Hadronic Decays Research Articles

Topological amplitudes of charmed baryon decays in the SU(3)F limit

Charmed baryon decay plays an important role in studying the weak and strong interactions. Topological diagram is an intuitive tool for analyzing the dynamics of heavy hadron decays. In this work, we investigate the topological diagrams of charmed baryon antitriplet (Bc3¯) decays into a light baryon octet (B8) and a light meson (M). A one-to-one mapping between the topological diagram and the invariant tensor is established. The topological diagrams of the Bc3¯→B8SM modes (where B8S and B8A are the q1↔q2 symmetric and antisymmetric octets) and the diagrams with a quark loop are presented for the first time. The completeness of topologies is confirmed by permutation. The linear relations of topologies are obtained by deriving the relation between the topological amplitudes constructed by the third- and second-rank octet tensors. It is found the topologies contributing to the Bc3¯→B8SM modes can be determined by the topologies contributing to the Bc3¯→B8AM modes, and vice versa. The equations of SU(3) irreducible amplitudes decomposed by topologies are derived through two different intermediate amplitudes. However, the inverse solution does not exist since the number of topologies exceeds number of SU(3) irreducible amplitudes. Applying this framework to the Standard Model, it is found there are thirteen independent SU(3) irreducible amplitudes contributing to the Bc3¯→B8M decays. Among these, four amplitudes associated with three-dimensional operators are significant for CP asymmetries. Considering the suppressions due to small Cabibbo-Kobayashi-Maskawa matrix elements and the Körner-Pati-Woo theorem, the branching fractions of charmed baryon decays are dominated by five SU(3) irreducible amplitudes in the SU(3)F limit. Quark-loop diagrams could enhance the U-spin breaking effects and increase the branching fraction difference of two decay channels. Systematic measurements of branching fractions of the singly Cabibbo-suppressed modes could help identify promising channels for searching for CP asymmetries in the charmed baryon sector. Published by the American Physical Society 2024

Test of lepton flavor universality with a measurement of R(D*) using hadronic B tagging at the Belle II experiment

The ratio of branching fractions R(D*)=B(B¯→D*τ−ν¯τ)/B(B¯→D*ℓ−ν¯ℓ), where ℓ is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of 189 fb−1 at the SuperKEKB asymmetric-energy e+e− collider. Data is collected at the ϒ(4S) resonance, and one B meson in the ϒ(4S)→BB¯ decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as B¯→D*τ−ν¯τ using leptonic τ decays. The normalization decay, B¯→D*ℓ−ν¯ℓ, produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields R(D*)=0.262−0.039+0.041(stat)−0.032+0.035(syst). This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society 2024

QCD chemistry: Remarks on diquarks

I review the status of “good diquarks” in light-quark hadronic structure and argue that the heavy-light diquarks play no role in the structure of b-containing hadrons (including exotics), and their role in c-containing hadrons is likely to be minimal if at all. I explain that a crucial phenomenological proof can be obtained from weak decays of b-containing hadrons, such Λb.

Model-independent predictions for decays of double-heavy hadrons into pairs of heavy hadrons

Double-heavy hadrons can decay into pairs of heavy hadrons through transitions from confining Born-Oppenheimer potentials to heavy-hadron-pair potentials with the same Born-Oppenheimer quantum numbers. The states of the double-heavy hadron are constrained by a Born-Oppenheimer exclusion principle from the identical heavy quarks. The states of a pair of identical heavy hadrons are constrained by exclusion principles from identical particles. The transitions are also constrained by conservation of angular momentum and parity. From these constraints, we derive model-independent selection rules for decays of double-heavy hadrons into pairs of heavy hadrons. The coupling potentials are expressed as sums of products of Born-Oppenheimer transition amplitudes and angular-momentum coefficients. If there is a single dominant Born-Oppenheimer transition amplitude, it factors out of the coupling potentials between double-heavy hadrons in the same Born-Oppenheimer multiplet and pairs of heavy hadrons in specific heavy-quark-spin-symmetry multiplets and out of the corresponding partial decay rates. As examples, we discuss the Born-Oppenheimer potentials and multiplets for conventional double-heavy baryons and for double-heavy tetraquark mesons. We also discuss the relative partial decay rates for conventional double-heavy baryons into pairs of heavy hadrons. Published by the American Physical Society 2024

