B-hadron Decays Research Articles

Reconstruction of semileptonically decaying beauty hadrons produced in high energy pp collisions

It is well known that in b-hadron decays with a single unreconstructible final state particle, the decay kinematics can be solved up to a quadratic ambiguity, without any knowledge of the b-hadron momentum. We present a method to infer the momenta of b-hadrons produced in hadron collider experiments using information from their reconstructed flight vectors. Our method is strictly agnostic to the decay itself, which implies that it can be validated with control samples of topologically similar decays to fully reconstructible final states. A multivariate regression algorithm based on the flight information provides a b-hadron momentum estimate with a resolution of around 60% which is sufficient to select the correct solution to the quadratic equation in around 70% of cases. This will improve the ability of hadron collider experiments to make differential decay rate measurements with semileptonic b-hadron decays.

LHCb results from the proton-lead collisions

The proton-lead (pPb) data taking during 2013 allows LHCb to expand its core physics program into the regime of heavy ion physics. Recent results of ψ(2S) mesons are shown, including the measurements of the nuclear modification factor and the forward-backward production ratio. The measurements are performed for prompt ψ(2S) mesons, originating from the pPb collision points, and ψ(2S) mesons from b-hadron decays. The results show that the suppression of prompt ψ(2S) mesons are much stronger than that of prompt J/ψ mesons. The results of ψ(2S) from b-hadron decays are consistent with those of J/ψ from b-hadron decays.

Open Heavy Flavor Measurements in Heavy Ion Collisions with CMS

The measurement of heavy flavor production is a powerful tool to study the properties of the high-density QCD medium created in heavy-ion collisions as heavy quarks are sensitive to the transport properties of the medium and may interact with the matter differently than light quarks. Heavy flavor jets, non-prompt J/ψ (J/ψ from B-hadron decay) and fully reconstructed B mesons have been studied in PbPb collisions at 2.76 TeV and pPb collisions at 5.02 TeV with CMS. Recently, the nuclear modification factor of prompt D0 mesons has been measured in PbPb collisions at 2.76 TeV with CMS as a function of both transverse momentum and collision centrality. These studies show that prompt D0 production is suppressed in semi-central to central PbPb collisions and the suppression is smaller at high pT. A comparison with the RAA of charged particle and non-prompt J/ψ hints a hierarchy of suppression as a function of flavor.

Heavy ion physics at LHCb

Proton-ion collisions are probed in the forward acceptance by the LHCb detector in a unique kinematic range, complementary to the other LHC experiments. The production of J/ψ and decaying into two muons is studied in proton-lead collisions at a proton-nucleon centre-of-mass energy . The results are based on a data sample corresponding to an integrated luminosity of 1.6 nb-1. The nuclear modification factor and the forward-backward production ratio are determined for J/ψ and (1S) mesons. Significant suppression of prompt J/ψ production is observed with respect to the production in pp collisions at large rapidity, while the suppression of J/ψ from b-hadron decays is less pronounced. The nuclear modification factor for (1S) mesons in the forward region is found to be similar to those for J/ψ from b-hadron decays. First observation of Z bosons in proton-lead collisions at a proton-nucleon centre-of-mass energy is also reported.

LHCb Topological Trigger Reoptimization

The main b-physics trigger algorithm used by the LHCb experiment is the so- called topological trigger. The topological trigger selects vertices which are a) detached from the primary proton-proton collision and b) compatible with coming from the decay of a b-hadron. In the LHC Run 1, this trigger, which utilized a custom boosted decision tree algorithm, selected a nearly 100% pure sample of b-hadrons with a typical efficiency of 60-70%; its output was used in about 60% of LHCb papers. This talk presents studies carried out to optimize the topological trigger for LHC Run 2. In particular, we have carried out a detailed comparison of various machine learning classifier algorithms, e.g., AdaBoost, MatrixNet and neural networks. The topological trigger algorithm is designed to select all ’interesting” decays of b-hadrons, but cannot be trained on every such decay. Studies have therefore been performed to determine how to optimize the performance of the classification algorithm on decays not used in the training. Methods studied include cascading, ensembling and blending techniques. Furthermore, novel boosting techniques have been implemented that will help reduce systematic uncertainties in Run 2 measurements. We demonstrate that the reoptimized topological trigger is expected to significantly improve on the Run 1 performance for a wide range of b-hadron decays.

LHCb results in proton-nucleus collisions at the LHC

The forward acceptance of the LHCb detector allows it to probe proton-ion collision in a unique kinematic range, complementary to the other LHC experiments. The production of J/Ψ and ϒ-mesons decaying into two muons is studied at the LHCb experiment in proton-lead collisions at a proton-nucleon centre-of-mass energy = 5 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 1.6 nb-1. The nuclear modification factor and the forward-backward production ratio are determined for J/Ψ and ϒ(1S) mesons. Clear suppression of prompt J/Ψ production is observed with respect to the production in pp collisions at large rapidity, while the suppression of J/Ψ from b-hadron decays is less pronounced. The nuclear modification factor for ϒ(1S) mesons in the forward region is found to be similar to those for J/Ψ from b-hadron decays. Furthermore, a first observation of Z bosons in proton-lead collisions is reported.

