Evidence for the decay B+c→J/ψ3π+2π−

JHEP05(2014)148 Published for SISSA by Springer Received:April 2, 2014 Accepted:April 28, 2014 Published:May 30, 2014 Evidence for the decay B+ c→J/ψ3π+2π− The LHCb collaboration E-mail: [email protected] Abstract: Evidence is presented for the decay B+ c→J/ψ3π+2π−using proton-proton collision data, corresponding to an integrated luminosity of 3 fb−1, collected with the LHCb detector. A signal yield of 32 ±8 decays is found with a significance of 4.5 standard deviations. The ratio of the branching fraction of the B+ c→J/ψ3π+2π−decay to that of the B+ c→J/ψ π+decay is measured to be B(B+ c→J/ψ3π+2π−) BB+ c→J/ψ π+= 1.74 ±0.44 ±0.24, where the first uncertainty is statistical and the second is systematic. Keywords: Hadron-Hadron Scattering, QCD, Branching fraction, B physics, Flavor physics ArXiv ePrint: 1404.0287 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP05(2014)148 JHEP05(2014)148 Contents 1 Introduction 1 2 Detector 2 3 Candidate selection 2 4 Signal and normalization yields 3 5 Efficiency and systematic uncertainties 5 6 Results and summary 8 The LHCb collaboration 12 1 Introduction The B+ cmeson is the only meson consisting of two heavy quarks of different flavours. It was discovered by the CDF collaboration through the semileptonic decay B+ c→J/ψ`+ν`X [1], where X denotes possible unobserved particles.1The CDF collaboration also observed the hadronic decay mode B+ c→J/ψπ+[2]. Recently, the LHCb experiment has observed several new channels including B+ c→J/ψπ+π+π−[3], B+ c→ψ(2S)π+[4], B+ c→J/ψD+ sand B+ c→J/ψD∗+ s[5], B+ c→J/ψK+[6], B+ c→J/ψK+K−π+[7] and B+ c→B0 sπ+[8]. The lifetime of the B+ cmeson [9,10] is about three times shorter than that of the B0and B+ mesons, confirming the important role played by the c quark in B+ cdecays. The decays of B+ cmesons into charmonia and light hadrons are expected to be well described by the factorization approximation [11,12]. In this scheme, the B+ c→J/ψ3π+2π−decay is characterized by the form factors of the B+ c→J/ψW+transition and the spectral functions for the virtual W+boson into light hadrons [13]. The predictions for the ratio of branching fractions R5π≡B(B+ c→J/ψ3π+2π−) BB+ c→J/ψπ+(1.1) are 0.95 and 1.1 [14], using form factor calculations from refs. [15] and [16], respectively. In this article, the first evidence for the decay B+ c→J/ψ3π+2π−and a measurement of R5πare reported. The analysis is based on a data sample of proton-proton (pp) collisions, corresponding to an integrated luminosity of 1 fb−1at a centre-of-mass energy of 7 TeV and 2 fb−1at 8 TeV, collected with the LHCb detector. 1The inclusion of charge conjugate modes is implicit throughout this paper. – 1 – JHEP05(2014)148 2 Detector The LHCb detector [17] 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 [18] placed downstream. The combined tracking system provides a momentum measurement with relative uncertainty that varies from 0.4% at 5 GeV/c to 0.6% at 100 GeV/c, and impact parameter resolution of 20 µm for tracks with large transverse momentum. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors [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. This analysis uses events collected by triggers that select the µ+µ−pair from the J/ψdecay with high efficiency. At the hardware stage either one or two muon candidates are required to trigger the event. In the case of single muon triggers, the transverse momentum, pT, of the muon candidate is required to be greater than 1.5 GeV/c. For dimuon candidates, the product of the pTof muon candidates is required to satisfy √pT1pT2>1.3 GeV/c. At the subsequent software trigger stage, two muons are selected with an invariant mass in the range 2.97 < mµ+µ−<3.21 GeV/c2and consistent with originating from a common vertex. The common vertex is required to be significantly displaced from the pp collision vertices. Simulated pp collisions are generated using Pythia 6.4 [22] with the configuration described in ref. [23]. Final-state QED radiative corrections are included using the Photos package [24]. The B+ cmesons are produced by a dedicated generator, Bcvegpy [25]. The decays of all hadrons are performed by EvtGen [26], and a specific model is implemented to generate the decays B+ c→J/ψ3π+2π−, assuming factorization [14]. The model allows the implementation of different form factors for this decay, calculated using QCD sum rules [15] or a relativistic quark model [16]. These predictions lead to very similar values and those based on the relativistic quark model are used in the simulation. The coupling of the five pion (3π+2π−) system to the virtual W+is taken from τ+lepton decays [27]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [28,29] as described in ref. [30]. 