B-hadron Decays Research Articles

Production of pentaquarks with hidden charm and double strangeness in Ξb and Ωb decays

Recently, several pentaquark states Pcss, with global flavor c¯cssn, have been predicted within a theoretical framework based on unitary coupled channels. We study theoretically the feasibility to observe the Pcss with I(JP)=12(12−) in the decays Ξb0→ηηcΞ0 and Ωb−→K−ηcΞ0. Indeed, within the model, the ηcΞ0 channel is the lowest mass pseudoscalar-baryon channel to which this pentaquark state couples, thus we can expect to observe its signal in the ηcΞ0 invariant mass distribution of the mentioned decays. We identify the dominant weak decay processes and then implement the hadronization into the different meson-baryon channels in the final state, linked by flavor symmetry. The dominant meson-baryon final state interaction is then implemented to generate the full amplitude, implicitly accounting for the dynamical emergence of the pentaquark states. We obtain a clear Breit-Wigner-like resonant signal in the spectrum of the Ωb− decay, exceeding that in the Ξb0 decay by two to three orders of magnitude. In the case of the latter decay, the resonant state would manifest as a significant dip in the spectrum. We study the feasibility of searching for these b-hadron decay modes and analyzing their resonant components using the current and future data samples from the LHCb experiment. Published by the American Physical Society 2024

Bilinear R-parity violating supersymmetry under the light of neutrino oscillation, Higgs and flavor data

In this work, we explore a well motivated beyond the Standard Model scenario, namely, R-parity violating Supersymmetry, in the context of light neutrino masses and mixing. We assume that the R-parity is only broken by the lepton number violating bilinear term. We try to fit two non-zero neutrino mass square differences and three mixing angle values obtained from the global χ2 analysis of neutrino oscillation data. We have also taken into account the updated data of the standard model (SM) Higgs mass and its coupling strengths with other SM particles from LHC Run-II along with low energy flavor violating constraints like rare b-hadron decays. We have used a Markov Chain Monte Carlo (MCMC) analysis to constrain the new physics parameter space. While doing so, we ensure that all the existing collider constraints are duly taken into account. Through our analysis, we have derived the most stringent constraints possible to date with existing data on the 9 bilinear R-parity violating parameters along with μ and tan β. We further explore the possibility of explaining the anomalous muon (g - 2) measurement staying within the parameter space allowed by neutrino, Higgs and flavor data while satisfying the collider constraints as well. We find that there still remains a small sub-TeV parameter space where the required excess can be obtained.

Machine Learning Approach to Analyze the Heavy Quark Diffusion Coefficient in Relativistic Heavy Ion Collisions.

The diffusion coefficient of heavy quarks in a deconfined medium is examined in this research using a deep convolutional neural network (CNN) that is trained with data from relativistic heavy ion collisions involving heavy flavor hadrons. The CNN is trained using observables such as the nuclear modification factor RAA and the elliptic flow v2 of non-prompt J/ψ from the B-hadron decay in different centralities, where B meson evolutions are calculated using the Langevin equation and the instantaneous coalescence model. The CNN outputs the parameters, thereby characterizing the temperature and momentum dependence of the heavy quark diffusion coefficient. By inputting the experimental data of the non-prompt J/ψ(RAA,v2) from various collision centralities into multiple channels of a well-trained network, we derive the values of the diffusion coefficient parameters. Additionally, we evaluate the uncertainty in determining the diffusion coefficient by taking into account the uncertainties present in the experimental data (RAA,v2), which serve as inputs to the deep neural network.

Measurement of the top-quark mass using a leptonic invariant mass in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

A measurement of the top-quark mass (mt) in the toverline{t} → lepton + jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in the top-quark decay chain. The distribution of the invariant mass mℓμ of the lepton, ℓ (with ℓ = e, μ), from the W-boson decay and the muon, μ, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract mt. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb−1 of sqrt{s} = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is mt = 174.41 ± 0.39 (stat.) ± 0.66 (syst.) ± 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup.

Energy-peak based method to measure top quark mass via B-hadron decay lengths

We develop a method for the determination of the top quark mass using the distribution of the decay length of the B-hadrons originating from its decay. This technique is based on our earlier observation regarding the location of the peak of the b quark energy distribution. Such “energy-peak” methods enjoy a greater degree of model-independence with respect to the kinematics of top quark production compared to earlier proposals. The CMS experiment has implemented the energy-peak method using associated b-jet energy as an approximation for b quark energy. The new method uses B-hadron decay lengths, which are related to b quark energies by convolution. The advantage of the new decay length method is that it can be applied in a way that evades jet-energy scale (JES) uncertainties. Indeed, CMS has measured the top quark mass using B-hadron decay lengths, but they did not incorporate the energy-peak method. Therefore, mismodeling of top quark transverse momentum remains a large uncertainty in their result. We demonstrate that, using energy-peak methods, this systematic uncertainty can become negligible. We show that with the current LHC data set, a sub-GeV statistical uncertainty on the top quark mass can be attained with this method. To achieve a comparable systematic uncertainty as is true for many methods based on exclusive or semi-inclusive observables using hadrons, we find that the quark-hadron transition needs to be described significantly better than is the case with current fragmentation functions and hadronization models.

