to exceed 2.7 GeV/c2. The techniques used in this analysis are validated using simulated events. In the simulation, pp collisions are generated using Pythia [41] with a specific LHCb configuration [42]. Decays of hadronic particles are described by EvtGen [43], in which final-state radiation – 2 – JHEP08(2018)131 is generated using Photos [44]. The interaction of the generated particles with the detector, and its response, are implemented using the Geant4 toolkit [45,46] as described in ref. [47]. 3 Event selection The signal Λ0 b→ψ(2S)pπ−and the normalization Λ0 b→ψ(2S)pK−decays are both reconstructed using the decay mode ψ(2S) →µ+µ−. Similar selection criteria, based on those used in ref. [16], are applied to both channels. Muon, proton, pion and kaon candidates are identified using combined information from the RICH, calorimeter and muon detectors. They are required to have a transverse momentum larger than 550, 900, 500 and 200 MeV/c, respectively. To allow for an efficient particle identification, kaons and pions are required to have a momentum between 3.2 and 150 GeV/c, whilst protons must have a momentum between 10 and 150 GeV/c. To reduce the combinatorial background due to particles produced in the pp interaction, only tracks that are inconsistent with originating from a PV are used. Pairs of oppositely charged muons consistent with originating from a common vertex are combined to form ψ(2S) →µ+µ−candidates. The mass of the dimuon candidate is required to be between 3.67 and 3.70 GeV/c2, where the asymmetric mass range around the known ψ(2S) mass [48] is chosen to account for final-state radiation. The position of the reconstructed dimuon vertex is required to be inconsistent with that of any of the reconstructed PVs. To form signal (normalization) Λ0 bcandidates, the selected ψ(2S) candidates are combined with a proton and a pion (kaon) of opposite charges. Each Λ0 bcandidate is associated with the PV with respect to which it has the smallest χ2 IP, where χ2 IP is defined as the difference in the vertex-fit χ2of a given PV reconstructed with and without the particle under consideration. To improve the Λ0 bmass resolution, a kinematic fit [49] is performed. This fit constrains the four charged final-state particles to form common vertex, the mass of the µ+µ−combination to the known ψ(2S) mass and the Λ0 bcandidate to originate from the associated PV. A good quality of this fit is required to further suppress combinatorial background. In addition, the measured decay time of the Λ0 bcandidate, calculated with respect to the associated PV, is required to be between 0.2 and 2.0 mm/c to suppress poorly reconstructed candidates and background from particles originating from the PV. To suppress cross-feed from B0→ψ(2S)K+π−decays with the positively charged kaon (negatively charged pion) misidentified as a proton (antiproton) for the signal (normalization) channel, a veto is applied on the Λ0 bcandidate mass recalculated with a kaon (pion) mass hypothesis for the proton. Any candidate with a recalculated mass consistent with the nominal B0mass is rejected. A similar veto is applied to suppress cross-feed from B0 s→ψ(2S)K−K+decays with the positively charged kaon misidentified as a proton, and additionally for the signal channel, the negatively charged kaon misidentified as a pion. Finally, to suppress cross-feed from the Λ0 b→ψ(2S)Λ decay, followed by a Λ→pπ−decay, candidates with a pπ−mass that is consistent with the nominal Λ mass [48] are rejected. – 3 – JHEP08(2018)131 5.6 5.65 0 10 20 30 40 50 60 5.6 5.65 1 10 2 10 3 10 Candidates/(5 MeV/c2) Candidates/(2 MeV/c2) mψ(2S)pπ−mψ(2S)pK− GeV/c2 GeV/c2 LHCb LHCb Λ0 b→ψ(2S)pπ− background total fit Λ0 b→ψ(2S)pK− background total fit qdata qdata Figure 1. Mass distributions of the (left) Λ0 b→ψ(2S)pπ−and (right) Λ0 b→ψ(2S)pK−candidates. 