Measurement of the χ b (3P) mass and of the relative rate of χ b1(1P) and χ b2(1P) production

JHEP10(2014)088 Published for SISSA by Springer Received:September 5, 2014 Accepted:October 1, 2014 Published:October 14, 2014 Measurement of the χb(3P)mass and of the relative rate of χb1(1P)and χb2(1P)production The LHCb collaboration E-mail: [email protected] Abstract: The production of χbmesons in proton-proton collisions is studied using a data sample collected by the LHCb detector, at centre-of-mass energies of √s= 7 and 8 TeV and corresponding to an integrated luminosity of 3.0 fb−1. The χbmesons are identified through their decays to Υ(1S)γand Υ(2S)γusing photons that converted to e+e−pairs in the detector. The relative prompt production rate of χb1(1P) and χb2(1P) mesons is measured as a function of the Υ(1S) transverse momentum in the χbrapidity range 2.0< y < 4.5. A precise measurement of the χb(3P) mass is also performed. Assuming a mass splitting between the χb1(3P) and the χb2(3P) states of 10.5 MeV/c2, the measured mass of the χb1(3P) meson is m(χb1(3P)) = 10515.7+2.2 −3.9(stat)+1.5 −2.1(syst) MeV/c2. Keywords: Quarkonium, Hadron-Hadron Scattering, Flavor physics ArXiv ePrint: 1409.1408 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP10(2014)088 JHEP10(2014)088 Contents 1 Introduction 1 2 Detector and data samples 2 3 Event reconstruction and selection 3 4 Sample composition and fit model 4 5χbmeson masses 6 5.1 Mass measurements 6 5.2 Systematic uncertainties 7 6 Relative rate of χb2(1P) and χb1(1P) production 9 6.1 Measurement of the relative rates 9 6.2 Systematic uncertainties 10 7 Results 12 8 Conclusion 13 The LHCb collaboration 17 1 Introduction The study of production and properties of heavy quark-antiquark bound states (quarkonia) provides an important test of the underlying mechanisms described by quantum chromodynamics (QCD). The quarkonium (cc and bb) states in which quarks have parallel spins include the S-wave (J/ψ,Υ) and the P-wave (χc,χb) states, where each of the latter comprises a closely spaced triplet of J= 0,1,2 spin states (χcJ ,χbJ ). In high-energy protonproton collisions at the LHC, qq pairs (q=c, b) are expected to be produced predominantly via a hard gluon-gluon interaction followed by the formation of bound quarkonium states. The production of the qq pair is described by perturbative QCD, while non-perturbative QCD is needed for the description of the evolution of the qq pair to the bound state. Several models have been developed for this non-perturbative part such as the colour singlet model [1–3] and the non-relativistic QCD (NRQCD) model [4,5], which also includes the production of quarkonium via the colour octet mechanism. Recent studies support the leading role of the colour singlet mechanism [6,7]. Measurements of the relative rate of J= 1 and J= 2 states provide information on the colour octet contribution. This relative rate is also predicted to have the same dependence on the meson transverse momentum (pT) in χband χcstates, once the pTof the χbmeson is scaled by the ratio of χcand χbmasses [8]. – 1 – JHEP10(2014)088 Measurements of χcproduction and the ratio of the χc1and χc2production crosssections have been made previously using various particle beams and energies [9–13]. All the χbstates are below the BB threshold (where Bstands for bmesons) and therefore can be studied through their radiative decays to the Υmesons, in the same way as the χc states were studied through their radiative decays to the J/ψ meson [13]. In this paper we report a measurement of the ratio of χb2(1P) to χb1(1P) production cross-sections σ(pp →χb2(1P)X)/σ(pp →χb1(1P)X) at centre-of-mass energies of √s= 7 and 8 TeV in the rapidity range 2.0< y < 4.5 as a function of the Υ(1S) transverse momentum from 5 to 25 GeV/c. The full LHCb sample is used, corresponding to an integrated luminosity