First observations of the rare decays B + → K + π + π − μ + μ − and B +→ ϕK + μ + μ −

JHEP10(2014)064 Published for SISSA by Springer Received:August 7, 2014 Accepted:September 16, 2014 Published:October 9, 2014 First observations of the rare decays B+→K+π+π−µ+µ−and B+→φK+µ+µ− The LHCb collaboration E-mail: [email protected] Abstract: First observations of the rare decays B+→K+π+π−µ+µ−and B+→φK+µ+µ−are presented using data corresponding to an integrated luminosity of 3.0 fb−1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The branching fractions of the decays are B(B+→K+π+π−µ+µ−) = 4.36 +0.29 −0.27 (stat) ±0.21 (syst) ±0.18 (norm)×10−7, B(B+→φK+µ+µ−) = 0.82 +0.19 −0.17 (stat) +0.10 −0.04 (syst) ±0.27 (norm)×10−7, where the uncertainties are statistical, systematic, and due to the uncertainty on the branching fractions of the normalisation modes. A measurement of the differential branching fraction in bins of the invariant mass squared of the dimuon system is also presented for the decay B+→K+π+π−µ+µ−. Keywords: Rare decay, Hadron-Hadron Scattering, B physics, Flavor physics ArXiv ePrint: 1408.1137 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP10(2014)064 JHEP10(2014)064 Contents 1 Introduction 1 2 The LHCb detector 2 3 Selection of signal candidates 2 4 Differential branching fraction of the decay B+→K+π+π−µ+µ−4 4.1 Systematic uncertainties 7 5 Branching fraction of the decay B+→φK+µ+µ−8 5.1 Systematic uncertainties 9 6 Conclusions 10 The LHCb collaboration 13 1 Introduction The B+→K+π+π−µ+µ−and B+→φK+µ+µ−decays proceed via b→sflavour changing neutral currents (FCNC).1In the Standard Model (SM), FCNC decays are forbidden at the tree level and are only allowed as higher-order electroweak loop processes. In extensions of the SM, new particles can significantly change the branching fractions and angular distributions of the observed final-state particles. Due to their sensitivity to effects beyond the SM, semileptonic Bdecays involving FCNC transitions are currently under intense study at the LHCb experiment [1–4]. The K+π+π−system in the final state of the B+→K+π+π−µ+µ−decay can result from the decay of several strange resonances. Its composition was studied by the Belle collaboration for the tree-level decay B+→J/ψ(→µ+µ−)K+π+π−[5], where the K1(1270)+ meson was found to have a prominent contribution. The K1(1270)+and the K1(1400)+ mesons are the mass eigenstates that result from mixing of the P-wave axial vector mesons 3P1(K1A) and 1P1(K1B) with the mixing angle θK1[6]. The value of θK1is either about −33◦or −57◦[6–11] with most recent determinations favouring the former [8–11]. The decay B+→J/ψφK+was first observed by the CLEO collaboration [12] and recently investigated in the search for the X(4140) [13–16]. The branching fraction of the rare decay B+→K1(1270)+µ+µ−, which is expected to contribute significantly to the K+π+π−µ+µ−final-state, is predicted to be B(B+→K1(1270)+µ+µ−) = (2.3+1.3 −1.0 +0.0 −0.2)×10−6[17]. Here, the first uncertainty originates from the form-factor calculations, while the second is from the uncertainty on the 1Charge conjugation is implied throughout this paper. – 1 – JHEP10(2014)064 mixing angle θK1. However, due to the unknown resonance structure of the final-state hadrons, there are no inclusive theoretical predictions available for the branching fractions of the decays B+→K+π+π−µ+µ−and B+→φK+µ+µ−. This paper presents the first observations of the decays B+→K+π+π−µ+µ−and B+→ φ(1020)K+µ+µ−, using a data sample collected by the LHCb experiment, corresponding to an integrated luminosity of 3.0 fb−1. The data were recorded in the years 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, respectively. In addition, a measurement of the differential branching fraction dB(B+→K+π+π−µ+µ−)/dq2, where q2is the invariant mass squared of the dimuon system, is presented. 2 The LHCb detector The LHCb detector [18] 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 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. 