Decay properties of b-hadrons with the ATLAS experiment

Recent results of the ATLAS experiment at LHC on decay properties of b-hadrons are reviewed. The timedependent CP asymmetry parameters have been measured in Bs → J/ψφ decays using flavour tagging. The parityviolating decay asymmetry parameter αb and the helicity amplitudes have been measured for the decay Λb → J/ψΛ0. The branching fraction B(B → χc1K) has been measured with χc1 reconstruction in the decay χc1 → J/ψγ. An excited Bc state has been observed through its decays to the ground B ± c state and two oppositely charged pions. The mass and decay of this state are consistent with expectations for the second S -wave state of the Bc meson, B ± c (2S ).


Introduction
The ATLAS detector [1] at the Large Hadron Collider (LHC) consists of several subsystems including the inner detector (ID), the electromagnetic and hadronic calorimeters, and the muon spectrometer (MS). Muon reconstruction at ATLAS makes use of both the ID and the MS, and covers the pseudorapidity range |η| < 2.5 A there-level trigger system allows ATLAS to effectively select events containing single muons with large transverse momentum p T (above ∼ 20 GeV), and events with two muons with the pair invariant mass between 2.5 GeV and 4.3 GeV. In such J/ψ trigger, the minimal muon p T thresholds of 4 − 6 GeV have been used in 2011-2012. During this period ATLAS has accumulated data samples of pp collisions corresponding to luminosities of ∼ 5 fb −1 at √ s = 7 GeV and ∼ 20 fb −1 at √ s = 8 GeV.
In this note, recent ATLAS results on decay properties of b-hadrons are presented. The time-dependent CP asymmetry parameters are measured in B 0 s → J/ψφ Email address: gladilin@mail.cern.ch (Leonid Gladilin ) 1 On behalf of the ATLAS Collaboration decays using flavour tagging [2]. The parity-violating decay asymmetry parameter α b and the helicity amplitudes are measured for the decay Λ 0 b → J/ψΛ 0 [3]. The branching fraction B(B + → χ c1 K + ) is measured with χ c1 reconstruction in the decay χ c1 → J/ψγ [4]. An excited B ± c state is observed through its decays to the ground B ± c state and two oppositely charged pions [5]. Corrections for detector effects are done with high-statistics Monte Carlo (MC) samples. Uncertainties due to uncertainties of simulation of physics processes and detector, MC statistic, luminosity measurement and assumptions of the analyses procedures are included into systematic errors. The measurements are compared to theoretical predictions and to the measurements by other experiments.