of 3.0 fb−1. The observation in LHCb data of the recently observed χb(3P) state [14,15] is also presented. The measurement of its mass and of the mass splitting between the χbJ (1P) states (J= 1 and J= 2) provide useful information for testing QCD models [16–18]. The kinematically allowed transitions χb(1P)→Υ(1S)γ,χb(2P)→Υ(1S)γ,χb(3P)→ Υ(1S)γand χb(3P)→Υ(2S)γare studied. The Υ(mS) (m= 1,2) meson is reconstructed in the dimuon final state and only photons that convert in the detector material are used. The converted photons are reconstructed using e+and e−tracks, allowing a separation of the χb1and χb2mass peaks, due to the improved energy resolution of converted photons with respect to that of photons identified with the calorimeter. Any contribution from the χb0mesons decays is neglected, as their radiative decay rate is expected to be suppressed by an order of magnitude compared to that of the χb2meson [17,19]. 2 Detector and data samples The LHCb detector [20] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing bor cquarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector (VELO) surrounding the pp interaction region, a large-area silicon-strip detector station 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 momentum, p, with a relative uncertainty that varies from 0.4% at low momentum to 0.6% at 100 GeV/c. The total material before the first tracking station corresponds to about 25% of a radiation length. The minimum distance of a track to a primary vertex, the impact parameter, is measured with a resolution of (15 + 29/pT)µm, where pTis in GeV/c. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter (ECAL) and a hadronic calorimeter. The reconstruction of converted photons is described in section 3. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers. The LHCb coordinate system is right-handed with its origin at the nominal interaction point, the zaxis aligned along the beam line towards the magnet and the yaxis pointing upwards. The magnetic field is oriented along the yaxis. – 2 – JHEP10(2014)088 The trigger 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. Events used in this analysis are first required to pass a hardware trigger that selects muon candidates with pT>1.76 GeV/c or dimuon candidates with a product of their pTlarger than (1.6 GeV/c)2. In the software trigger both muons are required to have pT>0.5 GeV/c, total momentum p > 6 GeV/c, and dimuon invariant mass greater than 4.7 GeV/c2. In the simulation, pp collisions are generated using Pythia [21,22] with a specific LHCb configuration [23]. Decays of hadronic particles are described by EvtGen [24], in which final state radiation is generated using Photos [25]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [26,27] as described in ref. [28]. The simulated samples consist of events containing at least one Υmeson that is forced to decay to two muons. In a sample used for background studies, no restriction on the Υmeson production mechanism is imposed. This sample is referred to as inclusive Υin the following. In another sample, used for the estimation of signal efficiencies and parametrisation, the Υis required to originate from a χbmeson. This simulated sample is about 10 times larger than the data sample. 3 Event reconstruction and selection The reconstruction and selection of χbcandidates closely follows ref. [13]. Photons that convert in the detector material are reconstructed from pairs of oppositely charged electron candidates. Since the acceptance is lower for photons that convert in the VELO and the energy resolution is worse, only γ→e+e−candidates without VELO hits are considered. This selection strongly favours conversions that occur between the downstream end of the VELO and the first tracking station upstream of the magnet. The e+e−candidates are required to be within the ECAL acceptance and to