Published for SISSA by Springer Received: March 24, 2016 Accepted: May 9, 2016 Published: May 23, 2016 The LHCb collaboration E-mail: Ivan.Belyaev@cern.ch Abstract: The decays Λ0b → ψ(2S)pK− and Λ0b → J/ψπ+ π− pK− are observed in a data sample corresponding to an integrated luminosity of fb−1 , collected in proton-proton collisions at and TeV centre-of-mass energies by the LHCb detector The ψ(2S) mesons are reconstructed through the decay modes ψ(2S) → µ+ µ− and ψ(2S) → J/ψπ+ π− The branching fractions relative to that of Λ0b → J/ψ pK− are measured to be B(Λ0b → ψ(2S)pK− ) = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10−2 , B(Λ0b → J/ψ pK− ) B(Λ0b → J/ψπ+ π− pK− ) = (20.86 ± 0.96 ± 1.34) × 10−2 , B(Λ0b → J/ψ pK− ) where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of J/ψ and ψ(2S) branching fractions The mass of the Λ0b baryon is measured to be M (Λ0b ) = 5619.65 ± 0.17 ± 0.17 MeV/c2 , where the uncertainties are statistical and systematic Keywords: B physics, Flavor physics, Hadron-Hadron scattering (experiments), Particle and resonance production, Spectroscopy ArXiv ePrint: 1603.06961 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration Article funded by SCOAP3 doi:10.1007/JHEP05(2016)132 JHEP05(2016)132 Observation of Λ0b → ψ(2S)pK− and Λ0b → J/ψπ+π−pK− decays and a measurement of the Λ0b baryon mass Contents Detector and simulation Event selection Measurement of branching fractions 4.1 Signal yields and efficiencies 4.2 Systematic uncertainties 4.3 Results 4 Measurement of Λ0b baryon mass Results and summary 12 The LHCb collaboration 17 Introduction The Λ0b baryon is the isospin singlet ground state of a bottom quark and two light quarks The rich phenomenology associated with decays of bottom baryons allows many measurements of masses, lifetimes and branching fractions, which test the theoretical understanding of weak decays of heavy hadrons in the framework of heavy quark effective theory (HQET) and the underlying QCD physics [1, 2] At the Tevatron, properties of the Λ0b baryon, such as mass and lifetime, have been measured using two-body modes, − decays [3–5].1 The high production rate of specifically Λ0b → J/ψ Λ0 and Λ0b → Λ+ c π b quarks at the Large Hadron Collider (LHC), along with the excellent momentum and mass resolution and the hadron identification capabilities of the LHCb detector, open up a host of multibody and Cabibbo-suppressed decay channels of Λ0b baryons, e.g the de− + − + − − cays Λ0b → D0 pK− , Λ0b → Λ+ c K [6], Λb → Λc D , Λb → Λc Ds [7] and Λb → J/ψ pπ [8] The high signal yield of the Λ0b → J/ψ pK− decay [9] allowed the precise measurement of the Λ0b lifetime [10, 11] The recent analysis of this decay mode uncovered a double resonant structure in the J/ψ p system consistent with two pentaquark states [12] LHCb has also measured several B meson decays into final states with charmonia [13–18] The first observation of Λ0b decays to excited charmonium, the Λ0b → ψ(2S)Λ0 decay, has been presented by the ATLAS collaboration [19] An experimental investigation of other similar multibody decays of the Λ0b baryon should lead to deeper insights into QCD The inclusion of charge-conjugate modes is implied throughout this paper –1– JHEP05(2016)132 Introduction In this paper, the first observations of the decays Λ0b → ψ(2S)pK− and Λ0b → J/ψ π+ π− pK− are reported, where ψ(2S) mesons are reconstructed in the final states µ+ µ− and J/ψ π+ π− The ratios of the branching fractions of these decays to that of the normalization decay Λ0b → J/ψ pK− , Rψ(2S) ≡ RJ/ψ π + π− ≡ B(Λ0b → ψ(2S)pK− ) , B(Λ0b → J/ψ pK− ) (1.1) B(Λ0b → J/ψ π+ π− pK− ) , B(Λ0b → J/ψ pK− ) (1.2) Detector and simulation The LHCb detector [20, 21] is a single-arm forward spectrometer covering the pseudorapidity range < 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 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet The polarity of the dipole magnet is reversed periodically throughout data-taking The tracking system provides a measurement of the momentum, p, 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, is measured with a resolution of (15 + 29/pT ) µm, where pT is the component of the momentum transverse to the beam, in GeV/c [22] Large samples of B+ → J/ψ K+ and J/ψ → µ+ µ− decays, collected concurrently with the data set, were used to calibrate the momentum scale of the spectrometer to a precision of 0.03 % [23] Different types of charged hadrons are distinguished using information from two ringimaging 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 trigger [24] comprises two stages Events are first required to pass the hardware trigger, which selects muon candidates with pT > 1.48 (1.76) GeV/c or pairs of opposite-sign muon candidates