CP violation for the B 0 s → J/ψφ decay
The decay B 0 s → J/ψφ is expected to be sensitive to new physics contributions. CP violation in the decay occurs due to interference between direct decays and decays with B 0 s −B 0 s mixing. The CP violating phase φ s is defined as the weak phase difference between the B 0 s −B 0 s mixing amplitude and the b → ccs decay ampli-  tude. The oscillation frequency of B 0 s meson mixing is characterized by the mass difference of the heavy (B H ) and light (B L ) mass eigenstates. In the Standard Model (SM), the phase φ s is small, φ s −0.037±0.002 rad [6]. Another physical quantity involved in B 0 s −B 0 s mixing is the width difference ∆Γ = Γ L −Γ H , which is predicted to be ∆Γ = 0.087 ± 0.021 ps −1 [7]. Although the ∆Γ value is not expected to be significantly affected by physics beyond the SM, its measurement allows theoretical predictions to be tested [8].
In this report, an update [2] to the previous ATLAS measurement [9] of the B 0 s → J/ψφ decay with the addition of flavour tagging, is presented. The analysis uses 4.9 fb −1 of pp data at √ s = 7 TeV. The CP states are separated statistically using an angular analysis of the final-state particles. Flavour tagging is used to reduce the uncertainty of the measured value of φ s . The determination of the initial flavour of neutral B-mesons is inferred using information from the B-meson that is typically produced from the other b-quark in the event.
To study and calibrate the opposite-site tagging, events containing the decays of B ± → J/ψK ± are used. Two methods are used to infer the flavour of the oppositeside b-quark, with varying efficiencies and discriminat-ing powers. The measured charge of a muon from the semileptonic decay of the B meson provides strong separation power. The separation power is enhanced by considering a weighted sum of the charge of the tracks in a cone around the muon. If no muon is present, a weighted sum of the charge of tracks associated with a b-tagged jet [10] provides some separation.
Candidates for B 0 s → J/ψ(µ + µ − )φ(K + K − ) are reconstructed by fitting two oppositely-charged muon tracks and two additional oppositely-charged tracks, assumed to be kaons, to a common vertex. The fit is constrained by fixing the invariant mass calculated from the two muon tracks to the nominal J/ψ mass [11]. The invariant mass of two kaon tracks is required to fall within the interval 1.0085 < m(K + K − ) < 1.0305 GeV. For each selected candidate the proper decay time t is estimated as t = L xy m(B 0 s )/p T , where p T is the reconstructed transverse momentum of the B 0 s meson candidate and m(B 0 s ) is the nominal B 0 s mass [11]. The transverse decay length, L xy , is the displacement in the transverse plane of the B 0 s meson decay vertex with respect to the primary vertex, projected onto the direction of the B 0 s transverse momentum.
An unbinned maximum likelihood fit is performed on the selected candidates to extract the parameters of the The fit uses information about the reconstructed B 0 s mass and its uncertainty, the measured proper decay time t and its uncertainty, the tag probability, and three transversity angles of each candidate. The transversity angles are defined in the J/ψ and φ rest frames [2]. Figure 1 shows the mass and the proper decay time fit projections for the B 0 s → J/ψφ. The number of signal B 0 s mesons extracted from the fit is 22670±150. The signal contribution from B 0 s → J/ψK + K − and B 0 s → J/ψ f 0 is measured to be consistent with zero. The results for φ s and ∆Γ s , assuming ∆Γ s is positive, are  [2] are consistent with those obtained in the untagged analysis [9]. The overall uncertainty on φ s is significantly reduced. 3. Parity violation for the Λ 0 b → J/ψΛ 0 decay The decay asymmetry parameter α enters into the angular distribution of a two-body spin 1/2 particle decay as where P is the polarization of the particle and θ is the angle between the polarization vector and the direction of the decay product in the particle's rest frame. Parity violation is not maximal in hadrons weak decays due to the presence of strongly bound spectator quarks. The spectator quarks effects can be computed for decays of heavy baryons, where the factorization theorem and perturbative QCD (pQCD) are applicable. The decay Λ 0 b → J/ψΛ 0 can be described by four helicity amplitudes: a , where first and second parameters represent the J/ψ and Λ 0 helicities, respectively. A sum of the amplitudes squared is normalised to unity, and the parity-violating decay asymmetry parameter α b is given by [12] The full angular probability density function (PDF) of the decay [12,13,14] connects the helicity amplitudes with functions of the angle θ, and polar and azimuthal angles of the J/ψ and Λ 0 decays with respect to their directions in the Λ 0 b rest frame. The analysis [3]  reconstructed by fitting two oppositely-charged muon tracks and two additional oppositely-charged tracks, assumed to be a proton and a pion, to their respective vertexes. The fit is constrained by fixing the invariant masses calculated from the two muon tracks and two additional tracks to the nominal J/ψ and Λ 0 masses [11], respectively. The combined momentum direction of the refitted Λ 0 track pair is constrained to point to the dimuon vertex. Figure 3 shows the invariant mass distribution of events passing all selection cuts [3], fitted with a threecomponent probability density function consisting of signal, combinatorial background, and residual con- −0.08 (stat) ± 0.06(syst). The statistical uncertainties are calculated by finding the range that satisfies χ 2 − χ 2 min < 1. Figure 4(right) shows χ 2 min as a function of the assumed α b value with the condition that the α b parameter is fixed in the fit.
The large |a − | and |b + | amplitudes correspond to the negative-helicity states of Λ 0 . The measured α b value is consistent with the recent LHCb measurement [16]. The expectations from pQCD [17] (from −0.17 to −0.14) and heavy quark effective theory [18,19] (0.78) disagree with the measured value at a level of ∼ 2.5 standard deviations.
Figure 5(left) shows the m(µ + µ − γK ± ) − m(µ + µ − γ) + m(χ c1 ) distribution of selected B ± decay candidates, where m(χ c1 ) is the nominal χ c1 mass [11]. The candidates are weighted to correct for trigger efficiency, muon reconstruction efficiency, conversion probability and converted-photon reconstruction efficiency. Also shown is the background template derived from MC simulation. The mass distribution for candidate B ± → χ c1 K ± decays is fitted with a B ± signal modelled by a Gaussian PDF where both the mean value and width are free parameters in the fit. The background distribution is modelled with the template derived from MC simulation.
The branching fraction B(B + → χ c1 K + ) is measured using the decay B ± → J/ψK ± as a reference channel. The final states of both channels are identical apart from the photon. The measured branching fraction is B(B + → χ c1 K + ) = (4.9 ± 0.9(stat) ± 0.6(syst)) × 10 4 . This value is in good agreement with the current worldaverage value [11] (dominated by measurements from Belle [20] and BaBar [21]). The precision of this measurement is significantly better than previous measurements from hadron collider experiments [11].

