Heavy Flavor Research Articles

2B or not 2B, a study of bottom-quark-philic semi-visible jets

Semi-visible jets arise in strongly interacting dark sectors, resulting in jets overlapping with the missing transverse momentum direction. The implementation of semi-visible jets is done using the Pythia Hidden Valley module to mimic the QCD sector showering in so-called dark shower. In this work, only heavy flavour Standard Model quarks are considered in dark shower, resulting in a much less ambiguous collider signature of semi-visible jets compared to the democratic production of all five quark flavours in dark shower. The constraints from available searches on this signature are presented, and it is shown the signal reconstruction can be improved by using variable-radius jets. Finally a search strategy is suggested.

Data-driven analysis of the beauty hadron production in pp collisions at the LHC with Bayesian unfolding

Heavy flavour production in proton-proton (pp) collisions provides insights into the fundamental properties of Quantum Chromodynamics (QCD). Beauty hadron production measurements are widely performed through indirect approaches based on their inclusive decay modes. A Bayesian unfolding data-driven analysis of the ALICE and LHCb data was performed in this study, which recovers the full kinematic information of the beauty hadrons via different inclusive decay channels. The corresponding beauty hadron production cross sections obtained after the Bayesian unfolding are found to be consistent within their uncertainties. The weighted average open beauty production cross sections are presented as a function of the transverse momentum and rapidity in pp 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} = 5.02 TeV and 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, respectively. The pT-integrated open beauty production dσ/dy and the total bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{b}\\overline{\ extrm{b}} $$\\end{document} cross section σbb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{\ extrm{b}\\overline{\ extrm{b}}} $$\\end{document} are also reported. The precision of these results significantly improves upon worldwide measurements, providing valuable validation and constraints on mechanisms of heavy flavour production in pp collisions at the LHC energies.

Heavy quark mass near the phase transition

Abstract Assuming that the number densities of heavy flavor in hadron gas and in QGP are same at $T_c$, we obtain the effective mass of heavy quark at $T_c$ from the comparison with the hadron resonance gas model which well describes particle yield in heavy-ion collisions. We find that charm quark mass at vanishing baryon chemical potential is around 1.8 GeV which is much heavier than QCD bare mass and close to $D$ meson mass. The mass slightly increases with increasing baryon chemical potential and then decreases.
On the other hand, anticharm quark mass constantly decreases with increasing baryon chemical potential. Bottom quark mass has a similar pattern. Extending the hadron resonance gas model to a bit higher temperature beyond $T_c$, the effective masses of charm and bottom quarks decrease with increasing temperature.

Investigation on the bottom analogs Tbb− of Tcc+

We investigate the doubly bottom state Tbb− composed of two bottom mesons with JP=1+. The potentials are obtained using the one-boson exchange model. With the heavy quark flavor symmetry, all the parameters in the model are determined by fitting the experimental data of doubly charmed state Tcc+ from our previous work. Our analysis indicates that the isospin symmetry is well preserved. We identify a deeply bound Tbb− state with quantum numbers I(JP)=0(1+), in contrast to the loosely bound Tcc+. Additionally, we discover a resonant Tbb− state with I(JP)=1(1+). Furthermore, our investigation into the B¯B¯*−B¯*B¯* coupled channel effect reveals its important impact. The binding energy of the bound I(JP)=0(1+) states becomes deeper, while the resonant Tbb− state with I(JP)=1(1+) remains nearly unchanged. Published by the American Physical Society 2024

Differentiable vertex fitting for jet flavor tagging

This work explores the use of differentiable programming to integrate domain knowledge, in the form of domain specific software, into neural networks to develop scientific machine learning systems. We propose a differentiable vertex fitting algorithm that estimates the crossing point of multiple curves. In the high energy physics setting, these curves are defined by particle equations of motion and the crossing point represents the origin of particle production. This differentiable vertex fitting algorithm can be seamlessly integrated into neural networks, and we show its utility and efficacy in the high energy physics application of the classification of jets, i.e., collimated streams of particles in particle detectors whose originating parent particle we aim to classify. We demonstrate how differentiable vertex fitting can be integrated into larger transformer-based models for jet flavor tagging and show improvements in heavy flavor jet classification when compared to baseline models. Published by the American Physical Society 2024