with a requirement that the product of the muon transverse momenta is √ larger than 1.7 (2.6) GeV2 /c2 for data collected at s = (8)TeV The subsequent software trigger is composed of two stages, the first of which performs a partial event reconstruction, –2– JHEP05(2016)132 are measured In measuring the branching fraction of Λ0b → J/ψ π+ π− pK− decays, contributions via intermediate resonances, such as Λ0b → ψ(2S)pK− , are implicitly included The low energy release in these decays allows a precise determination of the Λ 0b mass with a small systematic uncertainty This study is based on a data sample corresponding to an integrated luminosity of fb−1 , collected with the LHCb detector in pp collisions at centre-of-mass energies √ s = and 8TeV 3 Event selection The decays Λ0b → ψ(2S)pK− , Λ0b → J/ψ π+ π− pK− and Λ0b → J/ψ pK− are reconstructed using decay modes ψ(2S) → µ+ µ− , ψ(2S) → J/ψ π+ π− and J/ψ → µ+ µ− Common selection criteria, based on those used in refs [17, 33], are used for all channels, except for those related to the selection of two additional pions in the Λ0b → J/ψ π+ π− pK− and Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− channels Muon, proton, kaon and pion candidates are selected from well-reconstructed tracks within the acceptance of the spectrometer that are identified using information from the RICH, calorimeter and muon detectors [34, 35] Muons, protons, kaons and pions are required to have a transverse momentum larger than 550, 800, 500 and 200 MeV/c, respectively To allow good particle identification, kaons and pions are required to have a momentum between 3.2 GeV/c and 150 GeV/c whilst protons must have a momentum between 10 GeV/c and 150 GeV/c To reduce combinatorial background involving tracks from the primary pp interaction vertices, only tracks that exceed a minimum impact parameter χ2 with respect to every PV are used The impact parameter χ2 is defined as the difference between the χ2 of the PV reconstructed with and without the considered particle Pairs of oppositely-charged muons originating from a common vertex are combined to form J/ψ → µ+ µ− or ψ(2S) → µ+ µ− candidates The resulting dimuon candidates are required to have an invariant mass between −5σ and +3σ around the known J/ψ or ψ(2S) masses [36], where σ is the mass resolution An asymmetric mass interval is chosen to include part of the low-mass tail due to final-state radiation Candidate Λ0b baryons are formed from J/ψ pK− , ψ(2S)pK− and J/ψ π+ π− pK− combinations Each candidate is associated with the PV with respect to which it has the smallest impact parameter significance The Λ0b mass resolution is improved by employing a kinematic fit [37] that constrains the mass of the J/ψ candidate to its known value and requires the momentum of the Λ0b candidate to point back to the PV A requirement on the quality of this fit is applied to further suppress combinatorial background Finally, the measured decay time of the Λ0b candidate, calculated with respect to the associated primary vertex, is required to be between 0.5 and 6.7 ps The lower limit is used to suppress background from particles coming from the PV while the upper limit removes poorly reconstructed candidates –3– JHEP05(2016)132 while full event reconstruction is done at the second stage At the first stage of the software trigger the invariant mass of well-reconstructed pairs of oppositely charged muons forming a good-quality two-track vertex is required to exceed 2.7 GeV/c2 , and the two-track vertex is required to be significantly displaced from all PVs The analysis technique reported below has been validated using simulated events The pp collisions are generated using Pythia [25, 26] with a specific LHCb configuration [27] Decays of hadronic particles are described by EvtGen [28], in which final-state radiation is generated using Photos [29] The interaction of the generated particles with the detector, and its response, are implemented using the Geant4 toolkit [30, 31] as described in ref [32] Candidates/(1 MeV/c2 ) Candidates/(2 MeV/c2 ) 150 LHCb 100 50 m(ψ(2S)[→ 5.6 5.65 µ+ µ− ]pK− ) GeV/c2 3000 2000 1000 5.6 m(J/ψ pK− ) 5.65 GeV/c2 Figure Mass distributions of selected (left) Λ0b → ψ(2S)[→ µ+ µ− ]pK− and (right) Λ0b → J/ψ pK− candidates The total fit function (solid red), the Λ0b signal contribution (dotted magenta) and the combinatorial background (dashed blue) are shown The error bars show 68% Poissonian confidence intervals To suppress cross-feed from decays of the B0s meson into J/ψ K− K+ , ψ(2S)K− K+ and J/ψ π+ π− K− K+ final states, with the positively-charged kaon misidentified as a proton, a veto on the Λ0b candidate mass, recalculated with a kaon mass hypothesis for the proton, is applied Any