A Light Shed on Lepton Flavor Universality in B Decays

Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level b-hadron decays, b→clνl and b→sl+l− along with experimental environments. We revisit the world averages for RD(D*), RK(K*), RJ/ψ, and Rηc, for the semileptonic transitions and provide results within the framework of the relativistic independent quark model in addition to the results from model-independent studies. If the ongoing evaluation of the data of LHC Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel is likely to reach 5 σ. A confirmation of these measurements would soon turn out to be the first remarkable observation of physics beyond the Standard Model, providing a wider outlook on the understanding of new physics.

Charmless b-hadron decays at LHCb

Measurements of CP asymmetries in charmless two-body B-meson decays can provide stringent tests of the Standard Model. In multibody decays, short and long-distance dynamics along with a sizeable effective weak phase in the interference between tree and penguin amplitudes can lead to a rich structure of CP violation as a function of the phase space. We present here the latest studies of CP violation in charmless b-hadron decays, in particular those with baryons in the initial and final states.

Spectroscopy of the excited Ds mesons

The discoveries of the Ds0*(2317)+ and Ds1(2460)+ mesons challenge our understanding of quantum chromodynamics. After almost 20 years, the nature of these hadrons is still subject of debate and many models have been proposed to explain their unexpected masses: standard charmed-strange mesons, DK molecules and tetraquarks. The LHCb experiment has studied the production of excited Ds+ meson in prompt proton-proton collisions and from b-hadron decays. Precise measurement of their properties and observation of new Ds+ states have been reported. The latest results on the spectroscopy of the charmed-strange mesons and the prospect to investigate their nature will be presented.

Observation of Multiplicity Dependent Prompt χ_{c1}(3872) and ψ(2S) Production in pp Collisions.

The production of χ_{c1}(3872) and ψ(2S) hadrons is studied as a function of charged particle multiplicity in pp collisions at a center-of-mass energy of 8TeV, corresponding to an integrated luminosity of 2 fb^{-1}. For both states, the fraction that is produced promptly at the collision vertex is found to decrease as charged particle multiplicity increases. The ratio of χ_{c1}(3872) to ψ(2S) cross sections for promptly produced particles is also found to decrease with multiplicity, while no significant dependence on multiplicity is observed for the equivalent ratio of particles produced away from the collision vertex in b-hadron decays. This behavior is consistent with a calculation that models the χ_{c1}(3872) structure as a compact tetraquark. Comparisons with model calculations and implications for the binding energy of the χ_{c1}(3872) state are discussed.

Study of the lineshape of the χc1(3872) state

A study of the lineshape of the χc1(3872) state is made using a data sample corresponding to an integrated luminosity of 3 fb−1 collected in pp collisions at center-of-mass energies of 7 and 8 TeV with the LHCb detector. Candidate χc1(3872) and ψ(2S) mesons from b-hadron decays are selected in the J/ψπ+π− decay mode. Describing the lineshape with a Breit-Wigner function, the mass splitting between the χc1(3872) and ψ(2S) states, Δm, and the width of the χc1(3872) state, ΓBW, are determined to be Δm=185.598±0.067±0.068 MeV,ΓBW=1.39±0.24±0.10 MeV, where the first uncertainty is statistical and the second systematic. Using a Flatté-inspired model, the mode and full width at half maximum of the lineshape are determined to be mode=3871.69−0.04−0.13+0.00+0.05 MeV,FWHM=0.22−0.06−0.13+0.07+0.11 MeV. An investigation of the analytic structure of the Flatté amplitude reveals a pole structure, which is compatible with a quasibound D0D¯*0 state but a quasivirtual state is still allowed at the level of 2 standard deviations.4 MoreReceived 29 May 2020Accepted 21 September 2020DOI:https://doi.org/10.1103/PhysRevD.102.092005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2020 CERN, for the LHCb CollaborationPhysics Subject Headings (PhySH)Research AreasHadron-hadron interactionsHadronic decaysPropertiesMassTechniquesHadron collidersPrecision measurementsParticles & Fields

Tracking performance for long-lived particles at LHCb

The LHCb experiment is dedicated to the study of the c- and b-hadron decays, including long-lived particles such as Ks and strange baryons . These kind of particles are difficult to reconstruct by the LHCb tracking system since they escape detection in the first tracker. A new method to evaluate the performance of the different tracking algorithms for long-lived particles using real data samples has been developed. Special emphasis is laid on particles hitting only part of the tracking system of the new LHCb upgrade detector.