Heavy flavor production under a strong magnetic field

The magnetic field created in high energy nuclear collisions will affect the QCD dynamical processes, such as the heavy quark production which happens in the initial stage of the collisions. We calculate in a strong magnetic field the heavy quark production cross section of the process gg→QQ¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ gg\ o Q\\overline{Q} $$\\end{document} at leading order and the corresponding transverse momentum distribution in nucleus-nucleus collisions. In comparison to the QED process, the heavy quark production is dominated by the unique QCD channel with gluon self-interaction. Due to the dimension reduction of quark phase space in a strong magnetic field, the production is concentrated in a very narrow energy region above the threshold. Since the rotational invariance is broken in a magnetic field, the production becomes anisotropic.

Electric dipole moments in 5+3 flavor weak effective theory

A fully generic treatment of electric dipole moments (EDMs) is presented in the CP-violating and flavor-conserving weak effective field theory (WET) with five flavors of quarks and three flavors of leptons. We systematically analyze leading contributions to EDMs originating from QCD and QED renormalization group running between the electroweak scale and low energy scales of about 2 GeV. We include the full one-loop anomalous dimension and a subset of two-loop corrections, as well as threshold corrections at the bottom, charm and τ masses. This allows us to derive master formulae in the space of generic WET for the neutron and proton EDMs, for EDMs of diamagnetic atoms, and for the precession frequencies constrained in molecular EDM experiments, from which bounds on the electron EDM are extracted. In particular, our master formulae capture the contributions of WET CP-violating operators with heavy quark and lepton flavors. As an application, we study EDM constraints on the Yukawa couplings of the Higgs boson, in both the linear and non-linear realizations of electroweak symmetry breaking.

Study of multiplicity dependence of heavy flavor production in pp collisions using rope hadronization mechanism

The multiplicity dependence of the production of the charm mesons in pp collisions at [Formula: see text][Formula: see text]TeV and 13[Formula: see text]TeV as measured by ALICE experiment has been investigated using Pythia 8 event generator by studying the effect of various processes at partonic level such as the effect of different modes of color reconnections and rope hadronization. The relative yields ([Formula: see text]) of D-mesons and [Formula: see text] as a function of relative charged particle multiplicity for various transverse momentum ([Formula: see text]) ranges as measured by the ALICE experiment are in reasonable agreement with the estimations of Pythia 8 model within the framework of microscopic processes. The relative yields of B mesons for various [Formula: see text] intervals ([Formula: see text][Formula: see text]GeV/c) have also been predicted in pp collisions at [Formula: see text][Formula: see text]TeV and [Formula: see text][Formula: see text]TeV.

A roadmap for neutrino detection at LHC, HL-LHC and SPS

SND@LHC is a new detector for neutrino physics at LHC. Its experimental configuration makes it possible to distinguish between all three neutrino flavours, opening a unique opportunity to probe physics of heavy flavour production at the LHC in the region that is not accessible to ATLAS, CMS and LHCb. It can also explore lepton flavour universality in the neutral sector, and search for feebly interacting particles. The detector has been commissioned and installed in 2021–2022. A first set of data has since then been collected, providing the first observation of neutrinos produced at a collider. This paper discusses the detector technologies being used to study high-energy neutrinos at the LHC, and their performance in terms of physics reach. The necessary upgrades to operate at high-luminosity LHC are presented, as well as a proposed experiment to perform neutrino measurements at the newly approved Beam Dump Facility.

Bottom quark dynamics from nonprompt D0 and J/ψ production in Pb+Pb collisions at sNN=5.02 TeV