produce electromagnetic clusters that have compatible coordinates in the non bending plane. Any photon whose position in the ECAL is compatible with a straight line extrapolation of the electron track from the first tracking station is considered as a bremstrahlung photon. Its energy is added to the electron energy. If the same bremsstrahlung candidate is found for both the e+and the e−, the photon energy is added randomly to one of the tracks. The e+and e−tracks (corrected for bremsstrahlung) are then extrapolated backwards in order to determine the conversion point and a vertex fit is performed to reconstruct the photon momentum. The transverse momentum of the photon candidate (pγ T) is required to be larger than 600 MeV/c and the invariant mass of the e+e−pair is required to be less than 50 MeV/c2, which removes most of the combinatorial background. The resulting purity of the photon sample is determined from simulation to be about 99%. The Υcandidate is reconstructed in its decay to the µ+µ−final state. Each track must be identified as a muon with pT>2 GeV/c and p > 8 GeV/c. The two muons must originate from a common vertex with vertex fit χ2/ndf smaller than 25. Only Υcandidates with transverse momentum (pΥ T) greater than 4 GeV/c are kept. Figure 1shows the invariant mass of Υcandidates. The mass resolution is about 43 MeV/c2. The accepted mass ranges for the Υ(1S) and for the Υ(2S) candidates are given in table 1. – 3 – JHEP10(2014)088 (n, m) (1,1) (2,1) (3,1) (3,2) pΥ T( GeV/c)>4.0>4.0>5.0>6.0 pγ T( GeV/c)>0.6>0.9>1.3>0.7 Υmass range ( MeV/c2) 9360 < m(µ+µ−)<9560 9960 < m(µ+µ−)<10100 Low mass SB range ( MeV/c2) 9000 < m(µ+µ−)<9200 9650 < m(µ+µ−)<9850 High mass SB range ( MeV/c2) 9650 < m(µ+µ−)<9850 10150 < m(µ+µ−)<10250 Table 1. Selection criteria for each χb(nP)→Υ(mS)γtransition. SB indicates sideband. The Υand γcandidates are each associated with the primary vertex (PV) relative to which they have the smallest impact parameter χ2, defined as the difference between the χ2 of the PV reconstructed with and without the considered tracks. They are then combined to form a χbcandidate. The χbdecay time has to be smaller than 0.1 ps (about 5 times the observed resolution). Loose requirements are applied in order to reject combinatorial background and poorly reconstructed candidates using the following variables: the difference in z-positions of the primary vertices associated with the Υand γcandidates, the χ2of the χb candidate vertex fit and the difference between the χ2of the PV fitted with and without the χbcandidate. These requirements remove about 30% of the background and 8% of the signal. The cosine of the angle between the photon momentum in the χbrest frame and the χbmomentum is required to be positive. This requirement halves the background while preserving 92% of the signal. The χbcandidates are selected in the rapidity range 2.0< y < 4.5. The χbcandidates’ mass is defined as m∗(µ+µ−γ)≡m(µ+µ−γ)−m(µ+µ−) + m(Υ), where m(Υ(1S)) = 9460.3±0.3 MeV/c2and m(Υ(2S)) = 10023.3±0.3 MeV/c2are the known Υmass values [19]. This allows a nearly exact cancellation of the uncertainty due to the Υmass resolution and any possible bias on the Υcandidates mass. The χbmass resolution is therefore dominated by the resolution on the photon energy. The requirements on pΥ Tand pγ Tand the Υsignal mass ranges used for each χb(nP)→Υ(mS)γdecay mode are given in table 1. 