Production of quarkonium states at the ATLAS experiment

An overview is given of recent measurements of the production of various heavy quarkonium states in pp collisions at = 7 TeV, using the ATLAS detector at the LHC. These include the production cross sections of the C-even states χc1 and χc2 in their radiative decay modes into J/ψγ, and of the radially excited vector state ψ(2S) in its decay mode into J/ψπ+π−. Differential spectra in transverse momentum are measured separately for promptly produced charmonium states, originating from QCD sources, and for non-prompt states from the decays of B-hadrons. Results are compared to other existing measurements and various theoretical models.

Production of J/ψ and ϒ mesons in proton–lead collisions at sNN=5.02 TeV

Abstract The production of J / ψ and ϒ-mesons decaying into the dimuon final state is studied at the LHCb experiment, covering the rapidity intervals 1.5 y 4.0 and − 5.0 y − 2.5 and transverse momentum p T 15 GeV / c , in proton–lead collisions at a proton–nucleon centre-of-mass energy of s N N = 5.02 TeV . The analysis is based on a data sample corresponding to an integrated luminosity of 1.6 nb − 1 . The nuclear modification factor and the forward–backward production ratio are determined for J / ψ and ϒ ( 1 S ) mesons. At large rapidity a clear suppression of prompt J / ψ production is observed with respect to the production in pp collisions, while the suppression of J / ψ from b-hadron decays is less pronounced. The nuclear modification factor for ϒ ( 1 S ) mesons in the forward region is found to be similar to the one for J / ψ from b-hadron decays.

Measurement of χ c1 and χ c2 production with s $$ \sqrt{s} $$ = 7 TeV pp collisions at ATLAS

The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are measured in pp collisions at $$ \sqrt{s} $$ = 7 TeV with the ATLAS detector at the LHC using 4.5 fb−1 of integrated luminosity. The χ c states are reconstructed through the radiative decay χ c → J/ψγ (with J/ψ → μ + μ −) where photons are reconstructed from γ → e + e − conversions. The production rate of the χ c2 state relative to the χ c1 state is measured for prompt and non-prompt χ c as a function of J/ψ transverse momentum. The prompt χ c cross-sections are combined with existing measurements of prompt J/ψ production to derive the fraction of prompt J/ψ produced in feed-down from χ c decays. The fractions of χ c1 and χ c2 produced in b-hadron decays are also measured.

Open and closed heavy-flavour suppression in heavy-ion collisions with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure quarkonia in the dimuon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Quarkonia are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/ψ (from b-hadron decays), prompt J/ψ, inclusive ψ(2S), and the first three ϒ states in PbPb collisions at = 2.76 TeV. A suppression of non-prompt J/ψ, which is sensitive to the in-medium b-quark energy loss, has been measured at relatively high pT (6.5 < pT < 30 GeV/c) in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. For prompt J/ψ in the same kinematic range, a strong, centrality-dependent suppression is observed. Such strong suppression at high pT has previously not been observed at RHIC. At mid-rapidity and high pT, ψ(2S) show an even stronger suppression than J/ψ. Furthermore, CMS has measured the suppression of the three ϒ states, separately, down to pT =0 GeV/c. A clear ordering of the suppression with binding energy is observed, as expected from sequential melting of quarkonium states.

Formula omitted] production and B decay in heavy ion collisions at LHC

In comparison with J/ψ, the excited charmonium state ψ′ is loosely bounded and its yield is dominantly from the B-hadron decay. Based on the transport approach, we study the double ratio of N(ψ′)/N(J/ψ) from A+A collisions to that from p+p collisions at LHC energy. We found that the primordial production in the initial stage and the regeneration in the hot medium are not significant for ψ′ production in heavy ion collisions at LHC and the double ratio in semi-central and central collisions is controlled by the B decay.

Measurement of charmonium production in PbPb collisions at sNN=2.76TeV with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark–gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J / ψ (from b-hadron decays) and prompt J / ψ in PbPb collisions at s N N = 2.76 TeV . For prompt J / ψ with relatively high p T ( p T = 6.5 – 30 GeV/c ), a strong, centrality-dependent suppression is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. In the same kinematic range, a suppression of non-prompt J / ψ , which is sensitive to the in-medium b-quark energy loss, is measured for the first time. Results from the 2010 data taking period are reported and an outlook on the 2011 data analysis will be given.