We study bottom quark energy loss via the nuclear modification factor (RAA) and elliptic flow (v2) of nonprompt D0 and J/ψ in relativistic heavy-ion collisions at the Large Hadron Collider (LHC). The space-time profile of quark-gluon plasma is obtained from the hydrodynamics simulation, the dynamical evolution of heavy quarks inside the color deconfined QCD medium is simulated using a linear Boltzmann transport model that combines Yukawa and string potentials of heavy-quark-medium interactions, the hadronization of heavy quarks is performed using a hybrid coalescence-fragmentation model, and the decay of B mesons is simulated via . Using this numerical framework, we calculate the transverse momentum (pT) dependent RAA and v2 of direct D mesons, B mesons, and nonprompt D0 and J/ψ from B meson decay in Pb+Pb collisions at sNN=5.02 TeV. We find the mass hierarchy of the nuclear modification of prompt D and B mesons depends on their pT. Both RAA and v2 of heavy flavor particles show strong pT and centrality dependences due to the interplay between parton energy loss, medium geometry and flow, and hadronization of heavy quarks. Nonprompt D0 and J/ψ share similar patterns of RAA and v2 to B mesons except for a pT shift during the decay processes. Therefore, future more precise measurements on nonprompt D0 and J/ψ can help further pin down the bottom quark dynamics inside the quark-gluon plasma. Published by the American Physical Society 2024

Collisional energy loss of a heavy quark in a semiquark-gluon plasma

By utilizing a background field effective theory, we compute the collisional energy loss of a heavy quark moving through a semiquark-gluon plasma characterized by nontrivial holonomy for Polyakov loops. We consider the elastic scatterings between the incident heavy quark and the thermal partons with both hard and soft momentum transfers. As compared to the energy loss obtained from the perturbation theory, the hard processes get modified through the thermal distribution functions that depend on the background field, while the proper treatment of the soft processes strongly relies on the use of the hard-thermal-loop resummed gluon propagator derived from the background field effective theory. Our results show that the heavy quark energy loss is significantly suppressed in the semiquark-gluon plasma due to a background field that is self-consistently generated in the effective theory. On the other hand, the suppression has a strong dependence on the temperature of the plasma, which becomes negligible above 2–3 times the critical temperature. For a realistic coupling constant, ignoring a relatively weak dependence on the heavy quark velocity, the suppression on the collisional energy loss can be approximated by an overall factor determined solely by the background field. This simple conclusion is expected to be useful for phenomenological applications in the heavy flavor physics. Published by the American Physical Society 2024

In-beam performance of a Resistive Plate Chamber operated with eco-friendly gas mixtures

ALICE (A Large Ion Collider Experiment) studies the Quark-Gluon Plasma (QGP): a deconfined state of nuclear matter obtained in ultra-relativistic heavy-ion collisions. One of the key probes for QGP characterization is the study of quarkonia and open heavy flavor production, of which ALICE exploits the muonic decay. In particular, a set of Resistive Plate Chambers (RPCs), placed in the forward rapidity region of the ALICE detector, is used for muon identification purposes.The correct operation of these detectors is ensured by the choice of the proper gas mixture. Currently they are operated with a mixture of C2H2F4, i-C4H10 and SF6 but, starting from 2017, new EU regulations have enforced a progressive phase-out of C2H2F4 because of its large Global Warming Potential (GWP), which is making it difficult and costly to purchase. Moreover, CERN asked LHC experiments to reduce greenhouse gases emissions, to which RPC operation contributes significantly.A possible candidate for C2H2F4 replacement is the C3H2F4 (diluted with other gases, such as CO2), which has been extensively tested using cosmic muons. Promising gas mixtures have been devised; the next crucial steps are the detailed in-beam characterization of such mixtures as well as the study of their performance under increasing irradiation levels.This contribution will describe the methodology and results of beam tests carried out at the CERN Gamma Irradiation Facility (equipped with a high activity 137Cs source and muon beam) with an ALICE-like RPC prototype, operated with several mixtures with varying proportions of CO2, C3H2F4, i-C4H10 and SF6 . Absorbed currents, efficiencies, prompt charges, cluster sizes, time resolutions and rate capabilities will be presented, both from digitized (for detailed shape and charge analysis) and discriminated (using the same front-end electronics as employed in ALICE) signals.