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 minimum distance of a track to a primary pp interaction vertex (PV), the impact parameter (IP), is measured with a resolution of (15 + 29/pT)µm, where pTis the the component of ptransverse to the beam, in GeV/c. Charged hadrons are identified using two ring-imaging Cherenkov detectors (RICH) [19]. Photon, electron and hadron candidates 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 [20]. The trigger [21] 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. Simulated events are used to determine trigger, reconstruction and selection efficiencies. In addition, simulated samples are used to estimate possible backgrounds from Bmeson decays that can mimic the final states of the signal decays. Simulated events are generated using Pythia [22,23] with a specific LHCb configuration [24]. Decays of hadronic particles are described by EvtGen [25], in which final-state radiation is generated using Photos [26]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [27,28] as described in ref. [29]. 3 Selection of signal candidates The B+→K+π+π−µ+µ−and B+→φK+µ+µ−signal candidates are first required to pass the hardware trigger stage, which selects muons with pT>1.76 GeV/c. In the subsequent software trigger stage, at least one of the final-state hadrons (muons) is required to have both pT>1.6 GeV/c (1.0 GeV/c) and IP larger than 100 µm with respect to any PV in – 2 – JHEP10(2014)064 the event. A multivariate algorithm [30] is used to identify secondary vertices that are consistent with the decay of a bhadron with muons in the final state. Signal candidates are formed by combining two muons of opposite charge with three charged hadrons. Reconstructed signal candidate tracks must have significant displacement from any PV in the event. The signal candidate tracks are required to form a secondary vertex of good fit quality which is significantly displaced from the PV. Particle identification information from the RICH detectors (PID) is used to identify the final-state hadrons. For B+→K+π+π−µ+µ−decays, the invariant mass of the K+π+π−system is required to be below 2400 MeV/c2. For B+→φK+µ+µ−decays with φ→K+K−, the invariant mass of the K+K−system is required to be within 12 MeV/c2of the known φmeson mass [31]. This mass region contains almost entirely φ→K+K−meson decays with negligible background. The final states of the signal decays can be mimicked by other Bdecays, which represent potential sources of background. Resonant decays, where the muon pair originates from either J/ψ or ψ(2S) meson decays, are removed by rejecting events where the invariant mass of the dimuon system is in the veto regions 2946 < m(µ+µ−)<3176 MeV/c2or 3586 < m(µ+µ−)<3766 MeV/c2. The radiative tails of the J/ψ (ψ(2S)) decays are suppressed by extending the lower edge of these veto regions down by 250 MeV/c2(100 MeV/c2) if the reconstructed B+mass is smaller than 5230 MeV/c2. In the mass region 5330 < m(B+)<5450 MeV/c2the upper edge of the vetoes is extended up by 40 MeV/c2to reject a small fraction of misreconstructed J/ψ and ψ(2S) meson decays. The resonant decays can also be misreconstructed as signal if a muon from the charmonium decay is misidentified as a hadron and vice versa. To remove this potential background the invariant mass of the µ+π−or µ+K−system is calculated assigning the muon mass to the hadron. If the mass falls within 50 MeV/c2of the known J/ψ or ψ(2S) masses [31], the candidate is rejected. Potential background from the electroweak-penguin decay B0→K∗0µ+µ−, where the K∗0→K+π−decay is combined with a random π+meson, is studied and found to be negligible. Backgrounds from semileptonic b→c(→sµ+νµ)µ−¯νµcascade decays, as well as fully hadronic Bdecays such as B+→D0(→K+π+π−π−)π+where two hadrons are misidentified as muons, are also negligible. Combinatorial background is suppressed with a boosted decision tree (BDT) [32, 33]. The BDT training uses sWeighted [34] candidates from the control channel B+→J/ψ K+π+π−as a signal proxy and the high B+mass sideband (5529 < m(K+π+π−µ+µ−)<5780 MeV/c2) of B+→K+π+π−µ+µ−candidates as a background proxy. The BDT uses geometric and kinematic variables in the training, including the pTof the final state tracks and their displacement from the PV. Additionally, the pTof the reconstructed B+candidate, as well as information on the quality of the decay vertex and its displacement are used. Requirements on the BDT response and the PID criteria, which discriminate between kaons and pions for the reconstructed final-state hadrons, are optimised simultaneously using the metric S/√S+B. Here, Sand Bdenote the expected signal and background yields. The value of Sis calculated using an estimate for the branching fraction of the decay B+→K1(1270)+µ+µ−. This branching fraction is