candidate with a recalculated mass consistent with the nominal B 0s mass is rejected A similar veto is applied to suppress cross-feed from decays of B mesons into J/ψ K− π+ , ψ(2S)K− π+ and J/ψ π− π+ π+ K− decays with the positively-charged pion misidentified as a proton 4.1 Measurement of branching fractions Signal yields and efficiencies The mass distributions for selected Λ0b → ψ(2S)[→ µ+ µ− ]pK− candidates and candidates for the normalization channel Λ0b → J/ψ pK− are shown in figure Signal yields are determined using unbinned extended maximum likelihood fits to these distributions The signal is modelled with a modified Gaussian function with power-law tails on both sides [38, 39], where the tail parameters are fixed from simulation and the mass resolution parameter is allowed to vary The background is modelled with an exponential function multiplied by a first-order polynomial The resolution parameters obtained from the fits are found to be 3.82 ± 0.17 MeV/c2 for the channel Λ0b → ψ(2S)[→ µ+ µ− ]pK− and 6.12 ± 0.05 MeV/c2 for Λ0b → J/ψ pK− , in good agreement with expectations from simulation The mass distribution for selected Λ0b → J/ψ π+ π− pK− candidates is shown in figure 2(left), along with the result of an unbinned extended maximum likelihood fit using the model described above The mass resolution parameter obtained from the fit is 4.72 ± 0.23 MeV/c The mass distribution of the J/ψ π+ π− system from signal Λ0b → J/ψ π+ π− pK− decays is presented in figure 2(right) in the region 3.67 < m(J/ψ π+ π− ) < 3.7 GeV/c2 –4– JHEP05(2016)132 LHCb Candidates/(1 MeV/c2 ) Candidates/(2 MeV/c2 ) LHCb 150 100 50 LHCb 40 20 5.6 5.65 m(J/ψ π+ π− pK− ) 3.67 3.68 3.69 m(J/ψ π+ π− ) GeV/c2 3.7 GeV/c2 Figure Left: mass distribution of selected Λ0b → J/ψ π+ π− pK− candidates Right: backgroundsubtracted J/ψ π+ π− mass distribution for that mode The total fit function and the signal contributions are shown by solid red and dotted magenta lines, respectively The combinatorial background in the left plot and nonresonant contribution in the right plot are shown by dashed blue lines N (Λ0b ) Channel Λ0b → J/ψ pK− 28 834 ± 204 Λ0b → ψ(2S)[→ µ+ µ− ]pK− Λ0b → Λ0b → ψ(2S)[→ 665 ± 28 J/ψ π+ π− ]pK− 231 ± 17 J/ψ π+ π− pK− 793 ± 36 Table Signal yields of Λ0b decay channels Uncertainties are statistical only The background subtraction is performed with the sPlot technique [40] using the J/ψ π+ π− pK− mass as the discriminating variable The signal yield of + − − Λb → ψ(2S)[→ J/ψ π π ]pK decays is determined using an unbinned extended maximum likelihood fit to the J/ψ π+ π− invariant mass distribution The ψ(2S) component is modelled with a modified Gaussian function with power-law tails on both sides, where the tail parameters are fixed from simulation The nonresonant component is taken to be constant The mass resolution parameter obtained from the fit is 2.29±0.17 MeV/c2 The signal yields are summarized in table The ratio of branching fractions Rψ(2S) , defined in eq (1.1), is measured in two different decay modes, Λ0 R ψ(2S) ψ(2S)→µ+ µ− b εJ/ψ Nψ(2S)→µ+ µ− B(J/ψ → µ+ µ− ) = ì ì , NJ/ B((2S) à+ ) b εψ(2S)→µ + µ− Λ0 Rψ(2S) ψ(2S)→J/ψ π+ π− b εJ/ψ Nψ(2S)→J/ψ π+ π− = × Λ0 × , NJ/ψ B(ψ(2S) → J/ψ π+ π− ) b εψ(2S)→J/ψ + − π π (4.1) –5– JHEP05(2016)132 0 Value Λ0 Λ0 Λ0b Λ0b Λ0b Λ0b 1.188 ± 0.006 b b εJ/ψ /εψ(2S)→µ + µ− εJ/ψ /εψ(2S)→J/ψ π+ π− 8.84 ± 0.05 7.59 ± 0.04 εJ/ψ /εJ/ψ π+ π− Table Ratios of efficiencies The uncertainties reflect the limited size of the simulation sample + π− , defined in eq (1.2), is measured as Λ0 R J/ψ π+ π− b εJ/ψ NJ/ψ π+ π− = × Λ0 , NJ/ψ b εJ/ψ + − π π (4.2) Λ0 where NX represents the observed signal yield and εXb denotes the efficiency for the decay Λ0b → XpK− The ratio B(J/ψ→µ+ µ− ) B(ψ(2S)→µ+ µ− ) is taken to be equal to the more precisely + − B(J/ψ →e e ) measured ratio of dielectron branching fractions, B(ψ(2S)→e = 7.57 ± 0.17 [36] + e− ) + − For the ψ(2S) → J/ψ π π branching fraction the world average (34.46 ± 0.30)% [36] is taken The efficiency is defined as the product of the geometric acceptance and the detection, reconstruction, selection and trigger efficiencies The efficiencies for hadron identification as functions of kinematic parameters and event multiplicity are determined from data using calibration samples of low-background decays: D∗+ → D0 π+ followed by D0 → K− π− − + for kaons and pions, and Λ0 → pπ− and Λ+ c → pK π for protons [34] The remaining efficiencies are determined using simulation In the simulation of Λ0b → J/ψ pK− decays, the model established in ref [12] that includes pentaquark contributions is used, while in the simulation of the other decay modes the events are generated uniformly in phase space The simulation is corrected to