Spectrum of molecular hexaquarks

We investigate the mass spectra of molecular-type hexaquark states in the dibaryon systems. These systems are composed of the charmed baryons [Σc(*),Ξc(′,*)], doubly charmed baryons [Ξcc(*)], and hyperons [Σ(*),Ξ(*)]. We consider all possible combinations of particle-particle and particle-antiparticle pairs, including the S-wave spin multiplets in each combination. We establish the underlying connections among the molecular tetraquarks, pentaquarks, and hexaquarks with the effective quark-level interactions. We find that the existence of molecular states in DD*, DD¯*, and ΣcD¯(*) systems leads to the emergence of a large number of deuteronlike hexaquarks in the heavy flavor sectors. Currently, there have been several experimental candidates for molecular tetraquarks and pentaquarks. The experimental search for near-threshold hexaquarks will further advance the establishment of the underlying dynamical picture of hadronic molecules and deepen our understanding of the role of spin-flavor symmetry in near-threshold residual strong interactions. Published by the American Physical Society 2024

Production of hidden-heavy and double-heavy hadronic molecules at the Z factory of CEPC

With a clean environment and high collision energy, the Circular Electron Positron Collider (CEPC) would be an excellent facility for heavy flavor physics. Using the Monte Carlo event generator ythia, we simulate the production of the charmed (bottom) hadron pairs in the electron-positron collisions at the Z factory of CEPC, and the inclusive production rates for typical candidates of the hidden/double-charm and hidden/double-bottom S-wave hadronic molecules are estimated at an order-of-magnitude level with the final state interactions after the hadron pair production. The predicted cross sections for the hidden-charm meson-meson molecules X(3872) and Zc(3900) are at pb level, which are about two to three orders of magnitude larger than the production cross sections for the double-charm meson-meson molecules Tcc and Tcc*, as the double-charmed ones require the production of two pairs of cc¯ from the Z boson decay. The production cross sections for the hidden-charm pentaquark states Pc and Pcs as meson-baryon molecules are a few to tens of fb, which are about one magnitude larger than those of the possible hidden-charm baryon-antibaryon and double-charm meson-baryon molecules. In the bottom sector, the production cross sections for the Zb states as B(*)B¯* molecules are about tens to hundreds of fb, indicating 106–107 events from a two-year operation of CEPC, and the expected events from the double-bottom molecules are about 2–5 orders of magnitude smaller than the Zb states. Our results shows great prospects of probing heavy exotic hadrons at CEPC. Published by the American Physical Society 2024

Yadism: yet another deep-inelastic scattering module

We present yadism, a library for the evaluation of both polarized and unpolarized deep-inelastic scattering (DIS) structure functions and cross sections up to N3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document}LO in perturbative QCD. The package provides computations of observables in fixed-flavor and zero-mass variable flavor number schemes. The implementation of the general mass variable flavor number schemes is supported through the high virtuality limits for the heavy flavor coefficients. In addition, yadism provides a set of tools for the generation of interpolation grids in the PDF-independent PineAPPL format, allowing to test the PDF dependence on any DIS observable without needing to rerun the computation. This work is part of an ongoing effort to standardize the format of theory predictions in high-energy physics within the pineline framework. The code is open source, written in Python and documented to facilitate usage, integrations, and further extensions. Finally, the code has been benchmarked against the widely used APFEL++ and QCDNUM libraries.

The 1 → 3 massive splitting functions from QCD factorization and SCET

Splitting functions are universal functions describing the collinear dynamics of gauge theories, and as such are crucial ingredients for a wide variety of calculations in perturbative QCD. We present analytic results for the triple collinear splitting functions in QCD with a single massive parton. We derive the splitting functions using two distinct methods; first by expanding the squared matrix elements in the collinear limit, and secondly by using soft-collinear effective theory with massive quarks. We find agreement between these two approaches, providing a strong check of our results. Additionally, we also check all iterated and soft limits of our results, finding agreement with predictions from factorization. Our results provide an important ingredient for higher order perturbative calculations involving massive partons, and for the description of the collinear dynamics of heavy flavor jets.

Jet substructure

Jet substructure observables hold the keys to identifying the inner working of the quark–gluon plasma through its imprints in jet modification patterns. Because of the multi-scale nature of jet evolution, the strategy has focused on developing jet reclustering-based and jet shape-based observables to specifically probe different stages of jet evolution, which provide multi-dimensional quantification of jet particle distribution phase spaces. The developments of jet substructure in the past few years allowed us to explore the correlations among jet observables. The richer information beyond single-observable modifications could give provides a crucial step beyond the energy loss paradigm. This review surveys the theory and phenomenology of modern jet substructure observables, which help constraining the energy scales of medium interactions and identifying heavy flavor quarks within jets. Also, the peculiar soft activities point to more refined studies of medium responses to jets.