Search for production of a single vectorlike quark decaying to tH or tZ in the all-hadronic final state in pp collisions at s=13 TeV

A search for electroweak production of a single vectorlike T quark in association with a bottom (b) quark in the all-hadronic decay channel is presented. This search uses proton-proton collision data at s=13 TeV collected by the CMS experiment at the CERN LHC during 2016–2018, corresponding to an integrated luminosity of 138 fb−1. The T quark is assumed to have charge 2/3 and decay to a top (t) quark and a Higgs (H) or Z boson. Hadronic decays of the t quark and the H or Z boson are reconstructed from the kinematic properties of jets, including those containing b hadrons. No deviation from the standard model prediction is observed in the reconstructed tH and tZ invariant mass distributions. The 95% confidence level upper limits on the product of the production cross section and branching fraction of a T quark produced in association with a b quark and decaying via tH or tZ range from 1260 to 68 fb for T quark masses of 600–1200 GeV. © 2024 CERN, for the CMS Collaboration 2024 CERN

Search for the Exclusive W Boson Hadronic Decays W^{±}→π^{±}γ, W^{±}→K^{±}γ and W^{±}→ρ^{±}γ with the ATLAS Detector.

A search for the exclusive hadronic decays W^{±}→π^{±}γ, W^{±}→K^{±}γ, and W^{±}→ρ^{±}γ is performed using up to 140 fb^{-1} of proton-proton collisions recorded with the ATLAS detector at a center-of-mass energy of sqrt[s]=13 TeV. If observed, these rare processes would provide a unique test bench for the quantum chromodynamics factorization formalism used to calculate cross sections at colliders. Additionally, at future colliders, these decays could offer a new way to measure the W boson mass through fully reconstructed decay products. The search results in the most stringent upper limits to date on the branching fractions B(W^{±}→π^{±}γ)<1.9×10^{-6}, B(W^{±}→K^{±}γ)<1.7×10^{-6}, B(W^{±}→ρ^{±}γ)<5.2×10^{-6} at 95% confidence level.

Search for ηc(2S)→2(π+π−) and improved measurement of χcJ→2(π+π−)

We search for the hadronic decay ηc(2S)→2(π+π−) in the ψ(3686)→γηc(2S) radiative decay using (27.12±0.14)×108 ψ(3686) events collected by the BESIII detector at the BEPCII collider. No significant signal is found, and the upper limit of B[ψ(3686)→γηc(2S)]B[ηc(2S)→2(π+π−)] is determined to be 1.43×10−6 at the 90% confidence level. Using ψ(3686)→γχcJ transitions, we also measure the branching fractions of B[χcJ(J=0,1,2)→2(π+π−)], which are B[χc0→2(π+π−)]=(2.127±0.002(stat)±0.101(syst))%, B[χc1→2(π+π−)]=(0.685±0.001(stat)±0.031(syst))%, and B[χc2→2(π+π−)]=(1.153±0.001(stat)±0.063(syst))%. Published by the American Physical Society 2024

Search for Higgs boson pair production with one associated vector boson in proton-proton collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV

A search for Higgs boson pair (HH) production in association with a vector boson V (W or Z boson) is presented. The search is based on proton-proton collision data at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb−1. Both hadronic and leptonic decays of V bosons are used. The leptons considered are electrons, muons, and neutrinos. The HH production is searched for in the bb¯bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{b}\\overline{\ extrm{b}}\ extrm{b}\\overline{\ extrm{b}} $$\\end{document} decay channel. An observed (expected) upper limit at 95% confidence level of VHH production cross section is set at 294 (124) times the standard model prediction. Constraints are also set on the modifiers of the Higgs boson trilinear self-coupling, kλ, assuming k2V = 1, and vice versa on the coupling of two Higgs bosons with two vector bosons, k2V. The observed (expected) 95% confidence intervals of these coupling modifiers are −37.7 < kλ < 37.2 (−30.1 < kλ < 28.9) and −12.2 < k2V < 13.5 (−7.2 < k2V < 8.9), respectively.