4 Sample composition and fit model Two background sources are considered in the sample of χbcandidates. One source is the non-Υbackground originating mainly from the Drell-Yan process where the dimuon pair is combined with a photon. The second source is the combinatorial background where a genuine Υis combined with a random photon. The functions used for the fits are the sums of a background and signal functions. The χb1and χb2peaks are each parametrised with a double sided Crystal Ball (CB) function [29]: CBi(m∗)∝exp−1 2m∗−mi σi2if −αL<m∗−mi σi < αR – 4 – JHEP10(2014)088 ] 2 c) [MeV/ - µ + µ (m 9000 9500 10000 10500 11000 ) 2 cCandidates / (10 MeV/ 0 500 1000 1500 2000 LHCb Figure 1. Invariant dimuon mass of the Υcandidates after the event selection requirements and before the Υmass range requirement. The distribution is fitted with the sum (blue line) of a double-sided Crystal Ball function for each Υstate (dashed red line for Υ(1S), dotted pink line for Υ(2S), dash-dotted green line for Υ(3S)) and a second-order polynomial for the background (not shown). The hatched red bands show the signal regions and the hatched blue bands show the mass sidebands used for background studies. CBi(m∗)∝(nL/αL)nLexp(−1 2α2 L) (nL/αL−αL−(m∗−mi)/σi)nLif m∗−mi σi <−αL(4.1) CBi(m∗)∝(nR/αR)nRexp(−1 2α2 R) (nR/αR−αR+ (m∗−mi)/σi)nRif m∗−mi σi > αR, where the index i= 1(2) refers to the χb1(χb2) CB function. The CB left tail accounts for events with unreconstructed bremsstrahlung, while the right tail accounts for events with overcorrected bremsstrahlung. Simulation shows that the same tail parameters αR and nL,Rcan be used for all the χbi(nP) states, nL=nR= 2.5 and αR= 1.0, while different values of αLhave to be used: αL= 0.20, 0.25 and 0.30, for the χbi(1P), χbi(2P) and χbi(3P) shapes, respectively. Since in the study of χcstates it was found that the CB tail parameters were similar in data and simulation [13], the values found with simulation are used for the χb. The CB width, σ, increases with the mass difference between the considered χband Υstates. Fits to the mass distributions of χb(1P)→Υ(1S)γand χb(2P)→Υ(1S)γcandidates indicate that the width is 10% −20% larger in data than in simulation. Therefore, the CB width is fixed to the value found with simulated events increased by 10% and it is varied by ±10% for studies of the systematic effects. The shape of the non-Υbackground and its amplitude are estimated using the Υmass sidebands shown in figure 1and given in table 1. The mass distribution of these candidates is fitted with an empirical function fbkg(m∗)∝arctanm∗−m0 c+bm∗ m0−1+a , (4.2) – 5 – JHEP10(2014)088 where m0,a,band care free parameters. This function is then used to parametrise the non-Υbackground contribution with all parameters fixed to the fitted values. The shape of the combinatorial background is estimated using the inclusive Υsimulated sample and parametrised with eq. (4.2). All parameters are fixed to the values found with simulation except for the normalisation. In the case of the χb(3P)→Υ(2S)γtransition, this shape does not reproduce the data properly and the value of the m0parameter is therefore left free in the fit. This discrepancy is due to mismodeling of the pΥ Tspectrum in simulation and is accounted for in the systematic uncertainties. The fits have at most six free parameters: the mean mass value for the χb1peak m1, the mass difference between the χb2and χb1peaks ∆m12, the normalisation of the χb1CB function A1, the ratio of the χb2to χb1CB amplitudes r12, the normalisation of the combinatorial background Acomb and the m0parameter for the combinatorial background shape. 5χbmeson masses 5.1 Mass measurements The masses of the χbmesons are determined using unbinned maximum likelihood fits to the χbmass distributions using the parametrisation described in section 4. Figures 2(a) and (b) show the mass distributions for the χb(1P)→Υ(1S)γand χb(2P)→Υ(1S)γdecays with the fit results overlaid. In these fits the free parameters are m1,A1, ∆m12,r12 and Acomb. Table 2reports the resulting mass determinations for these states compared to the world average values [19]. A small bias is expected on the measured masses, attributed to unreconstructed bremsstrahlung of the e+e−pair. This bias is proportional to the Q-value of the transition and is expected, from simulation, to