3 Candidate selection The decays B+ c→J/ψ3π+2π−and B+ c→J/ψπ+are reconstructed using the J/ψ→µ+µ−decay mode. The selection criteria chosen are similar for both channels. – 2 – JHEP05(2014)148 All tracks are required to be in the pseudorapidity range 2 <η<4.9. Good track quality of charged particles is ensured by requiring the χ2per number of degrees of freedom, χ2/ndf, provided by the track fit, to be less than 3. Suppression of fake tracks created by the reconstruction is achieved by a neural network trained with simulated samples to discriminate between fake tracks and tracks associated with real particles [31], ensuring the rate of fake tracks below 0.3 %. Two dedicated neural networks are used for muon and pion identification. These networks use the information from the Cherenkov detectors [19], muon chambers [32] and the calorimeter system [33], together with the tracking information. The momentum of the pion candidates is required to be between 3.2 GeV/c and 150 GeV/c in order to ensure good quality particle identification in Cherenkov detectors. The requirements on the neural network output are chosen to ensure good agreement between data and simulation and significant reduction of the background due to misidentification. Pairs of oppositely charged muons, originating from a common vertex, are combined to form J/ψ→µ+µ−candidates. The pTof each muon is required to be greater than 550 MeV/c. Good vertex reconstruction is ensured by requiring the χ2of the vertex fit, χ2 vtx, to be less than 20. To select dimuon vertices that are well-separated from the reconstructed pp interaction vertices, the decay length is required to be at least three times its uncertainty. The invariant mass of the dimuon combination is required to be between 3.020 and 3.135 GeV/c2. The asymmetric mass range with respect to the known J/ψmeson mass [9] is chosen to include the QED radiative tail. The selected J/ψcandidates are combined with pions to form B+ c→J/ψ3π+2π−and B+ c→J/ψπ+candidates. The transverse momentum of each pion is required to be greater than 400 MeV/c. To ensure that the pions are inconsistent with being directly produced in a pp interaction, the impact parameter χ2, defined as the difference between the χ2values of the fits of the pp collision vertex formed with and without the considered pion track, is required to satisfy χ2 IP >4. When more than one primary vertex is reconstructed, the vertex with the smallest value of χ2 IP is chosen. Good vertex reconstruction for the B+ ccandidate vertex is ensured by requiring the χ2 vtx/ndf to be less than 12. To suppress the large combinatorial background in the B+ c→J/ψ3π+2π−sample, the χ2of the vertex fit for all J/ψπ±combinations, as well as for all dipion combinations, is required to be less than 20. To improve the invariant mass resolution, a kinematic fit [34] is performed that constrains the µ+µ−pair to the known mass of the J/ψmeson. It is also required that the B+ ccandidate’s momentum vector points back to from the associated pp interaction vertex. The χ2per number of degrees of freedom of the fit, χ2 fit/ndf, is required to be less than 5. The measured decay time of the B+ ccandidate, calculated with respect to the associated primary vertex, is required to be between 150 µm/c and 1 mm/c. 4 Signal and normalization yields The mass distribution for the selected J/ψ3π+2π−candidates is shown in figure 1. To estimate the signal yield, an extended maximum likelihood fit to the unbinned mass distribution is made. The B+ c→J/ψ3π+2π−signal is modelled by a Gaussian distribution – 3 – JHEP05(2014)148 6.2 6.3 6.4 0 5 10 15 mJ/ψ3π+2π−[GeV/c2] Candidates/(10 MeV/c2) LHCb Figure 1. Mass distribution for selected B+ c→J/ψ3π+2π−candidates. The result of a fit using the model described in the text (red solid line) is shown together with the background component (blue dashed line). and the background by a constant function. The fit results for the fitted mass and mass resolution of B+ csignal, mB+ cand σB+ c, and signal yield NB+ c→J/ψ3π+2π−, are listed in table 1, The statistical significance for the observed signal is determined as Sσ=q−2 log LB LS+B where LS+B and LBdenote the likelihood associated with the signal-plus-background and background-only hypothesis, respectively. The likelihoods are calculated with the peak position fixed to the known mass of B+ cmeson [5,9] and the mass resolution fixed to 10.1 MeV/c2as expected from simulation. The statistical significance of the B+ c→J/ψ3π+2π−signal is 4.5 standard deviations. For the selected B+ ccandidates, the existence of resonant