Measurement of {psi {(2S)}} production cross-sections in proton-proton collisions at sqrt{s} =7 and 13text { TeV}

The cross-sections of psi {(2S)} meson production in proton-proton collisions at sqrt{s} =13text { TeV} are measured with a data sample collected by the LHCb detector corresponding to an integrated luminosity of 275text { pb} ^{-1} . The production cross-sections for prompt psi {(2S)} mesons and those for psi {(2S)} mesons from b-hadron decays ({psi {(2S)}} text{-from- }{b} ) are determined as functions of the transverse momentum, p_{mathrm {T}}, and the rapidity, y, of the psi {(2S)} meson in the kinematic range 2<p_{mathrm {T}} <20text { GeV/}c and 2.0<y<4.5. The production cross-sections integrated over this kinematic region are σ(promptψ(2S),13TeV)=1.430±0.005(stat)±0.099(syst)μb,σ(ψ(2S)-from-b,13TeV)=0.426±0.002(stat)±0.030(syst)μb.\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$\\begin{aligned} \\begin{aligned}&\\sigma (\\text{ prompt } {\\psi {(2S)}},13\\text { TeV})\\\\&\\quad =1.430\\pm 0.005\\text { (stat)} \\pm 0.099\\text { (syst)} \\,\\upmu \\text {b} ,\\\\&\\sigma ({\\psi {(2S)}} \\text{-from- }{b},13\\text { TeV})\\\\&\\quad =0.426\\pm 0.002\\text { (stat)} \\pm 0.030\\text { (syst)} \\,\\upmu \\text {b} . \\end{aligned} \\end{aligned}$$\\end{document}A new measurement of psi {(2S)} production cross-sections in pp collisions at sqrt{s}=7text { TeV} is also performed using data collected in 2011, corresponding to an integrated luminosity of 614text { pb} ^{-1} . The integrated production cross-sections in the kinematic range 3.5<p_{mathrm {T}} <14text { GeV/}c and 2.0<y<4.5 are σ(promptψ(2S),7TeV)=0.471±0.001(stat)±0.025(syst)μb,σ(ψ(2S)-from-b,7TeV)=0.126±0.001(stat)±0.008(syst)μb.\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$\\begin{aligned} \\begin{aligned}&\\sigma (\\text{ prompt } {\\psi {(2S)}},7\\text { TeV})\\\\&\\quad =0.471\\pm 0.001\\text { (stat)} \\pm 0.025\\text { (syst)} \\,\\upmu \\text {b} ,\\\\&\\sigma ({\\psi {(2S)}} \\text{-from- }{b},7\\text { TeV})\\\\&\\quad =0.126\\pm 0.001\\text { (stat)} \\pm 0.008\\text { (syst)} \\,\\upmu \\text {b} . \\end{aligned} \\end{aligned}$$\\end{document}All results show reasonable agreement with theoretical calculations.

Study of the Rare B-meson Decays with the ATLAS Experiment

The searching for New Physics beyond the Standard Model of particles is one of the main aims of the current particle experiments. Rare b-hadron decays, occurring at the first level through loop Feynman diagrams, are strongly sensitive to the possible existence of new particles. Recent analyses from the LHCb experiment, measuring decay angles distributions in rare B-meson decay into a kaon K0* and two muons, hints a deviation from the Standard Model prediction. The ATLAS experiment works on a similar analysis of the decay Bd0 → K*μ−μ+ in order to confirm this deviation. The new particles’ existence could be observed in the deviations from the predicted decay’s angular distribution. This work provides input for the future analysis of this rare decay in the data-taking period of Run 2. To be more specific, observing of the fine deviations requires the precise description of the measured data and the Standard Model’s predictions. The improvement to the Run 1 analysis was achieved by an expansion of the fit function in a series of spherical harmonic functions. Next, the effect of the remaining background events in the signal dataset was repressed by adding masses mB and mK* to the final fit. That required to know functions’ prescriptions of the mB and mK* distributions for the signal decay and the expected background hence looking for their prescriptions was a part of this work too. The invariant B mass was described by the Gauss distribution with a per-event error. The per-event fit has to take into account the so-called Punzi effect in some cases which was also discussed here.