be about −0.5 and −1.5 MeV/c2 for the χb(1P)→Υ(1S)γand χb(2P)→Υ(1S)γdecays, respectively. The measurements given in table 2are not corrected for this bias and are consistent with such a bias. On the other hand the χb(3P) mass measured using the χb(3P)→Υ(mS)γtransitions is corrected for the bias estimated with simulation, −3.0±2.0 MeV/c2and −0.5±0.5 MeV/c2for m= 1 and m= 2, respectively, where the uncertainties cover possible discrepancies between data and simulation. In the case of the χb(3P) meson, the mass splitting and the relative yields are also fixed, as the spin-1 and spin-2 peaks cannot be separated. Theory predictions vary from 9 to 12 MeV/c2[16,17] for ∆m12 and this parameter is fixed to 10.5 MeV/c2. The value of r12 is fixed based on theoretical predictions [17] and our experimental measurement. It can be expressed as the product of the ratio of branching fractions to Υγ and of the ratio of production cross-sections of the χb2(3P) and χb1(3P) states. Predictions for branching fractions are found in refs. [17,18]. The predictions from ref. [17] agree well with the experimental measurements for the χb(1P) and the χb(2P) mesons. The model of ref. [17] predicts similar values for the two transitions, B(χb2(3P)→Υ(mS)γ))/B(χb1(3P)→Υ(mS)γ)≈0.47 (m= 1,2). According to ref. [8] the ratio of production cross-sections is expected to be the same for the χb(3P) and χb(1P) mesons and thus, using the measurement detailed in section 6, we obtain σ(χb2(nP))/σ(χb1(nP)) = 0.9±0.2. – 6 – JHEP10(2014)088 (n, m) (1,1) (2,1) m19892.3±0.5 10254.7±1.3 m1world average 9892.8±0.4 10255.5±0.6 ∆m12 19.81 ±0.65 12.3±2.6 ∆m12 world average 19.43 ±0.37 13.5±0.6 Table 2. Fitted values of the χb(nP) (n= 1,2) masses (in MeV/c2) from the χb(nP)→Υ(1S)γ transitions, compared to the world average values. The uncertainties are statistical only. (n, m) (3,1) (3,2) (3,1)+(3,2) m110509.0+5.0 −2.610518.5+1.9 −1.310515.7+2.2 −3.9 ∆m12 10.5 (fixed) 10.5 (fixed) 10.5 (fixed) N(χb) 107 ±19 41 ±12 169 ±25 Table 3. Fitted values of the χb(3P) mass (in MeV/c2) for the χb(3P)→Υ(mS)γ(m= 1,2) transitions. The last column gives the result of the simultaneous fit to the two transitions. The values are corrected for the mass bias (−3 MeV/c2and −0.5 MeV/c2for the Υ(1S) and Υ(2S) transitions, respectively). The last row gives the total χbyields. The uncertainties are statistical only. To summarise, the value r12 = 0.47 ×0.9 = 0.42 is used in the fits to the mass distributions associated with the transitions of the χb(3P) meson to Υ(1S) and Υ(2S) mesons. Table 3gives the result of the fits to the mass distributions for the χb(3P)→ Υ(1S)γand χb(3P)→Υ(2S)γtransitions. A simultaneous fit to these two distributions is also performed and the result is reported in the last column of table 3. Figure 2shows the results of these fits. The χb(3P)→Υ(1S)γand χb(3P)→Υ(2S)γdecays are seen with a statistical significance, determined from the likelihood ratio of the fits with background only and with signal plus background hypotheses, of 6.0σand 3.6σrespectively. The total statistical significance determined with the simultaneous fit is 6.9σ. 5.2 Systematic uncertainties The systematic uncertainties on the measurement of the χb(nP) (n= 1,2) mass splitting and of the χb(3P) mass are detailed as follows. First the systematic uncertainties related to the signal parametrisation are considered. The χb0contribution is expected to be small because its branching fraction to Υ(1S)γis less than 2% for χb(1P) and χb(2P) mesons [19]. In order to estimate the systematic uncertainty due to the presence of a χb0or another unknown state, a third CB function is added to the fit, with a peak position fixed to the world average value for the χb(nP) for n= 1,2 and left free for the χb(3P). The resulting yield of χb0mesons is compatible with zero. The Gaussian width of the CB function is