Precision studies in charm and new physics searches at BESIII

The investigation of charm physics plays an important role in checking the Standard Model’s description of strong and weak interactions, validating its predictions for particle physics, and exploring new physics beyond the Standard Model. Using a large dataset from [Formula: see text] annihilation, BESIII has conducted comprehensive and extensive studies in the areas of hadronic, leptonic and semileptonic decays. This proceeding presents an overview of the recent researches and discoveries by BESIII on decays of charmed baryons, charmed mesons and charmonium states, highlighting their significance in advancing the understanding of new physics, experimental determination of physical parameters and improvement of measurement precision.

Search for a resonance decaying to a W boson and a photon in proton-proton collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV using leptonic W boson decays

A search for a new charged particle X with mass between 0.3 and 2.0 TeV decaying to a W boson and a photon is presented, using proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb−1. Particle X has electric charge ±1 and is assumed to have spin 0. The search is performed using the electron and muon decays of the W boson. No significant excess above the predicted background is observed. The upper limit at 95% confidence level on the product of the production cross section of the X and its branching fraction to a W boson and a photon is found to be 94 (137) fb for a 0.3 TeV resonance and 0.75 (0.81) fb for a 2.0 TeV resonance, for an X width-to-mass ratio of 0.01% (5%). This search presents the most stringent constraints to date on the existence of such resonances across the probed mass range. A statistical combination with an earlier study based on the hadronic decay mode of the W boson is also performed, and the upper limit at 95% confidence level for a 2.0 TeV resonance is reduced to 0.50 (0.63) fb for an X width-to-mass ratio of 0.01% (5%).

Unveiling the secrets of new physics through top quark tagging

AbstractThe ubiquity of top-rich final states in the context of beyond the Standard Model (BSM) searches has led to their status as extensively studied signatures at the LHC. Over the past decade, numerous endeavours have been undertaken in the literature to develop methods for efficiently distinguishing boosted top quark jets from QCD jets. Although cut-based strategies for boosted top tagging, which rely on substructure information from fat jets resulting from the hadronic decay of boosted top quarks, were introduced in the literature as early as 2008, recent years have witnessed a surge in the utilization of machine learning-based approaches for the classification of top-jets from QCD jets. The review focuses on the present status of boosted top tagging and its application for BSM searchers.

Search for pair-produced vectorlike quarks coupling to light quarks in the lepton plus jets final state using 13 TeV pp collisions with the ATLAS detector

A search is presented for the pair production of heavy vectorlike quarks (VLQs) that each decay into a W boson and a light quark. This study focuses on events where one W boson decays into leptons and the other into hadrons. The search analyzed 140 fb−1 of pp collision data with s=13 TeV, recorded by the ATLAS detector from 2015 to 2018 during run 2 of the Large Hadron Collider. The final state is characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, multiple small-radius jets, and a single large-radius jet identified as originating from the hadronic decay of a boosted W boson. With higher center-of-mass energy and integrated luminosity than in the run 1 search, and improved analysis tools, this analysis excludes VLQs (Q) with masses below 1530 GeV at 95% confidence level for the branching ratio B(Q→Wq)=1, an improvement of 840 GeV on the previous ATLAS limit. © 2024 CERN, for the ATLAS Collaboration 2024 CERN