Observation of an excited B ±
c meson state Excited states of the B ± c meson have not previously been observed. The spectrum and properties of the B ± c family are predicted by non-relativistic potential models, perturbative QCD, and lattice calculations [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. The second S -wave state, B ± c (2S ), is predicted to have a mass in the range 6835 − 6917 MeV. Both the 1S and 2S states have pseudoscalar B ± c (0 − ) and vector B * ± c (1 − ) spin states that are predicted to differ in mass by about 20 − 50 MeV. Transitions between the spin states occur through soft photon radiation. The dominant B ( * )± (2S ) decay is expected to be B ( * )± c (2S ) → B ( * )± c (1S )π + π − . The mass differences of 2S and 1S states for the pseudoscalar and vector B ± c mesons can be similar. A search for excited B ± c states is performed using 4.9 fb −1 and 19.2 fb −1 of pp data at √ s = 7 TeV and √ s = 8 TeV, respectively. The B ± c mesons are reconstructed through their decays to J/ψπ ± with J/ψ → µ + µ − . B ± c candidates are reconstructed by fitting two muon tracks from the J/ψ candidate together with a pion candidate track to a common vertex. The invariant mass of the two muons is constrained to the world average of J/ψ mass. The transverse momentum of the B ± c candidates is required to be above 15 GeV for 7 TeV data and above 18 GeV for 8 TeV data. Figure 6 shows invariant mass distributions of the reconstructed B ± c → J/ψπ ± candidates in 7 TeV and 8 TeV data. Both distributions are fitted separately using an extended unbinned maximum likelihood fit, with a Gaus-  The reconstruction of the excited state candidates uses the B ± c ground state candidates within ±3σ of the fitted mass values. The excited state candidates are reconstructed in the decay to the B ± c meson and two oppositely charged tracks associated with the corresponding primary vertex and assumed to be pions. The transverse momentum threshold of the pion candidates is 400 MeV. The three tracks from the B ± c candidate vertex and the two additional tracks from the primary vertex are refitted simultaneously with the following constraints given by the decay topology: the refitted triplet of the B ± c tracks and the pair of additional tracks must intersect in two separate vertexes. THe a The invariant mass of the refitted muon tracks is constrained to the nominal J/ψ mass, and the combined momentum of the refitted B ± c tracks must point to the vertex of the excited candidate. Figure 7 shows the m(B ± c ) − m(B ± ) − 2m(π + ) distributions for the right-charge combinations and for the same (wrong) pion charge combinations in 7 TeV and 8 TeV data. A structure is observed in the mass difference distributions. In order to characterize it, an unbinned maximum likelihood fit to the right-charge combinations is performed. The fit includes a third-order polynomial to model the background and a Gaussian function for the structure. The fit finds the peak at a mass difference value of 288.2 ± 5.1 MeV in the 7 TeV data and 288.4 ± 4.8 MeV in the 8 TeV data. The fit yields 22 ± 6 and 35 ± 13 signal events in the 7 TeV and 8 TeV data, respectively. The peak mass difference values, combined as the error weighted mean, correspond to a mass of 6842 ± 4(stat) ± 5(syst) MeV. Within the uncertainties, the mass of the resonance corresponding to the observed structure is consistent with the predicted mass of the B ± c (2S ) state. The significance of the new structure is evaluated with pseudo-experiments. For the combined 7 TeV and 8 TeV dataset the total significance of the observation is found to be 5.2σ. The local significance of the observation, obtained by fixing the mean value of the signal component, is 5.4σ.

Summary
A measurement of time-dependent CP asymmetry parameters in B 0 s → J/ψ(µ + µ − )φ(K + K − ) decays has been performed using flavour tagging. The results are consistent with those obtained in the previous untagged analysis and significantly reduce the overall uncertainty on the CP violating phase φ s . The results are consistent with the values predicted in the Standard Model.
A measurement of the parity-violating decay asymmetry parameter α b and the helicity amplitudes for the decay Λ 0 b → J/ψ(µ + µ − )Λ 0 (pπ − ) has been performed. The measured α b value is consistent with the recent measurement by LHCb, and does not support available theoretical predictions.
The branching fraction B(B + → χ c1 K + ) = (4.9 ± 0.9(stat) ± 0.6(syst)) × 10 4 has been measured. The measured value agrees well with the world average and is significantly better than previous measurements from hadron collider experiments. An excited B ± c state has been observed through its hadronic transition to the ground state with a significance of 5.2 standard deviations. The mass of the observed state is 6842 ± 4(stat) ± 5(syst) MeV. The mass and decay of this state are consistent with expectations for the second S -wave state of the B ± c meson, B ± c (2S ).