varied within ±10% to cover possible differences between data and simulation. For these two fit variations, the differences between results of the nominal and alternative fits are taken as systematic uncertainties, added in quadrature and referred to as signal uncertainty in table 4. – 7 – JHEP10(2014)088 ] 2 c)) [MeV/1S( Υ (m)+ µµ (m)- γµµ (m 9750 9800 9850 9900 9950 ) 2 cCandidates / (3.0 MeV/ 0 20 40 60 80 100 120 (a) γ )1S( Υ →)1P( b χ LHCb ] 2 c)) [MeV/1S( Υ (m)+ µµ (m)- γµµ (m 10100 10200 10300 10400 ) 2 cCandidates / (5.0 MeV/ 0 10 20 30 40 50 60 70 80 90 (b) γ )1S( Υ →)2P( b χ LHCb ] 2 c)) [MeV/1S( Υ (m)+ µµ (m)- γµµ (m 10400 10500 10600 10700 10800 ) 2 cCandidates / (6.0 MeV/ 0 5 10 15 20 25 30 35 40 (c) γ )1S( Υ →)3P( b χ LHCb ] 2 c)) [MeV/2S( Υ (m)+ µµ (m)- γµµ (m 10400 10500 10600 10700 10800 ) 2 cCandidates / (6.0 MeV/ 0 5 10 15 20 25 (d) γ )2S( Υ →)3P( b χ LHCb ] 2 c)) [MeV/1S( Υ (m)+ µµ (m)- γµµ (m 10400 10500 10600 10700 10800 ) 2 cCandidates / (6.0 MeV/ 0 5 10 15 20 25 30 35 40 45 (e) γ )1S( Υ →)3P( b χ LHCb γ )1S,2S( Υ →)3P( b χ Simultaneous fit to ] 2 c)) [MeV/2S( Υ (m)+ µµ (m)- γµµ (m 10400 10500 10600 10700 10800 ) 2 cCandidates / (6.0 MeV/ 0 5 10 15 20 25 30 (f) γ )2S( Υ →)3P( b χ LHCb γ )1S,2S( Υ →)3P( b χ Simultaneous fit to Figure 2. Distribution of m∗(µ+µ−γ)≡m(µ+µ−γ)−m(µ+µ−) + m(Υ) for χbcandidates with fit projections overlaid for (a) χb(1P)→Υ(1S)γ, (b) χb(2P)→Υ(1S)γ, (c,e) χb(3P)→Υ(1S)γ and (d,f) χb(3P)→Υ(2S)γchannels. The result of the simultaneous fit to the χb(3P)→Υ(1S)γ and χb(3P)→Υ(2S)γmass distributions is shown in (e) and (f). The cyan dotted line shows the non-Υbackground, the grey dashed line shows the combinatorial background, the red dashed line the χb1contribution, the green dash-dotted line the χb2contribution, and the blue full line the sum of all these contributions. – 8 – JHEP10(2014)088 Kingdom); NSF (U.S.A.). The Tier1 computing centres are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC (Spain), GridPP (United Kingdom). We are indebted to the communities behind the multiple open source software packages on which we depend. We are also thankful for the computing resources and the access to software R&D tools provided by Yandex LLC (Russia). Individual groups or members have received support from EPLANET, Marie Sk lodowskaCurie Actions and ERC (European Union), Conseil g´en´eral de Haute-Savoie, Labex ENIGMASS and OCEVU, R´egion Auvergne (France), RFBR (Russia), XuntaGal and GENCAT (Spain), Royal Society and Royal Commission for the Exhibition of 1851 (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] R. Baier and R. Ruckl, Hadronic collisions: a quarkonium factory,Z. Phys. C 19 (1983) 251 [INSPIRE]. [2] V.G. Kartvelishvili, A.K. Likhoded and S.R. Slabospitsky, Dmeson and ψmeson production in hadronic interactions,Sov. J. Nucl. Phys. 28 (1978) 678 [Yad. Fiz. 28 (1978) 1315] [INSPIRE]. 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Zvyagin38. 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 4LAPP, Universit´e de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France 5Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France 7LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France 8LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France 9Fakult¨at Physik, Technische Universit¨at Dortmund, Dortmund, Germany 10 Max-Planck-Institut f¨ur Kernphysik (MPIK), Heidelberg, Germany 11 Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 12 School of Physics, University College Dublin, Dublin, Ireland 13 Sezione INFN di Bari, Bari, Italy 14 Sezione INFN di Bologna, Bologna, Italy 15 Sezione INFN di Cagliari, Cagliari, Italy 16 Sezione INFN di Ferrara, Ferrara, Italy 17 Sezione INFN di Firenze, Firenze, Italy 18 Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 19 Sezione INFN di Genova, Genova, Italy 20 Sezione INFN di Milano