structures is searched for in the π+π−,π+π+π−,π+π−π−, 2π+2π−, 3π+2π−and J/ψπ+π−combinations of final state particles using the sPlot technique [35], with the reconstructed J/ψ3π+2π−mass as discriminating variable, to subtract the background. No significant narrow structures are observed; in particular, no indication of a contribution from B+ c→ψ(2S)π+π+π−, followed by the ψ(2S)→J/ψπ+π−decay, is seen. The background-subtracted five-pion mass distribution is shown in figure 2, along with the theoretical prediction in ref. [14], which describes the data well. The consistency between data and the model prediction is estimated using a χ2-test and gives a p-value of 14 %. The corresponding p-value for the phase space decay model is 4 %. – 4 – JHEP05(2014)148 Parameter Value mB+ cMeV/c26273 ±3 σB+ cMeV/c211.4±3.4 NB+ c→J/ψ3π+2π−32 ±8 Table 1. Signal parameters of the unbinned extended maximum likelihood fit to the J/ψ3π+2π−mass distribution. Uncertainties are statistical only. 2 2.5 3 0 5 10 15 20 m3π+2π−[GeV/c2] Yield/(400 MeV/c2) LHCb Figure 2. Background-subtracted distribution of five-pion mass from B+ c→J/ψ3π+2π−events (points with error bars). The model prediction from ref. [14] is shown by a red solid line, and the expectation from the phase space model is shown by a blue dashed line. The mass distribution of the selected B+ c→J/ψπ+candidates is shown in figure 3, together with the result of an extended unbinned maximum likelihood fit. The B+ csignal is modelled by a Gaussian distribution and the background by an exponential function. The fit gives a yield of 2271 ±63 events. 5 Efficiency and systematic uncertainties The overall efficiency for each decay is the product of the geometrical acceptance of the detector, reconstruction, selection and trigger efficiencies. These are estimated using simula- – 5 – JHEP05(2014)148 6.2 6.3 6.4 0 200 400 600 800 mJ/ψπ+[GeV/c2] Candidates/(10 MeV/c2) LHCb Figure 3. Mass distribution for selected B+ c→J/ψπ+candidates. The result of a fit using the model described in the text (red solid line) is shown together with the background component (blue dashed line). tion and the ratio of the efficiencies is found to be ε(B+ c→J/ψπ+) ε(B+ c→J/ψ3π+2π−)= 123.8±5.6±15.1,(5.1) where the first uncertainty is statistical, due to the finite size of the simulated sample, and the second one is systematic, as discussed below. The large difference in efficiencies is due to the reconstruction of four additional low-pTpions in the B+ c→J/ψ3π+2π−mode. The efficiencies for the data samples collected at a centre-of-mass energy of 7 TeV and 8 TeV are found to be similar and a luminosity-weighted average is used, with the corresponding systematic uncertainty discussed below. Many sources of systematic uncertainty cancel in the ratio, in particular those related to the muon and J/ψreconstruction and identification. Those that do not cancel are discussed below and summarized in table 2. A systematic uncertainty arises from the imperfect knowledge of the shape of the signal and background in the J/ψ3π+2π−and J/ψπ+mass distributions. The dependence of the signal yields on the fit model is studied by varying the signal and background parameterizations. This is assessed by using Crystal Ball [36] and double-sided Crystal Ball [37] functions for the parameterization of the B+ csignals. The background parametrization – 6 – JHEP05(2014)148 Source Uncertainty [%] Fit model 6.6 Decay model m3π+2π−reweighting 7.7 ψ(2S) mass veto 3.1 Data-simulation agreement Hadron interactions 4 ×2.0 Track quality selection 4 ×0.6 Trigger 1.1 Pion identification 0.7 Selection variables 1.0 B+ clifetime 0.9 Stability for various data taking conditions 2.5 Acceptance 0.8 Total 13.9 Table 2. Relative systematic uncertainties for the ratio R5π. The total uncertainty is the quadratic sum of the individual components. is performed using both exponential and polynomial functions. The maximum observed change of 6.6 % in the ratio of B+ c→J/ψ3π+2π−and B+ c→J/ψπ+yields is assigned as a systematic uncertainty. To assess the systematic uncertainty related to the B+ c→J/ψ3π+2π−decay model used in the simulation [14], the reconstructed mass distribution of the five-pion system in simulated events is reweighted to reproduce the distribution observed in data. As a cross-check the efficiency is also recalculated using a phase space model for the B+ c→J/ψ3π+2π−decays. There is a maximal change in efficiency of 7.7 %, which is taken as the systematic uncertainty for the decay model. In addition, the analysis is repeated with the removal of all B+ ccandidates where the J/ψπ+π−mass is compatible with originating from