determined by scaling that of the rare decay B0→K∗0µ+µ−[1] by the branching fraction ratio of the radiative decays B+→K1(1270)+γand B0→K∗0γ[31]. – 3 – JHEP10(2014)064 To determine the branching fractions of the signal decays, the normalisation modes B+→ψ(2S)K+, with the subsequent decay ψ(2S)→J/ψ(→µ+µ−)π+π−, and B+→ J/ψ φK+are used. The branching fraction of the decay B+→ψ(2S)K+is (6.27 ±0.24) × 10−4[31], and the branching fraction of the decay B+→J/ψφK+is (5.2±1.7)×10−5[31]. The final states of the normalisation modes are identical to those of the signal decays, which is beneficial since many systematic effects are expected to cancel. Both normalisation modes are selected in analogy to the signal decays except for additional mass requirements. For the ψ(2S) decay, the reconstructed π+π−µ+µ−mass is required to be within 60 MeV/c2of the known ψ(2S) mass. The reconstructed invariant mass of the dimuon system originating from the J/ψ meson decay is required to be within 50 MeV/c2of the known J/ψ mass. 4 Differential branching fraction of the decay B+→K+π+π−µ+µ− The determination of the differential branching fraction dB(B+→K+π+π−µ+µ−)/dq2is performed in bins of q2, as given in table 1. Figure 1shows the invariant mass distribution of B+→K+π+π−µ+µ−candidates in each q2bin studied. Signal yields are determined using extended maximum likelihood fits to the unbinned K+π+π−µ+µ−mass spectra. The m(K+π+π−µ+µ−) distribution of the signal component is modelled using the sum of two Gaussian functions, each with a power-law tail on the low-mass side. The background component is modelled with an exponential function, where the reductions in efficiency due to the vetoes of the radiative tails of the charmonium decays are accounted for by using scale factors. The signal yield integrated over the full q2range is NKππµµ = 367 +24 −23. The statistical significance of the signal is in excess of 20 standard deviations, according to Wilks’ theorem [35]. Figure 2a shows the fit to the mass distribution of the control channel B+→J/ψ K+π+π−that is used to determine the parameters describing the mass distribution of the B+→K+π+π−µ+µ−signal decay. To account for partially reconstructed decays at low masses, a Gaussian function is used in addition to the exponential to describe the background component. The yield of the control channel is 59 335 ±343. Figure 2b shows the fit for the normalisation channel B+→ψ(2S)K+. To describe the mass shape, the same components are used as for the fit of the control decay and all mass shape parameters are allowed to vary in the fit. The yield of the normalisation channel is 5128 ±67. The differential branching fraction dB(B+→K+π+π−µ+µ−)/dq2in a q2bin of width ∆q2is dB(B+→K+π+π−µ+µ−) dq2=1 ∆q2·Nsig Nnorm ·norm sig ·B(B+→ψ(2S)K+) ·Bψ(2S)→J/ψ (→µ+µ−)π+π−.(4.1) Here, Nsig is the yield of the signal channel in the given q2bin and Nnorm the yield of the normalisation channel. The efficiencies for the reconstruction and selection of the signal and normalisation channels are denoted by sig and norm, respectively. The efficiency for the signal decay is determined using simulated B+→K1(1270)+µ+µ−events generated according to ref. [17]; a separate efficiency ratio is calculated for each q2bin. – 4 – JHEP10(2014)064 ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 2 cCandidates per 10 MeV/ 0 10 20 30 40 50 LHCb < 2.00 2 q0.10 < ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 2 cCandidates per 10 MeV/ 0 5 10 15 20 25 LHCb < 4.30 2 q2.00 < ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 2 cCandidates per 10 MeV/ 0 5 10 15 20 25 30 35 40 45 LHCb < 8.68 2 q4.30 < ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 2 cCandidates per 10 MeV/ 0 5 10 15 20 25 30 LHCb < 12.86 2 q10.09 < ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 2 cCandidates per 10 MeV/ 0 1 2 3 4 5LHCb < 19.00 2 q14.18 < ] 2 c) [MeV/ - µ + µ - π + π + Km( 5200 5300 5400 5500 5600 2 cCandidates per 10 MeV/ 0 20 40 60 80 100 120 140 LHCb - µ + µ - π + π + K → + B Figure 1. Invariant mass of B+→K+π+π−µ+µ−candidates in bins of q2with fit projections overlaid. The signal component (shaded light blue) is modelled by the sum of two Gaussian functions, each with a power-law tail at low mass. The background component (shaded dark blue) is modelled by an exponential function. In the q2ranges 4.30 < q2<8.68 GeV2 /c4, 10.09 < q2<12.86 GeV2 /c4, and 14.18 < q2<19.00 GeV2 /c4, scaling factors are applied to account for the vetoes of the radiative tails of the charmonium resonances, resulting in steps in the background mass shape. The lower right plot shows a separate fit to the signal decay integrated over all q2bins. The branching fraction for the ψ(2S) meson to decay to the final state π+π−µ+µ−is B(ψ(2S)→J/ψ (→µ+µ−)π+π−) = (2.016 ±0.031) ×10−2[31]. The resulting differential branching fractions for the decay B+→K+π+π−µ+µ−are shown in figure 3with numerical values given in table 1. Summation over all q2bins yields an integrated branching fraction of 3.43 +0.23 −0.21 (stat) ±0.15 (syst) ±0.14 (norm)× 10−7, where the uncertainties are statistical, systematic, and due to the uncertainty on the normalisation channel. The fraction of signal events removed by the vetoes of the charmonium regions is determined from simulated B+→K1(1270)+µ+µ−events to be – 5 – JHEP10(2014)064 ] 2 c) [MeV/ - µ + µ - π + π + Km( 5100 5200 5300 5400 5500 5600 2 cCandidates per 10 MeV/ 0 2000 4000 6000 8000 10000 12000 14000 16000 LHCb - π + π + K ψ / J → + B (a) ] 2 c) [MeV/ - µ + µ - π + π + Km( 5100 5200 5300 5400 5500 5600 2 cCandidates per 10 MeV/ 0 200 400 600 800 1000 1200 LHCb + (2S)K ψ → + B (b) Figure 2. Invariant mass distribution of (a) the control decay B+→J/ψ K+π+π−and (b) the normalisation mode B+→ψ(2S)K+with fit projections overlaid. q2bin [ GeV2 /c4]Nsig dB dq2[×10−8GeV−2c4] [ 0.10,2.00] 134.1+12.9 −12.37.01 +0.69 −0.65 ±0.47 [ 2.00,4.30] 56.5+ 9.7 −9.12.34 +0.41 −0.38 ±0.15 [ 4.30,8.68] 119.9+14.6 −13.72.30 +0.28 −0.26 ±0.20 [10.09,12.86] 54.0+10.1 −9.41.83 +0.34 −0.32 ±0.17 [14.18,19.00] 3.3+ 2.8 −2.10.10 +0.08 −0.06 ±0.01 [ 1.00,6.00] 144.8+14.9 −14.32.75 +0.29 −0.28 ±0.16 Table 1. Signal yields for the decay B+→K+π+π−µ+µ−and resulting differential branching fractions in bins of q2. The first contribution to the uncertainty is statistical, the second systematic, where the uncertainty due to the branching fraction of the normalisation channel is included. The q2binning used corresponds to the binning used in previous analyses of b→sµ+µ−decays [1– 3]. Results are also presented for the q2range from 1 to 6 GeV2 /c4, where theory predictions are expected to be most reliable. (21.3±1.5)%. The uncertainty on this number is determined from a variation of the angle θK1and the form-factor parameters within their uncertainties. Correcting for the charmonium vetoes yields a total branching fraction of B(B+→K+π+π−µ+µ−) = 4.36 +0.29 −0.27 (stat) ±0.21 (syst) ±0.18 (norm)×10−7. Since the systematic uncertainty due to the normalisation channel is significant, we also report the branching ratio of the signal channel with respect to its normalisation mode, which is determined to be B(B+→K+π+π−µ+µ−) B(B+→ψ(2S)K+)=6.95 +0.46 −0.43 (stat) ±0.34 (syst)×10−4. Due to the low signal yield, no attempt is made to resolve the different contributions to the K+π+π−system in the K+π+π−µ+µ−final state. However, it is possible to obtain the m(K+π+π−) distribution using the sPlot [34] technique. Figure 4shows this distribution for the signal decay in the full q2region, as well as for the control decay B+→J/ψK+π+π−. – 6 – JHEP10(2014)064 ] 4 c/ 2 [GeV 2 q 0 5 10 15 ) 4 c -2 GeV -8 10× ( 2 q/dBd 0 1 2 3 4 5 6 7 8 LHCb Figure 3. Differential branching fraction dB(B+→K+π+π−µ+µ−)/dq2. Errors shown include both statistical and systematic uncertainties. Shaded regions indicate the vetoed charmonium resonances. ] 2 c) [MeV/ - π + π + Km( 1000 1500 2000 2500 2 cCandidates per 35 MeV/ 0 10 20 30 40 50 60 LHCb - µ + µ - π + π + K → + B (a) ] 2 c) [MeV/ - π + π + Km( 1000 1500 2000 2500 2 cCandidates per 35 MeV/ 0 1000 2000 3000 4000 5000 LHCb - π + π + K ψ / J → + B (b) Figure 4. Background-subtracted m(K+π+π−) distributions for (a) the signal decay B+→K+π+π−µ+µ−and (b) the control channel B+→J/ψ K+π+π−. The vertical lines indicate the masses of the K1(1270)+and K1(1400)+resonances. For the signal decay B+→K+π+π−µ+µ−the data are consistent with the presence of several broad and overlapping resonances. 