reproduce the transverse momentum and rapidity distributions of the Λ0b baryons observed in data [9] and to account for small discrepancies between data and simulation in the reconstruction of charged tracks [41] The ratios of efficiencies to those in the Λ0b → J/ψ pK− channel are presented in table 4.2 Systematic uncertainties Most systematic uncertainties cancel in the measurements of the ratios of branching fractions, notably those related to the reconstruction, identification and trigger efficiencies of the J/ψ → µ+ µ− and ψ(2S) → µ+ µ− candidates [13], due to the similarity of the muon and dimuon spectra for these modes The remaining systematic uncertainties are summarized in table and discussed below Alternative parametrizations for the signal and background are used to estimate the systematic uncertainties related to the fit model A modified Novosibirsk function [42], –6– JHEP05(2016)132 and the ratio RJ/ψ π Source Rψ(2S) ψ(2S)→µ+ µ− Rψ(2S) ψ(2S)→J/ψ π+ π− RJ/ψ π + π− Fit model 0.8 3.0 3.5 Cross-feed 0.8 0.9 0.9 0.3 0.8 0.8 Hadron interaction — × 2.0 × 2.0 Track efficiency correction — 3.2 2.7 Hadron identification 0.1 0.1 0.2 Trigger 1.1 1.1 1.1 Selection criteria 0.6 0.9 0.2 Simulation sample size 1.0 1.6 1.7 2.0 6.4 6.4 Efficiency calculation: Λ0b decay model Reconstruction of additional pions: Table Systematic uncertainties (in %) on the ratios of branching fractions Rψ(2S) and RJ/ψ π + π− an Apolonios function [43], an asymmetric variant of the Apolonios function and the Student’s t-distribution are used for the Λ0b signal shape, and an exponential function multiplied by a second-order polynomial is used for the background The ratio of event yields is remeasured with the cross-check models, and the maximum deviation with respect to the nominal value is assigned as a systematic uncertainty The uncertainty associated with the B0s and B0 cross-feed is estimated by varying the widths of the rejected regions and recomputing the signal yields, taking into account the changes in efficiencies As an additional cross-check, a veto is applied also on possible contributions from Λ0b → J/ψ pK+ , Λ0b → ψ(2S)pK+ and Λ0b → J/ψ π+ π− pK+ decays where the positive kaon is misidentified as a proton and the antiproton is misidentified as a negative kaon The maximum of the observed differences from the nominal values is assigned as the corresponding systematic uncertainty The remaining systematic uncertainties are associated with the efficiency determination The systematic uncertainty related to the decay model for Λ0b → ψ(2S)pK− and Λ0b → J/ψ π+ π− pK− decays is estimated using the simulated samples, corrected to reproduce the invariant mass of the pK− and ψ(2S)p or J/ψ π+ π− p systems observed in data The largest change in efficiency is taken as the corresponding systematic uncertainty The decay modes Λ0b → J/ψ π+ π− pK− and Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− have two additional pions to reconstruct compared to the reference mode Λ0b → J/ψ pK− The uncertainty associated with the reconstruction of these additional low-pT tracks has two independent contributions First, the uncertainties in the amount and distribution of material in the detector result in an uncertainty of 2.0% per additional final-state pion due to –7– JHEP05(2016)132 Sum in quadrature tio RJ/ψ π 4.3 + π− Results Using eq (4.1) and the ratios of yields and efficiencies determined above, the ratio Rψ(2S) is measured for each ψ(2S) decay mode separately: Rψ(2S) Rψ(2S) ψ(2S)→µ+ µ− ψ(2S)→J/ψ π+ π− = (20.74 ± 0.88 ± 0.41 ± 0.47) × 10−2 , = (20.55 ± 1.52 ± 1.32 ± 0.18) × 10−2 , (4.3) where the first uncertainty is statistical, the second is systematic and the third is related to the uncertainties on the dielectron J/ψ and ψ(2S) branching fractions and the ψ(2S) → J/ψ π+ π− branching fraction The average of the ratios in eq (4.3) is Rψ(2S) = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10−2 (4.4) In this average the systematic uncertainties related to the normalization channel, Λ0b → J/ψ pK− , and the trigger efficiency are considered to be 100% correlated while other systematic uncertainties are treated as uncorrelated The ratio of the branching fractions of Λ0b → J/ψ π+ π− pK− and Λ0b → J/ψ pK− is found to be + − RJ/ψ π π = (20.86 ± 0.96 ± 1.34) × 10−2 , (4.5) where contributions via intermediate resonances are included –8– JHEP05(2016)132 the modelling of hadron interactions [41] Second, the small difference in the track finding efficiency between data and simulation is corrected using a data-driven technique [41] The uncertainties in the correction factors are propagated to the efficiency ratios by means of pseudoexperiments This results in a systematic uncertainty of 3.2% for the