ψ(2S) →J/ψπ+π−decays. The observed difference of 3.1 % is assigned as an additional systematic uncertainty. A large class of uncertainties arises from the differences between data and simulation, in particular those affecting the efficiency for reconstruction of charged-particle tracks. The largest of these arises from the simulation of hadronic interactions in the detector, which has an uncertainty of 2 % per track [31,38,39]. An additional uncertainty associated with the track quality requirements for the additional four pions in the signal decay is estimated to be 0.6 % per track [5,7]. The trigger efficiency for events with J/ψ→µ+µ−produced in beauty hadron decays is studied on data in high-yield modes [5,40] and a systematic uncertainty of 1.1 % is assigned based on the comparison of the ratio of trigger efficiencies for high-yield samples of B+→J/ψK+and B+→ψ(2S)K+decays on data and simulation [40]. – 7 – JHEP05(2014)148 The systematic uncertainty associated with pion identification is studied using a sample of B+→J/ψK+π+π−decays. The efficiency to identify a π+π−pair is compared for data and simulation. This comparison shows a 0.35% difference between the data and simulation in the efficiency to identify a pion pair. As a result of this study an uncertainty of 0.7 % is assigned for the four additional pions in the analysis. The transverse momentum and rapidity spectra for the selected B+ c→J/ψπ+candidates, as well their daughter J/ψmesons and pions, are found to be in good agreement with the predictions from the Bcvegpy generator. Good agreement in efficiencies determined from the data and simulation has been observed for all variables used in the selection of B+ c→J/ψπ+candidates. The differences do not exceed 1 %, which is used as a conservative estimate for the systematic uncertainty from the selection variables. The agreement between data and simulation has also been cross-checked using the B+ c→J/ψ3π+2π−signal by varying the selection criteria to the values that correspond to a 20 % change in the signal yield in simulation. No unexpectedly large deviation is found. The different acceptance as a function of decay time for the B+ c→J/ψ3π+2π−and B+ c→J/ψπ+decay modes results in an additional systematic uncertainty related to the imprecise knowledge of the B+ clifetime. To assess the related uncertainty, the decay time distributions for simulated events are reweighted after changing the B+ clifetime by one standard deviation around the value of 509 ±8±12 fs [10] measured by LHCb and the efficiencies are recomputed. The observed 0.9 % variation in the ratio of efficiencies is used as the systematic uncertainty. The uncertainty related to the stability of the analysis results against variations of the detector and trigger configurations occuring in different data-taking periods are tested by studying the ratio of the yields of B+→J/ψK+π+π−and B+→J/ψK+decays as a function of the data-taking period. According to this study an additional systematic uncertainty of 2.5 % is assigned [5]. The last systematic uncertainty originates from the dependence of the geometrical acceptance on both the beam crossing angle and the position of the luminosity region. The resulting 0.8 % difference in the efficiency ratios is taken as an estimate of the systematic uncertainty. A summary of systematic uncertainties is presented in table 2. The total systematic uncertainty on the ratio of the branching fractions R5πis 13.9 %. 6 Results and summary The first evidence for the decay B+ c→J/ψ3π+2π−is found using pp collisions, corresponding to an integrated luminosity of 3 fb−1, collected with the LHCb detector A signal yield of 32±8 events is found. The significance, taking into account the systematic uncertainties due to the fit function, peak position and mass resolution in the fit, is estimated to be 4.5 standard deviations. Using the B+ c→J/ψπ+mode as a normalization channel, the ratio of branching fractions is calculated as R5π=N(B+ c→J/ψ3π+2π−) NB+ c→J/ψπ+×ε(B+ c→J/ψπ+) ε(B+ c→J/ψ3π+2π−),(6.1) – 8 – JHEP05(2014)148 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 iUniversit`a di Modena e Reggio Emilia, Modena, Italy – 15 – JHEP05(2014)148 jUniversit`a di Genova, Genova, Italy kUniversit`a di Milano Bicocca, Milano, Italy lUniversit`a di Roma Tor Vergata, Roma, Italy mUniversit`a della Basilicata, Potenza, Italy nLIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain oHanoi University of Science, Hanoi, Viet Nam pUniversit`a di Padova, Padova, Italy qUniversit`a di Pisa, Pisa, Italy rScuola Normale Superiore, Pisa, Italy sUniversit`a degli Studi di Milano, Milano, Italy tPolitecnico di Milano, Milano, Italy – 16 –