Bicocca, Milano, Italy 21 Sezione INFN di Milano, Milano, Italy 22 Sezione INFN di Padova, Padova, Italy 23 Sezione INFN di Pisa, Pisa, Italy 24 Sezione INFN di Roma Tor Vergata, Roma, Italy 25 Sezione INFN di Roma La Sapienza, Roma, Italy 26 Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ow, Poland 27 AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ow, Poland 28 National Center for Nuclear Research (NCBJ), Warsaw, Poland – 19 – JHEP10(2014)088 29 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 30 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 31 Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 32 Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia 33 Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 34 Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia 35 Institute for High Energy Physics (IHEP), Protvino, Russia 36 Universitat de Barcelona, Barcelona, Spain 37 Universidad de Santiago de Compostela, Santiago de Compostela, Spain 38 European Organization for Nuclear Research (CERN), Geneva, Switzerland 39 Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland 40 Physik-Institut, Universit¨at Z¨urich, Z¨urich, Switzerland 41 Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands 42 Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands 43 NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 44 Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 45 University of Birmingham, Birmingham, United Kingdom 46 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 47 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 48 Department of Physics, University of Warwick, Coventry, United Kingdom 49 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 50 School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 51 School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 52 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 53 Imperial College London, London, United Kingdom 54 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 55 Department of Physics, University of Oxford, Oxford, United Kingdom 56 Massachusetts Institute of Technology, Cambridge, MA, United States 57 University of Cincinnati, Cincinnati, OH, United States 58 University of Maryland, College Park, MD, United States 59 Syracuse University, Syracuse, NY, United States 60 Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to 2 61 Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to 3 62 Institut f¨ur Physik, Universit¨at Rostock, Rostock, Germany, associated to 11 63 National Research Centre Kurchatov Institute, Moscow, Russia, associated to 31 64 Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain, associated to 36 65 Van Swinderen Institute, University of Groningen, Groningen, The Netherlands, associated to 41 66 Celal Bayar University, Manisa, Turkey, associated to 38 aUniversidade Federal do Triˆangulo Mineiro (UFTM), Uberaba-MG, Brazil bP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia cUniversit`a di Bari, Bari, Italy dUniversit`a di Bologna, Bologna, Italy eUniversit`a di Cagliari, Cagliari, Italy fUniversit`a di Ferrara, Ferrara, Italy gUniversit`a di Firenze, Firenze, Italy hUniversit`a di Urbino, Urbino, Italy – 20 – JHEP10(2014)088 iUniversit`a di Modena e Reggio Emilia, Modena, Italy jUniversit`a di Genova, Genova, Italy kUniversit`a di Milano Bicocca, Milano, Italy lUniversit`a di Roma Tor Vergata, Roma, Italy mUniversit`a di Roma La Sapienza, Roma, Italy nUniversit`a della Basilicata, Potenza, Italy oAGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ow, Poland pLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain qHanoi University of Science, Hanoi, Viet Nam rUniversit`a di Padova, Padova, Italy sUniversit`a di Pisa, Pisa, Italy tScuola Normale Superiore, Pisa, Italy uUniversit`a degli Studi di Milano, Milano, Italy vPolitecnico di Milano, Milano, Italy – 21 –