ratio + − Rψ(2S) ψ(2S)→J/ψ π+ π− and 2.7% for the ratio RJ/ψ π π The systematic uncertainties related to the hadron identification efficiency, 0.1 (0.2)% + − for Rψ(2S) (RJ/ψ π π ) ratios, reflect the limited sizes of the calibration samples, and are + − propagated to the ratios Rψ(2S) and RJ/ψ π π by means of pseudoexperiments The trigger efficiency for events with J/ψ → µ+ µ− and ψ(2S) → µ+ µ− produced in beauty hadron decays is studied in data A systematic uncertainty of 1.1% is assigned based on a comparison between data and simulation of the ratio of trigger efficiencies for high-yield samples of B+ → J/ψ K+ and B+ → ψ(2S)K+ decays [13] 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 by varying the selection criteria in ranges that lead to as much as ±20% change in the measured signal yields The stability is tested by comparing the efficiency-corrected yields within these variations The largest deviations range between 0.2% and 0.9% and are taken as systematic uncertainties Finally, a systematic uncertainty due to the limited size of the simulation sample is assigned With all the systematic uncertainties added in quadrature, the total is 2.0% for the ratio Rψ(2S) ψ(2S)→µ+ µ− , 6.4% for the ratio Rψ(2S) ψ(2S)→J/ψ π+ π− and 6.4% for the ra- The absolute branching fractions Λ0b → ψ(2S)pK− and Λ0b → J/ψ π+ π− pK− are derived −4 using the branching fraction B(Λ0b → J/ψ pK− ) = (3.04 ± 0.04 ± 0.06 ± 0.33 +0.43 −0.27 ) × 10 , measured in ref [9], where the third uncertainty is due to the uncertainty on the branching ∗ fraction of the decay B0 → J/ψ K (892)0 and the fourth is due to the knowledge of the ratio of fragmentation fractions fΛ0 /fd They are found to be b −5 B(Λ0b → ψ(2S)pK− ) = (6.29 ± 0.23 ± 0.14 +1.14 , −0.90 ) × 10 −5 B(Λ0b → J/ψ π+ π− pK− ) = (6.34 ± 0.29 ± 0.41 +1.15 , −0.91 ) × 10 (4.6) Λ0 b εψ(2S)→J/ψ Nψ(2S)→µ+ µ− B(ψ(2S) → µ+ µ− ) + = ì ì B(J/ à+ µ− ) + − B(ψ(2S) → J/ψ π π ) Nψ(2S)→J/ψ π+ π− b εψ(2S)→µ+ µ− = (2.30 ± 0.20 ± 0.12 ± 0.01) × 10−2 , (4.7) where the third uncertainty is related to the uncertainty of the known branching fraction B(J/ψ → µ+ µ− ) = (5.961 ± 0.033)% [36] This result is in agreement with the world average of (2.29 ± 0.25) × 10−2 [36] based on results of the E672/E706 [44] and BaBar [45] collaborations, and has similar precision Measurement of Λ0b baryon mass The low energy release in Λ0b → ψ(2S)pK− and Λ0b → J/ψ π+ π− pK− decays allows the Λ0b mass to be determined with a small systematic uncertainty The mass is measured using four decay channels: Λ0b → ψ(2S)[→ µ+ µ− ]pK− , + − − + − − Λb → ψ(2S)[→ J/ψ π π ]pK , Λb → J/ψ π π pK and Λb → J/ψ pK− The mass distributions for the Λ0b → ψ(2S)[→ µ+ µ− ]pK− and Λ0b → J/ψ pK− channels are shown in figure In the Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− channel, the J/ψ π+ π− system is constrained to the nominal ψ(2S) mass [36] to improve the precision In the Λ0b → J/ψ π+ π− pK− channel, to avoid overlap with the Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− channel the ψ(2S) region is vetoed, i.e the mass of the J/ψ π+ π− combination is required to be outside the range 3670 < m(J/ψ π+ π− ) < 3700 MeV/c2 The mass distributions for these two samples, along with the result of an unbinned extended maximum likelihood fit using the model described in section 4.1, are shown in figure The systematic uncertainties on the measurement of the Λ0b baryon mass for all four channels are listed in table The precision of the absolute momentum scale calibration of 0.03% is the dominant source of uncertainty [23, 46] This uncertainty is proportional to the energy release in the decay and is minimal for the processes with a ψ(2S) in the final state A further uncertainty is related to the energy loss in the material of the tracking –9– JHEP05(2016)132 where the third uncertainty comes from the uncertainties in the branching fractions of Λ0b → J/ψ pK− , ψ(2S) → J/ψ π+ π− , ψ(2S) → e+ e− and J/ψ → e+ e− decays From the two separate measurements of the ratio Rψ(2S) via different decay modes of the ψ(2S) meson (eq (4.3)), the ratio of the ψ(2S) → µ+ µ− and ψ(2S) → J/ψ π+ π− branching fractions is calculated as Candidates/(2 MeV/c2 ) Candidates/(2 MeV/c2 ) LHCb 40 20 5.6 50 5.65 m(ψ(2S)pK− ) 100 5.6 5.65 m(J/ψ π+ π− pK− ) GeV/c2 GeV/c2 Figure Left: mass distribution of selected Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− candidates with an additional constraint for the ψ(2S) mass [36] Right: mass distribution of selected Λ0b → J/ψ π+ π− pK− candidates with a requirement of the J/ψ π+ π− combination mass to be outside the range 3670 < m(J/ψ π+ π− ) < 3700 MeV/c2 The total fit function (solid red), the Λ0b signal contribution (dotted magenta) and the combinatorial background (dashed blue) are shown J/ψ ψ(2S) → µ+ µ− ψ(2S) → J/ψ π+ π− ✘✘ J/ψ π+ π− , ✘ ψ(2S) Momentum scale 0.34 0.19 0.15 0.26 Energy loss correction 0.03 0.02 0.06 0.07 Fit model 0.04 0.03 0.08 0.05 Sum in quadrature 0.34 0.19 0.18 0.27 Table Systematic uncertainties (in MeV/c2 ) on the Λ0b mass using the decay modes Λb → J/ψ pK− , Λ0b → ψ(2S)[→ µ+ µ− ]pK− , Λ0b → ψ(2S)[→ J/ψ π+ π− ]pK− and + − − + − Λb → J/ψ π π pK with the J/ψ π π mass outside the ψ(2S) region system [47], which is known with an accuracy of 10% [48] This effect is estimated by varying the energy loss correction in the reconstruction by 10% and taking the observed mass shift as an uncertainty The uncertainty due to the fit model is estimated using the same set of cross-check models for the signal and background parameterization as considered in section 4, with the maximum deviation in the mass assigned as a systematic uncertainty The uncertainties on the masses of the J/ψ and ψ(2S) mesons [36] are small and are therefore neglected √ As a cross-check, the data sample is divided into four parts, for data collected at s = and 8TeV and with different magnet polarities The measured masses are consistent among these subsamples, and therefore no systematic uncertainty is assigned To check the effect of the selection criteria (see section 3), the high-yield Λ0b → J/ψ pK− decay channel is used No sizeable dependence of the mass on the selection criteria is observed and no additional uncertainty is assigned The results from the four decay channels are presented in table To combine them, correlations must be taken into account The statistical uncertainties and those related – 10 – JHEP05(2016)132 LHCb M (Λ0b ) MeV/c2 Channel Λ0b → J/ψ pK− 5619.62 ± 0.04 ± 0.34 Λ0b → ψ(2S)[→ µ+ µ− ]pK− Λ0b → Λ0b → ψ(2S)[→ 5619.84 ± 0.18 ± 0.19 J/ψ π+ π− ]pK− J/ψ π+ π− pK− excluding ψ(2S) 5619.38 ± 0.33 ± 0.18 5619.08 ± 0.30 ± 0.27 Table Measured Λ0b mass in different decay channels The first uncertainty is statistical and the second is systematic M (Λ0b ) = 5619.65 ± 0.17 ± 0.17 MeV/c2 , (5.1) where the first uncertainty is statistical and the second systematic The χ2 /ndf calculated for the individual measurements with respect to the combined value is 3.0/3 This is the most precise measurement of any b-hadron mass reported to date Previous direct measurements of the Λ0b mass by LHCb were made using the decay Λ0b → J/ψ Λ0 [23, 47] and are statistically independent of the results of this study The combination obtained here is consistent with, and more precise than, the results of these earlier studies The LHCb results are combined, taking the statistical uncertainties and those related to the fit procedure to be uncorrelated and those due to the energy loss correction to be fully correlated The uncertainty due to the momentum scale in ref [23] is also taken to be fully correlated, whereas in ref [47] a different alignment and calibration procedure was used and so the corresponding uncertainty is considered to be uncorrelated with the other measurements The result of the combination is dominated by the measurements of this analysis and is M (Λ0b ) = 5619.65 ± 0.16 ± 0.14 MeV/c2 , (5.2) where the uncertainties are statistical and systematic The χ2 /ndf calculated for the individual measurements with respect to the combined value is 3.4/5 The measured mass is in agreement with, but more precise than, the results of the ATLAS [49] and CDF [5] collaborations From the value of the Λ0b mass in eq (5.2) and a precise measurement of the mass difference between the Λ0b and B0 hadrons reported in ref [7], the mass of the B0 meson is calculated to be M (B0 ) = 5279.93 ± 0.39 MeV/c2 , (5.3) where the correlation of 41% between the LHCb measurements of the Λ0b mass and the Λ0b –B0 mass splitting has been taken into account This is in agreement with the current world average of 5279.61 ± 0.16 MeV/c2 [36] – 11 – JHEP05(2016)132 to the fit procedure are treated as uncorrelated while those due to the momentum scale and energy loss correction are considered to be fully correlated The combined value of the Λ0b mass is Results and summary The Λ0b → ψ(2S)pK− and Λ0b → J/ψ π+ π− pK− decay modes are observed using a sample of pp collisions at centre-of-mass energies of and 8TeV, corresponding to an integrated luminosity of fb−1 With results from the channels ψ(2S) → µ+ µ− and ψ(2S) → J/ψ π+ π− combined, the ratio of branching fractions is measured: Rψ(2S) = B(Λ0b → ψ(2S)pK− ) = (20.70 ± 0.76 ± 0.46 ± 0.37) × 10−2 , B(Λ0b → J/ψ pK− ) RJ/ψ π + π− = B(Λ0b → J/ψ π+ π− pK− ) = (20.86 ± 0.96 ± 1.34) × 10−2 , B(Λ0b → J/ψ pK− ) where the first uncertainty is statistical, the second is systematic and contributions via intermediate resonances are included