Production of J/ψ and $ \varUpsilon $ mesons in pp collisions at $ \sqrt{s}=8 $ TeV

Abstract The production of J/ψ and $ \varUpsilon $ mesons in pp collisions at $ \sqrt{s}=8 $ TeV is studied with the LHCb detector. The J/ψ and $ \varUpsilon $ mesons are reconstructed in the μ + μ − decay mode and the signal yields are determined with a fit to the μ + μ − invariant mass distributions. The analysis is performed in the rapidity range 2. 0 &lt; y &lt; 4. 5 and transverse momentum range 0 &lt; p T &lt; 14 (15) GeV/c of the J/ψ $ \left( \varUpsilon \right) $ mesons. The J/ψ and $ \varUpsilon $ production cross-sections and the fraction of J/ψ mesons from b-hadron decays are measured as a function of the meson p T and y.

Investigating charmonium production at LHC with the [formula omitted] final state

We propose to investigate various charmonium states using their common decay channel to pp¯ at LHC. Having the branching ratios for charmonium decaying into the pp¯ final state measured or calculated, we propose to measure the charmonium production rate for both hadroproduction including soft-diffraction and inclusive production from b-hadron decays. We discuss the experimental prospects at LHCb, in particular, those for yet unmeasured ηc and hc.

Measurements of the CKM angleγin tree-dominated decays at LHCb

We review the first measurements of the CKM angle gamma from LHCb. These measurements have been performed with b-hadron decays dominated by b -> u and b -> c tree-level amplitudes, from which gamma can be determined without theoretical uncertainties. Precision is achieved by averaging results from B- -> Dh- (h= K,pi) decays with D -> h+h-, D -> K+pi-, and D -> K+pi-pi+pi-, and D -> K_S^0h+h-. Prospects for these and future measurements of gamma using neutral b-hadron decays are briefly discussed.

Quarkonium production in 2.76 TeV PbPb collisions in CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure hard probes in the di-muon decay channel in the high multiplicity environment of nucleus-nucleus collisions. Such probes are especially relevant for studying the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. CMS has measured the nuclear modification factors of non-prompt J/Ψ (from b-hadron decays), prompt J/Ψ and Y(1S) in PbPb collisions at = 2.76 TeV. Suppression of the excited Y-states has also been measured with respect to the Y(1S). Results from the 2010 data taking period are reported and an outlook on the 2011 data analysis will be given.

Investigating charmonium production at the LHC with thepp¯final state

We propose to investigate various charmonium states using their common decay channel to p pbar at LHC. Having the branching ratios for charmonium decaying into the p pbar final state measured or calculated, we propose to measure the charmonium production rate for both hadroproduction including soft-diffraction and inclusive production from b-hadron decays. We discuss the theoretical impacts in QCD of measuring different charmonium production rates and also the experimental prospects at LHCb, in particular, those for yet unmeasured eta_c and h_c.

Measurement of ψ(2S) meson production in pp collisions at [Formula: see text

The differential cross-section for the inclusive production of ψ(2S) mesons in pp collisions at sqrt{s}=7 mbox{TeV} has been measured with the LHCb detector. The data sample corresponds to an integrated luminosity of 36 pb−1. The ψ(2S) mesons are reconstructed in the decay channels ψ(2S)→μ+μ− and ψ(2S)→J/ψπ+π−, with the J/ψ meson decaying into two muons. Results are presented both for promptly produced ψ(2S) mesons and for those originating from b-hadron decays. In the kinematic range pT(ψ(2S))≤16 GeV/c and 2<y(ψ(2S))≤4.5 we measure where the last uncertainty on the prompt cross-section is due to the unknown ψ(2S) polarization. Recent QCD calculations are found to be in good agreement with our measurements. Combining the present result with the LHCb J/ψ measurements we determine the inclusive branching fraction where the last uncertainty is due to the mathcal{B}(b rightarrow J/psi X), mathcal{B}(J/psirightarrowmu^{+} mu^{-}) and mathcal{B}(psi(2S) rightarrow e^{+} e^{-}) branching fraction uncertainties.

Time alignment of the front end electronics of the LHCb calorimeters

LHCb is the experiment at the Large Hadron Collider at CERN designed for performing studies of CP-symmetry violation and rare decays of B-hadrons. Its calorimeter system allows to trigger on photons and electrons by associating the information from a scintillating pad signing charged particle (SPD), a pre-shower tagging electromagnetic particles (PS), an electromagnetic calorimeter (ECAL) and a hadronic calorimeter (HCAL). We present the principles and procedures for the fine time-alignment throughout the commissioning and the first collision phases. We give a particular emphasis to the choices made in the electronic design of the calorimeters to deal with the signal shape and spill over. Also we summarise the achieved levels of synchronisation.

Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at $\sqrt{s}=7\mbox{~TeV}$ with the ATLAS detector

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb^-1. The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.