4.1 Systematic uncertainties The dominant systematic uncertainty comes from the branching fraction of the normalisation mode B+→ψ(2S)K+, which is known to a precision of 6%. This uncertainty is fully correlated between the q2bins and is quoted separately. The systematic uncertainty introduced by the choice of signal mass model is estimated by re-evaluating the signal yield using a single Gaussian function with a power-law tail. To estimate the uncertainty of the background mass model, a linear mass shape is used instead of the nominal exponential function. The total systematic uncertainty assigned due to the modelling of the mass distribution is approximately 2%. The majority of systematic effects bias the efficiency ratio norm/sig, which is determined using simulation. To account for differences between data and simulation, correc- – 7 – JHEP10(2014)064 tions based on data are applied to simulated events. The efficiency to identify kaons is corrected by using large D∗+→D0(→K−π+)π+control samples. Muon identification performance and tracking efficiency are corrected using J/ψ →µ+µ−decays. In addition, track multiplicity and vertex fit quality are weighted according to the control channel B+→J/ψ K+π+π−. The systematic uncertainties associated with these corrections are evaluated by determining the branching fraction without the correction and taking the full observed deviation as a systematic uncertainty. In total, they constitute a systematic uncertainty of around 1%. The software trigger is observed to be well described in simulation, but slight discrepancies are observed for the hardware stage. These are corrected by weighting the simulated samples according to the maximum muon pT. The branching fraction is recalculated without these weights, and the observed difference of 1% is assigned as the systematic uncertainty from the trigger simulation. Additional systematic uncertainties stem from the fact that simulated B+→K1(1270)+µ+µ−events, modelled according to ref. [17], are used to determine the efficiency ratio norm/sig. To account for contributions other than the K1(1270)+to the K+π+π−system, events are weighted according to the m(K+π+π−) distribution shown in figure 4. This results in a systematic uncertainty of 1–2%, depending on the q2range considered. The effect of a potentially different q2distribution of the signal decay is evaluated by defining the efficiency ratio using B+→K1(1270)+µ+µ−events generated according to a phase-space model. The observed deviation results in a systematic uncertainty of 1–2%. 5 Branching fraction of the decay B+→φK+µ+µ− The signal decay B+→φK+µ+µ−is expected to be rarer than the decay B+→K+π+π−µ+µ−as an s¯squark pair must be created from the vacuum. Therefore, only the total branching fraction of this decay mode is determined. Figure 5a shows the B+→φK+µ+µ−signal candidates after the full selection. The signal yield is determined to be Nsig = 25.2+6.0 −5.3using an extended maximum likelihood fit to the unbinned φK+µ+µ− mass distribution. The statistical significance of the signal, calculated using Wilks’ theorem, is 6.6σ. The signal component is modelled using the sum of two Gaussian functions with a tail described by a power law on the low-mass side. The background mass shape is modelled using a second-order Chebychev polynomial. The parameters describing the signal mass shape are fixed to those determined using the normalisation mode B+→J/ψφK+, as shown in figure 5b. The yield of the normalisation mode is Nnorm = 1908 ±63. To determine the total branching fraction of the decay B+→φK+µ+µ−, the formula B(B+→φK+µ+µ−) = N0 sig Nnorm ·B(B+→J/ψφK+)·B(J/ψ →µ+µ−) (5.1) is used. Here, N0 sig denotes the signal yield determined in a fit where signal candidates are weighted by the relative efficiency norm/sig(q2), according to their q2value. This is necessary since the efficiency ratio varies significantly over the full q2range. The weights are determined in bins of q2, with the same choice of q2bins as in table 1. Using the branching fraction of the normalisation channel, the integrated branching fraction is determined to be – 8 – JHEP10(2014)064 G. Simi22, M. Sirendi47, N. Skidmore46, T. Skwarnicki59, N.A. Smith52, E. Smith55,49, E. Smith53, J. Smith47, M. Smith54, H. Snoek41, M.D. Sokoloff57, F.J.P. Soler51, F. Soomro39, D. Souza46, B. Souza De Paula2, B. Spaan9, A. Sparkes50, P. Spradlin51, S. Sridharan38, F. Stagni38, M. Stahl11, S. Stahl11, O. 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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 – 15 – JHEP10(2014)064 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 KVI - 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 – 16 – JHEP10(2014)064 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 – 17 –