Flavor-spin symmetry of the PψN/HΩcccN and PψsΛ/HΩcccsΛ molecular states

Based on a contact Lagrangian that incorporates the SU(3) flavor and SU(2) spin symmetries, we discuss the symmetry properties of the interactions among the heavy flavor meson-baryon PψN, PψsΛ (with quark components [nc¯][nnc], [sc¯][nnc], or [nc¯][nsc]) systems and dibaryon HΩcccN, HΩcccsΛ (with quark components [nnc][ncc], [nnc][scc], or [nsc][ncc]) systems (n=u, d). The light quark components of the PψN (PψsΛ) and HΩcccN (HΩcccsΛ) systems have identical flavors, the interactions generated from the exchanges of light mesons in the PψN (PψsΛ) systems should be very similar to that of the HΩcccN (HΩcccsΛ) systems. We perform the single-channel and multichannel calculations on the PψN/PψsΛ/HΩcccN/HΩcccsΛ systems and introduce the SU(3) breaking effect to identify the different mass spectra among the PψN (HΩcccN) and PψsΛ (HΩcccsΛ) systems. We suggest two kinds of evidences for the existence of the flavor-spin symmetry among the heavy flavor PψN/HΩcccN/PψsΛ/HΩcccsΛ molecule community, i.e., the mass arrangements of the PψN/HΩcccN/PψsΛ/HΩcccsΛ mass spectra and the binding energies of the heavy flavor meson-baryon (dibaryon) systems attributed to the same contact potentials. Published by the American Physical Society 2024

Dynamics of heavy flavor in a weakly magnetized hot QCD medium

We obtain the spatial and momentum-diffusion coefficients (Ds and κ) and the collisional energy loss (dE/dx) of a heavy quark (HQ) traversing through a thermal medium of quarks and gluons in a weak magnetic field (B), for the two cases of the HQ moving either parallel or perpendicular to B. For that purpose, we consider Coulomb scatterings (t-channel) of the HQ with the light quarks, obtained from the imaginary part of the HQ self-energy via the cutting rules. Both the normalized (by T3) κ, and dE/dx, for charm quarks are larger than that for bottom quarks due to the larger mass of the latter. Also, the effect of B is more feeble on the bottom quark, compared to the charm quark. Comparatively, the magnitudes of both κ and dE/dx are significantly smaller for the case of v⊥B. For both the cases, our results show that the momentum transfer between the HQ and the medium takes place preferentially along the direction of HQ velocity, thus leading to a significant increase in the momentum-diffusion anisotropy, compared to B=0. We also calculate the (scaled) spatial diffusion coefficient, which we find to be independent of the heavy flavor mass and is almost unaffected by changes in B. Published by the American Physical Society 2024

Elliptic anisotropy of open-charm hadrons from parton scatterings in p -Pb collisions at energies available at the CERN Large Hadron Collider

The elliptic azimuthal anisotropy coefficient (v2) of open-charm hadrons at midrapidity (|η<1|) was studied in p-Pb collisions at sNN=8.16 TeV using a multiphase transport () model including an additional charm quark–antiquark pair production trigger. The model provides a simultaneous description of the measured pT spectrum and v2 of the D0 meson, as well as the v2 of light flavor meson Ks0. We found that the D0 and Ks0 v2 are both significantly affected by different parton scatterings among charm and light quarks. In addition, the predictions for the v2 of other charm hadrons including D+, Ds+, and Λc+ in p-Pb collisions are given for the first time. The v2 of open-charm hadrons reasonably follows the number-of-constituent-quark (NCQ) scaling up to 2.5 GeV, strongly indicating the importance of partonic degrees of freedom for the collectivity of heavy flavors in high-multiplicity p-Pb collisions. These findings may hint at the formation of deconfined matter in small collision systems, and provide referential value for future measurements of azimuthal anisotropy at energies available at the CERN Large Hadron Collider. Published by the American Physical Society 2024