From measurements of the ratio Rψ(2S) via two different decay modes of the ψ(2S) meson it is determined that B(ψ(2S) → µ+ µ− ) = (2.30 ± 0.20 ± 0.12 ± 0.01) × 10−2 , B(ψ(2S) → J/ψ π+ π− ) where the first uncertainty is statistical, the second is systematic and the third is related to the uncertainty on B(J/ψ → µ+ µ− ) This is the most precise direct measurement of this ratio to date Using Λ0b → ψ(2S)pK− , Λ0b → J/ψ π+ π− pK− and Λ0b → J/ψ pK− decays, the mass of the Λ0b baryon is measured to be M (Λ0b ) = 5619.65 ± 0.17 ± 0.17 MeV/c2 , where the first uncertainty is statistical and the second is systematic Combining this result with previous LHCb measurements that used the channel Λ0b → J/ψ Λ0 [23, 47] gives M (Λ0b ) = 5619.65 ± 0.16 ± 0.14 MeV/c2 , (6.1) where the first uncertainty is statistical and the second is systematic This is the most precise determination of the mass of any b hadron to date 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 – 12 – JHEP05(2016)132 where the first uncertainty is statistical, the second is systematic and the third is related to the uncertainties of the known dielectron J/ψ and ψ(2S) branching fractions and of the branching fraction of the ψ(2S) → J/ψ π+ π− decay The ratio of branching fractions for Λ0b → J/ψ π+ π− pK− and Λ0b → J/ψ pK− is 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) 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V Salustino Guimaraes2 , C Sanchez Mayordomo68 , B Sanmartin Sedes38 , R Santacesaria26 , C Santamarina Rios38 , M Santimaria19 , E Santovetti25,l , A Sarti19,m , C Satriano26,n , 10 11 12 13 14 15 16 17 18 19 20 21 Centro Brasileiro de Pesquisas F´ısicas (CBPF), Rio de Janeiro, Brazil Universidade Federal Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil Center for High Energy Physics, Tsinghua University, Beijing, China LAPP, Universit´e Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France Clermont Universit´e, Universit´e Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France LAL, Universit´e Paris-Sud, CNRS/IN2P3, Orsay, France LPNHE, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, CNRS/IN2P3, Paris, France I Physikalisches Institut, RWTH Aachen University, Aachen, Germany Fakultă at Physik, Technische Universită at Dortmund, Dortmund, Germany Max-Planck-Institut fă ur Kernphysik (MPIK), Heidelberg, Germany Physikalisches Institut, Ruprecht-Karls-Universită at Heidelberg, Heidelberg, Germany School of Physics, University College Dublin, Dublin, Ireland Sezione INFN di Bari, Bari, Italy Sezione INFN di Bologna, Bologna, Italy Sezione INFN di Cagliari, Cagliari, Italy Sezione INFN di Ferrara, Ferrara, Italy Sezione INFN di Firenze, Firenze, Italy Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy Sezione INFN di Genova, Genova, Italy Sezione INFN di Milano Bicocca, Milano, Italy – 19 – JHEP05(2016)132 A Satta25 , D.M Saunders47 , D Savrina32,33 , S Schael9 , M Schiller39 , H Schindler39 , M Schlupp10 , M Schmelling11 , T Schmelzer10 , B Schmidt39 , O Schneider40 , A Schopper39 , M Schubiger40 , M.-H Schune7 , R Schwemmer39 , B Sciascia19 , A Sciubba26,m , A Semennikov32 , A Sergi46 , N Serra41 , J Serrano6 , L Sestini23 , P Seyfert21 , M Shapkin36 , I Shapoval17,44,g , Y Shcheglov31 , T Shears53 , L Shekhtman35 , V Shevchenko66 , A Shires10 , B.G Siddi17 , R Silva Coutinho41 , L Silva de Oliveira2 , G Simi23,s , M Sirendi48 , N Skidmore47 , T Skwarnicki60 , E Smith54 , I.T Smith51 , J Smith48 , M Smith55 , H Snoek42 , M.D Sokoloff58,39 , F.J.P Soler52 , F Soomro40 , D Souza47 , B Souza De Paula2 , B Spaan10 , P Spradlin52 , S Sridharan39 , F Stagni39 , M Stahl12 , S Stahl39 , S Stefkova54 , O Steinkamp41 , O Stenyakin36 , S Stevenson56 , S Stoica30 , S Stone60 , B Storaci41 , S Stracka24,t , M Straticiuc30 , U Straumann41 , L Sun58 , W Sutcliffe54 , K Swientek28 , S Swientek10 , V Syropoulos43 , M Szczekowski29 , T Szumlak28 , S T’Jampens4 , A Tayduganov6 , T Tekampe10 , G Tellarini17,g , F Teubert39 , C Thomas56 , E Thomas39 , J van Tilburg42 , V Tisserand4 , M Tobin40 , J Todd58 , S Tolk43 , L Tomassetti17,g , D Tonelli39 , S Topp-Joergensen56 , E Tournefier4 , S Tourneur40 , K Trabelsi40 , M Traill52 , M.T Tran40 , M Tresch41 , A Trisovic39 , A Tsaregorodtsev6 , P Tsopelas42 , N Tuning42,39 , A Ukleja29 , A Ustyuzhanin67,66 , U Uwer12 , C Vacca16,39,f , V Vagnoni15 , G Valenti15 , A Vallier7 , R Vazquez Gomez19 , P Vazquez Regueiro38 , C V´ azquez Sierra38 , S Vecchi17 , M van Veghel42 , J.J Velthuis47 , M Veltri18,h , G Veneziano40 , M Vesterinen12 , B Viaud7 , D Vieira2 , M Vieites Diaz38 , X Vilasis-Cardona37,p , V Volkov33 , A Vollhardt41 , D Voong47 , A Vorobyev31 , V Vorobyev35 , C Voß65 , J.A de Vries42 , R Waldi65 , C Wallace49 , R Wallace13 , J Walsh24 , J Wang60 , D.R Ward48 , N.K Watson46 , D Websdale54 , A Weiden41 , M Whitehead39 , J Wicht49 , G Wilkinson56,39 , M Wilkinson60 , M Williams39 , M.P Williams46 , M Williams57 , T Williams46 , F.F Wilson50 , J Wimberley59 , J Wishahi10 , W Wislicki29 , M Witek27 , G Wormser7 , S.A Wotton48 , K Wraight52 , S Wright48 , K Wyllie39 , Y Xie63 , Z Xu40 , Z Yang3 , H Yin63 , J Yu63 , X Yuan35 , O Yushchenko36 , M Zangoli15 , M Zavertyaev11,c , L Zhang3 , Y Zhang3 , A Zhelezov12 , Y Zheng62 , A Zhokhov32 , L Zhong3 , V Zhukov9 , S Zucchelli15 22 23 24 25 26 27 28 29 30 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 – 20 – JHEP05(2016)132 31 Sezione INFN di Milano, Milano, Italy Sezione INFN di Padova, Padova, Italy Sezione INFN di Pisa, Pisa, Italy Sezione INFN di Roma Tor Vergata, Roma, Italy Sezione INFN di Roma La Sapienza, Roma, Italy Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krak´ ow, Poland AGH - University of Science and Technology, Faculty of Physics and Applied Computer Science, Krak´ ow, Poland National Center for Nuclear Research (NCBJ), Warsaw, Poland Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia Institute for High Energy Physics (IHEP), Protvino, Russia Universitat de Barcelona, Barcelona, Spain Universidad de Santiago de Compostela, Santiago de Compostela, Spain European Organization for Nuclear Research (CERN), Geneva, Switzerland Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Lausanne, Switzerland Physik-Institut, Universită at Ză urich, Ză urich, Switzerland Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine University of Birmingham, Birmingham, United Kingdom H.H Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom Department of Physics, University of Warwick, Coventry, United Kingdom STFC Rutherford Appleton Laboratory, Didcot, United Kingdom School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom Imperial College London, London, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom Department of Physics, University of Oxford, Oxford, United Kingdom Massachusetts Institute of Technology, Cambridge, MA, United States University of Cincinnati, Cincinnati, OH, United States University of Maryland, College Park, MD, United States Syracuse University, Syracuse, NY, United States Pontif´ıcia Universidade Cat´ olica Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil, associated to2 University of Chinese Academy of Sciences, Beijing, China, associated to Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China, associated to3 Departamento de Fisica , Universidad Nacional de Colombia, Bogota, Colombia, associated to Institut fă ur Physik, Universită at Rostock, Rostock, Germany, associated to 12 National Research Centre Kurchatov Institute, Moscow, Russia, associated to 32 Yandex School of Data Analysis, Moscow, Russia, associated to32 68 69 a b c d e f g i j k l m n o p q r s t u Universidade Federal Triˆ angulo Mineiro (UFTM), Uberaba-MG, Brazil Laboratoire Leprince-Ringuet, Palaiseau, France P.N Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia Universit` a di Bari, Bari, Italy Universit` a di Bologna, Bologna, Italy Universit` a di Cagliari, Cagliari, Italy Universit` a di Ferrara, Ferrara, Italy Universit` a di Urbino, Urbino, Italy Universit` a di Modena e Reggio Emilia, Modena, Italy Universit` a di Genova, Genova, Italy Universit` a di Milano Bicocca, Milano, Italy Universit` a di Roma Tor Vergata, Roma, Italy Universit` a di Roma La Sapienza, Roma, Italy Universit` a della Basilicata, Potenza, Italy AGH - University of Science and Technology, Faculty of Computer Science, Electronics and Telecommunications, Krak´ ow, Poland LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain Hanoi University of Science, Hanoi, Viet Nam Universit` a di Padova, Padova, Italy Universit` a di Pisa, Pisa, Italy Scuola Normale Superiore, Pisa, Italy Universit` a degli Studi di Milano, Milano, Italy † Deceased – 21 – JHEP05(2016)132 h Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain, associated to37 Van Swinderen Institute, University of Groningen, Groningen, The Netherlands, associated to 42 ... interval is chosen to include part of the low -mass tail due to final-state radiation Candidate Λ0b baryons are formed from J/ ψ pK− , ψ(2S)pK− and J/ ψ π+ π− pK− combinations Each candidate is associated... candidates The resulting dimuon candidates are required to have an invariant mass between −5σ and +3σ around the known J/ ψ or ψ(2S) masses [36], where σ is the mass resolution An asymmetric mass. .. bottom baryons allows many measurements of masses, lifetimes and branching fractions, which test the theoretical understanding of weak decays of heavy hadrons in the framework of heavy quark effective