PAIReD jet: A multi-pronged resonance tagging strategy across all Lorentz boosts

We propose a new approach of jet-based event reconstruction that aims to optimally exploit correlations between the products of a hadronic multi-pronged decay across all Lorentz boost regimes. The new approach utilizes clustered small-radius jets as seeds to define unconventional jets, referred to as PAIReD jets. The constituents of these jets are subsequently used as inputs to machine learning-based algorithms to identify the flavor content of the jet. We demonstrate that this approach achieves higher efficiencies in the reconstruction of signal events containing heavy-flavor jets compared to other event reconstruction strategies at all Lorentz boost regimes. Classifiers trained on PAIReD jets also have significantly better background rejections compared to those based on traditional event reconstruction approaches using small-radius jets at low Lorentz boost regimes. The combined effect of a higher signal reconstruction efficiency and better classification performance results in a two to four times stronger rejection of light-flavor jets compared to conventional strategies at low Lorentz-boosts, and rejection rates similar to classifiers based on large-radius multi-pronged jets at high Lorentz-boost regimes.

The phase space distance between collider events

How can one fully harness the power of physics encoded in relativistic N-body phase space? Topologically, phase space is isomorphic to the product space of a simplex and a hypersphere and can be equipped with explicit coordinates and a Riemannian metric. This natural structure that scaffolds the space on which all collider physics events live opens up new directions for machine learning applications and implementation. Here we present a detailed construction of the phase space manifold and its differential line element, identifying particle ordering prescriptions that ensure that the metric satisfies necessary properties. We apply the phase space metric to several binary classification tasks, including discrimination of high-multiplicity resonance decays or boosted hadronic decays of electroweak bosons from QCD processes, and demonstrate powerful performance on simulated data. Our work demonstrates the many benefits of promoting phase space from merely a background on which calculations take place to being geometrically entwined with a theory’s dynamics.

Observation of γγ→ττ in proton–proton collisions and limits on the anomalous electromagnetic moments of the τ lepton

The production of a pair of τ leptons via photon-photon fusion,γγ→ττ, is observed for the first time in proton-proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb-1. Events with a pair of τ leptons produced via photon-photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section ofγγ→ττisσobsfid=12.4-3.1+3.8fb. Constraints are set on the contributions to the anomalous magnetic moment (aτ) and electric dipole moments (dτ) of the τ lepton originating from potential effects of new physics on theγττvertex:aτ=0.0009-0.0031+0.0032and|dτ|<2.9×10-17ecm(95% confidence level), consistent with the standard model.

Quantum corrections to the decay law in flight

The deviation of the decay law from the exponential is a well known effect of quantum mechanics. Here we analyze the relativistic survival probabilities, S(t,p), where p is the momentum of the decaying particle and provide analytical expressions for S(t,p) in the exponential (E) as well as the nonexponential (NE) regions at small and large times. Under minimal assumptions on the spectral density function, analytical expressions for the critical times of transition from the NE to the E at small times and the E to NE at large times are derived. The dependence of the decay law on the relativistic Lorentz factor, γ=1/1−v2/c2, reveals several interesting features. In the short time regime of the decay law, the critical time, τst, shows a steady increase with γ, thus implying a larger NE region for particles decaying in flight. Comparing S(t,p) with the well known time dilation formula, e−Γt/γ, in the exponential region, an expression for the critical γ where S(t,p) deviates most from e−Γt/γ is presented. This is a purely quantum correction. Under particular conditions on the resonance parameters, there also exists a critical γ at large times which decides if the NE region shifts backward or forward in time as compared to that for a particle at rest. All the above analytical results are supported by calculations involving realistic decays of hadrons and leptons.

Measurements of the branching fractions of the P -wave charmonium spin-singlet state hc(P11)→h+h−π0/η

Based on (2712.4±14.3)×106 ψ(3686) events, we investigate four hadronic decay modes of the P-wave charmonium spin-singlet state hc(1P1)→h+h−π0/η (h=π or K) via the process ψ(3686)→π0hc at BESIII. The hc→π+π−π0 decay is observed with a significance of 9.6σ after taking into account systematic uncertainties. Evidences for hc→K+K−π0 and hc→K+K−η are found with significances of 3.5σ and 3.3σ, respectively, after considering the systematic uncertainties. The branching fractions of these decays are measured to be B(hc→π+π−π0)=(1.36±0.16±0.14)×10−3, B(hc→K+K−π0)=(3.26±0.84±0.36)×10−4, and B(hc→K+K−η)=(3.13±1.08±0.38)×10−4, where the first uncertainties are statistical and the second are systematic. No significant signal of hc→π+π−η is found, and the upper limit of its decay branching fraction is determined to be B(hc→π+π−η)&lt;4.0×10−4 at the 90% confidence level. Published by the American Physical Society 2024