4 Signal yields and efficiencies The mass distributions for the selected Λ0 b→ψ(2S)pπ−and Λ0 b→ψ(2S)pK−candidates are shown in figure 1. The signal yields are determined using unbinned extended maximum-likelihood fits to these distributions. For each distribution the Λ0 bcomponent is described by a modified Gaussian function with power-law tails on both sides [50,51]. The tail parameters are fixed to values obtained from simulation, and the peak position and resolution of the Gaussian function are free to vary in the fit. The combinatorial background component is described by a monotonic second-order polynomial function with positive curvature. The resolution parameters obtained from the fits are found to be 5.23 ±0.55 MeV/c2 for the Λ0 b→ψ(2S)pπ−channel and 3.96 ±0.13 MeV/c2for the Λ0 b→ψ(2S)pK−channel, which are in good agreement with expectations from simulation. The signal yields are determined to be 121±13 and 806±29 for the Λ0 b→ψ(2S)pπ−and Λ0 b→ψ(2S)pK−decay modes, respectively. The resonance structure of the Λ0 b→ψ(2S)pπ−decay is investigated using the sPlot technique [52] for background subtraction, with the reconstructed ψ(2S)pπ−mass as the discriminating variable. The background-subtracted mass distributions of ψ(2S)p, ψ(2S)π−and pπ−combinations are shown in figure 2, along with those obtained from simulated decays generated according to a phase-space model. The ψ(2S)p and ψ(2S)π−mass distributions show no evidence for contributions from exotic states. The mass distribution of the pπ−combination differs from the phase-space model, indicating possible contributions from excited N0and ∆0states. Further studies with a larger data sample will provide a deeper insight into the underlying structure of the Λ0 b→ψ(2S)pπ−decay. The ratio of branching fractions Rπ/K, defined in eq. (1.1), is measured as Rπ/K=NΛ0 b→ψ(2S)pπ− NΛ0 b→ψ(2S)pK− εΛ0 b→ψ(2S)pK− εΛ0 b→ψ(2S)pπ− ,(4.1) – 4 – JHEP08(2018)131 4.8 5 5.2 5.4 0 5 10 15 20 25 30 35 40 4 4.2 4.4 4.6 0 5 10 15 20 25 30 35 40 1.2 1.4 1.6 1.8 0 5 10 15 20 25 30 35 40 NΛ0 b/(50 MeV/c2) NΛ0 b/(50 MeV/c2) NΛ0 b/(50 MeV/c2) mψ(2S)p mψ(2S)π−mpπ− GeV/c2 GeV/c2 GeV/c2 LHCb LHCb LHCb simulation simulation simulation qdata qdata qdata Figure 2. Background-subtracted mass distributions of the (left) ψ(2S)p, (centre) ψ(2S)π−and (right) pπ+combinations in the Λ0 b→ψ(2S)pπ−decay compared with distributions obtained from a phase-space simulation. where Nrepresents the measured yield and εdenotes the efficiency of the corresponding decay. The efficiency is defined as the product of the geometric acceptance and the detection, reconstruction, selection and trigger efficiencies. The hadron-identification efficiencies as functions of kinematics and the event multiplicity are determined from data using the following calibration samples of low-background decays: D∗+→D0(→K−π+)π+, K0 S→π+π−and D+ s→φ(→K+K−)π+for kaons and pions; and Λ →pπ−and Λ+ c→pK+π−for protons [53,54]. The remaining efficiencies are determined using simulation. The pTand rapidity spectra and the lifetime of the Λ0 bbaryons in simulated samples are adjusted to match those observed in a high-yield low-background sample of reconstructed Λ0 b→J/ψpK−decays. The simulated samples are produced according to a phase-space decay model. The simulated Λ0 b→ψ(2S)pK−decays are corrected to reproduce the pK−mass and cos θpK−distributions observed in data, where θpK−is the helicity angle of the pK−system, defined as the angle between the momentum vectors of the kaon and Λ0 bbaryon in the pK−rest frame. To account for imperfections in the simulation of charged tracks, corrections obtained using data-driven techniques are also applied [55]. The efficiencies are determined separately for each data-taking period and are combined according to the corresponding luminosity [56] for each period and the known production cross-section of bb pairs in the LHCb acceptance [1–5]. The