CP violation and material interaction of neutral kaons in measurements of the CKM angle \u03b3 using B\xb1 \u2192 DK\xb1 decays where D \u2192 Ks0\u03c0+\u03c0\u2212

As measurements of the CKM angle γ in decays of b-hadrons become increasingly precise, it is important to consider the impact of processes that affect secondary and tertiary decay products and can contribute to the observed CP violation. The golden decay mode used to measure γ is B → DK, where D → Ks0π+π−. Due to the presence of a Ks0 meson in the final state, γ measurements based on this mode are affected by neutral kaon CP violation and matter regeneration, and this study examines the potential size of the impact. Previous studies determine the impact of kaon CP violation to be a few degrees in size, but make simplifying assumptions about how experimental measurements are implemented. These assumptions are lifted in the present study, and the matter regeneration effect is included for the first time. The results are presented within the context of γ measurements to be made using the LHCb and Belle II detectors. It is found that the expected biases due to ignoring the effects of CP violation and matter regeneration in neutral kaons is small and less than 0.5° degrees in all experimental scenarios considered.

Tests of Lepton Flavour Universality at LHCb

In the Standard Model of particle physics the three charged leptons are identical copies of each other, apart from mass differences, and the electroweak coupling of the gauge bosons to leptons is independent of the lepton flavour. This prediction is called lepton flavour universality (LFU) and is well tested. In tree level decays, any violation of LFU would be a clear sign of physics beyond the Standard Model. Experimental tests of LFU in semileptonic decays of b-hadrons or rare b decays are highly sensitive to New Physics particles which preferentially couple to the 2nd and 3rd generations of leptons. Recent results from LHCb on lepton flavour universality in transitions and rare decays are discussed. The results are based on 3 fb−1 of proton-proton collisions collected at centre of mass energies and 8 TeV.

Rareb-hadron decays as probe of new physics

The unexpected absence of unambiguous signals of New Physics (NP) at the TeV scale at the Large Hadron Collider (LHC) puts today flavor physics at the forefront. In particular, rare decays of b-hadrons represent a unique probe to challenge the Standard Model (SM) paradigm and test models of NP at a scale much higher than that accessible by direct searches. This article reviews the status of the field.

Measurement of Z → bb̄ cross section and search for a Higgslike particle produced in association with b quarks at CDF

We present a measurement of the Z → bb̄ production cross section in pp collisions at √s = 1.96 TeV . We use a data set of 5.46 fb-1 collected by the CDF experiment at the Tevatron collider during Run II using a dedicated trigger path which required a displaced vertex compatible with a b-hadron decay. A data-driven procedure is applied to estimate the dijet mass spectrum of the non-resonant multijet background. Using a similar strategy we set one of the most stringent upper limits on the production of a Higgs-like particle in association with b quarks. We also set a limit on the inclusive SM H → bb̄.

Measurement of R(D* ) with hadronic τ decays

Lepton universality, described in the Standard Model (SM), predicts equal coupling between gauge bosons and the three lepton families. SM extensions give additional interactions, implying in some cases a stronger coupling with the third generation of leptons. Semileptonic decays of b-hadrons provide a sensitive probe to such New Physics effects. The presence of additional charged Higgs bosons, required by such SM extensions, can have significant effect on the semileptonic decay rate of B 0 → D*τν. A probe of new physics effects is the measurement of the quantities: and . The combination of experimental measurements performed by BaBar, Belle and LHCb observing the channel where the τ decays in leptons, gives a deviation from the SM prediction of about 4 σ. It is therefore important to perform additional measurements in this sector in order to improve the precision and confirm or disprove this deviation. Another possibility is to perform this measurement using the channel where the semileptonic τ decays in 3 pions. This in LHCb allows to have e better reconstruction of vertices and other kinematic variables. Results obtained by LHCb on B 0 → D*τν decays, where the τ decays hadronically, are reported.

Lepton polarization asymmetries in rare semi-tauonic b rightarrow s exclusive decays at FCC-ee

We consider measurements of exclusive rare semi-tauonic b-hadron decays, mediated by the b rightarrow s tau ^+ tau ^- transition, at a future high-energy circular electron–positron collider (FCC-ee). We argue that the high boosts of b-hadrons originating from on-shell Z boson decays allow for a full reconstruction of the decay kinematics in hadronic tau decay modes (up to discrete ambiguities). This, together with the potentially large statistics of Zrightarrow bbar{b}, opens the door for the experimental determination of tau polarizations in these rare b-hadron decays. In the light of the current experimental situation on lepton flavor universality in rare semileptonic B decays, we discuss the complementary short-distance physics information carried by the tau polarizations and suggest suitable theoretically clean observables in the form of single- and double-tau polarization asymmetries.

Measurements of charge and CP asymmetries in b-hadron decays using top-quark events collected by the ATLAS detector in pp collisions at sqrt{s}=8 TeV

Same- and opposite-sign charge asymmetries are measured in lepton+jets toverline{t} events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb−1 from proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.