Machine learning-based study of open-charm hadrons in proton-proton collisions at the Large Hadron Collider

In proton-proton and heavy-ion collisions, the study of charm hadrons plays a pivotal role in understanding the QCD medium and provides an undisputed testing ground for the theory of strong interaction, as they are mostly produced in the early stages of collisions via hard partonic interactions. The lightest open charm, D0 meson (cu¯), can originate from two separate sources. The prompt D0 originates from either direct charm production or the decay of excited open charm states, while the nonprompt stems from the decay of beauty hadrons. In this paper, using different machine learning (ML) algorithms such as XGBoost, CatBoost, and Random Forest, an attempt has been made to segregate the prompt and nonprompt production modes of the D0 meson signal from its background. The ML models are trained using the invariant mass through its hadronic decay channel, i.e., D0→π+K−, pseudoproper time, pseudoproper decay length, and distance of closest approach of D0 meson, using 8 simulated pp collisions at s=13 TeV. The ML models used in this analysis are found to retain the pseudorapidity, transverse momentum, and collision energy dependence. In addition, we report the ratio of nonprompt to prompt D0 yield, the self-normalized yield of prompt and nonprompt D0, and explore the charmonium, J/ψ to open charm, D0 yield ratio as a function of transverse momenta and normalized multiplicity. The observables studied in this paper are well predicted by all the ML models compared to the simulation. Published by the American Physical Society 2024

Interaction of exotic states in a hadronic medium: the Z c (3900) case

We investigate the interactions of the charged exotic state Z c (3900) in a hadronic medium composed of light mesons. We study processes such as Zcπ→DD¯ , Zcπ→D*D¯* , Zcπ→DD¯* and the inverse ones. Using effective Lagrangians and form factors calculated with Quantum Chromodynamics (QCD) sum rules (treating the Z c (3900) as a tetraquark) we estimate the vacuum and thermally-averaged cross-sections of these reactions. We find that the Z c (3900) has relatively large interaction cross sections with the constituent particles of the hadronic medium. After that, we use the production and suppression cross sections in a rate equation to estimate the time evolution of the Z c multiplicity. We include the Z c decay and regeneration terms The coalescence model is employed to compute the initial Z c multiplicity for the compact tetraquark configuration. Our results indicate that the combined effects of hadronic interactions, hydrodynamical expansion, decay and regeneration affect the final yield, which is bigger than the initial value. Besides, the dependence of the Z c final yield with the centrality, center-of-mass energy and the charged hadron multiplicity measured at midrapidity [dNch/dη(η<0.5)] is also investigated.

First results on the performance of the upgraded LHCb RICH detectors

With the start of LHC Run 3 the upgraded LHCb experiment, designed to run at an instantaneous luminosity five times larger with respect to previous running periods, is aiming to collect L=50fb−1 of pp collisions data by the end of Run 4. In addition, thanks to a trigger-less readout and a full software trigger, the selection efficiency for fully hadronic decay channels of heavy hadrons increases up to a factor two. The LHCb RICH detectors have been renewed to manage the increased detector occupancy, ensuring outstanding charged hadron discrimination in the more demanding Run 3 environment and at the maximum LHC interaction rate. This process is detailed, outlining the automated calibration procedures of the RICH system to achieve optimal signal efficiency and background rejection during the initial stages of the Run 3 data-taking. The early performance of the newly installed detectors is presented, including the determination of performance parameters such as the Cherenkov angle resolution. It is also reported that the particle identification capabilities are already approaching the design level and outperforming those of Run 1 and 2.