ratio of the total efficiency of the normalization channel to that of the signal channel is determined to be εΛ0 b →ψ(2S)pK− εΛ0 b →ψ(2S)pπ− = 0.761 ±0.004 ,(4.2) where only the uncertainty that arises from the sizes of the simulated samples is given. Additional sources of uncertainty are discussed in the following section. The kaon identification efficiency, entering into εΛ0 b →ψ(2S)pK−, is the main factor causing non-equality of the total efficiencies for the signal and normalization channels. – 5 – JHEP08(2018)131 5 Systematic uncertainties Since the signal and normalization decay channels have similar kinematics and topologies, most systematic uncertainties cancel in the ratio Rπ/K, e.g. those related to muon identification. The remaining contributions to the systematic uncertainty are listed in table 1 and discussed below. To estimate the systematic uncertainty related to the fit model, pseudoexperiments are sampled from the baseline fit models with all parameters fixed from those obtained from the fits to the data. For each pseudoexperiment fits are performed with a number of alternative models for the signal and background components and the ratio Rπ/Kis computed. A generalized Student’s t-distribution [57] and an Apollonios function [58] are used as alternative models for the signal component, while polynomial functions of the second and the third order with various constraints for monotonicity and convexity are used as alternative backgrounds. The maximum relative bias found for Rπ/Kis 0.7%, which is assigned as a relative systematic uncertainty. The uncertainty related to the imperfect knowledge of the Λ0 bdecay model used for the simulation of the Λ0 b→ψ(2S)pK−decays is estimated by varying the correction factors obtained from kinematic distributions observed in data. Changing these correction factors within their statistical uncertainties causes a negligible variation of the efficiency εΛ0 b →ψ(2S)pK−. For the Λ0 b→ψ(2S)pπ−signal decays the observed two-body mass distributions are in agreement with the phase-space model used in the simulation. The corresponding uncertainty due to the unknown decay kinematics of the Λ0 b→ψ(2S)pπ−signal decays is small and therefore neglected. An additional uncertainty arises from the differences between data and simulation, in particular those affecting the efficiency for the reconstruction of charged-particle tracks. The small difference in the track-finding efficiency between data and simulation is corrected using a data-driven technique [55]. The uncertainties in these correction factors together with the uncertainties in the hadron-identification efficiencies, related to the finite size of the calibration samples [53,54], are propagated to the ratio of total efficiencies by means of pseudoexperiments. This results in a systematic uncertainty of 0.2% associated with the track reconstruction and hadron identification. The systematic uncertainty on the efficiency of the trigger has been previously studied using high-yield B+→J/ψK+and B+→ψ(2S)K+decays by comparing ratios of trigger efficiencies in data and simulation [59]. Based on these comparisons a relative uncertainty of 1.1% is assigned. Another source of uncertainty is the potential disagreement between data and simulation in the estimation of efficiencies, due to effects not considered above. This is studied using a high-yield low-background sample of Λ0 b→J/ψpK−decays, by varying the selection criteria in ranges that lead to as much as ±20% differences in the measured signal yields. The resulting variations in the efficiency-corrected yields do not exceed 1% for all inspected selection criteria. The value of 1% is taken as a corresponding systematic uncertainty. Finally, the 0.5% relative uncertainty in the ratio of efficiencies from eq. (4.2) is assigned as a systematic uncertainty due to the finite size of the simulated samples. – 6 – JHEP08(2018)131 Source Uncertainty [%] Fit model 0.7 Track reconstruction and hadron identification 0.2 Trigger 1.1 Selection criteria 1.0 Size of the simulation samples 0.5 Total 1.7 Table 1. Relative systematic uncertainties for the ratio of branching fractions. The total uncertainty is the quadratic sum of the individual contributions. 6 Results and summary The Cabibbo-suppressed decay Λ0 b→ψ(2S)pπ−is observed using a data sample collected by the LHCb experiment in proton-proton collisions corresponding to 1.0, 2.0 and 1.9 fb−1 of integrated luminosity at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The observed yield of Λ0 b→ψ(2S)pπ−decays is 121 ±13. Using the Λ0 b→ψ(2S)pK−decay as a normalization channel, the ratio of the branching fractions is measured to be Rπ/K=BΛ0 b→ψ(2S)pπ− BΛ0 b→ψ(2S)pK−= (11.4±1.3±0.2)% , where the first uncertainty is statistical and the second is systematic. Neglecting the resonance structures in the Λ0 b→ψ(2S)pπ−and Λ0 b→ψ(2S)pK−decays, the calculated value for the ratio Rπ/Kis Rth π/K≈Φ3Λ0 b→ψ(2S)pπ− Φ3Λ0 b→ψ(2S)pK−×tan2θC≃11% , where Φ3denotes the full three-body phase-space and θCis the Cabibbo angle [60]. The measured value is in a good agreement with this estimate. The branching fraction BΛ0 b→ψ(2S)pπ−is calculated using the value of BΛ0 b→ψ(2S)pK−=6.29 ±0.23 ±0.14+1.14 −0.90×10−5[16] as BΛ0 b→ψ(2S)pπ−=7.17 ±0.82 ±0.33+1.30 −1.03×10−6, where the first uncertainty is statistical, the second systematic (including the statistical and systematic uncertainties from BΛ0 b→ψ(2S)pK−) and the third arises from the uncertainties in the branching fractions of the Λ0 b→J/ψpK−,ψ(2S) →J/ψπ+π−,ψ(2S) →e+e− and J/ψ→e+e−decays [48]. The ψ(2S)p and ψ(2S)π−mass spectra are investigated and no evidence for contributions from exotic states is found. With a larger data sample a detailed amplitude analysis of this decay could be performed, making it possible to search for small contributions from exotic states. – 7 – JHEP08(2018)131 Acknowledgments We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); MOST and NSFC (China); CNRS/IN2P3 (France); BMBF, DFG and MPG (Germany); INFN (Italy); NWO (Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FASO (Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); NSF (U.S.A.). We acknowledge the computing resources that are provided by CERN, IN2P3 (France), KIT and DESY (Germany), INFN (Italy), SURF (Netherlands), PIC (Spain), GridPP (United Kingdom), RRCKI and Yandex LLC (Russia), CSCS (Switzerland), IFIN-HH (Romania), CBPF (Brazil), PL-GRID (Poland) and OSC (U.S.A.). We are indebted to the communities behind the multiple open-source software packages on which we depend. Individual groups or members have received support from AvH Foundation (Germany), EPLANET, Marie Sk lodowska-Curie Actions and ERC (European Union), ANR, Labex P2IO and OCEVU, and R´egion Auvergne-Rhˆone-Alpes (France), Key Research Program of Frontier Sciences of CAS, CAS PIFI, and the Thousand Talents Program (China), RFBR, RSF and Yandex LLC (Russia), GVA, XuntaGal and GENCAT (Spain), Herchel Smith Fund, the Royal Society, the English-Speaking Union and the Leverhulme Trust (United Kingdom). Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. References [1] LHCb collaboration, Measurement of σ(pp →bbX) at √s= 7 TeV in the forward region, Phys. Lett. B 694 (2010) 209 [arXiv:1009.2731] [INSPIRE]. [2] LHCb collaboration, Measurement of J/ψproduction in pp collisions at √s= 7 TeV ,Eur. Phys. J. 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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 – 15 – JHEP08(2018)131 54 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 55 Imperial College London, London, United Kingdom 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 – 16 – JHEP08(2018)131 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 – 17 –