Quark Hadron Research Articles

The distribution amplitude of the ηc-meson at leading twist from lattice QCD

Distribution amplitudes are functions of non-perturbative matrix elements describing the hadronization of quarks and gluons. Thanks to factorization theorems, they can be used to compute the scattering amplitude of high-energy processes. Recently, new ideas have allowed their computation using lattice QCD, which should provide us with a general, fully relativistic determination. We present the first lattice calculation of the ηc-meson distribution amplitude at leading twist. Starting from the relevant matrix element in discrete Euclidean space on a set of Nf = 2 CLS ensembles, we explain the method to connect to continuum Minkowski spacetime. After addressing several sources of systematic uncertainty, we compare to Dyson-Schwinger and non-relativistic QCD determinations of this quantity. We find significant deviations between the latter and our result even at small Ioffe times.

Nonuniversality of heavy quark hadronization in elementary high-energy collisions

It has been traditionally hypothesized that the heavy quark (charm c and bottom b) fragmentation is universal across different collision systems, based on the notion that hadronization as a soft process should occur at the characteristic nonperturbative quantum chromodynamics (QCD) scale, ΛQCD. However, this universality hypothesis has recently been challenged by the observation that the c- and b-baryon production relative to their meson counterparts in minimum bias proton-proton (pp) collisions at the CERN Large Hadron Collider (LHC) energies is significantly enhanced as compared to the electron-positron (e+e−) collisions. The conception of nonuniversality is unambiguously reinforced by the latest measurement of the charged-particle multiplicity dependence of the b-baryon–to–meson yield ratio, Λb/B, by the LHCb experiment in s=13TeVpp collisions at the LHC, evolving continuously from the saturation value in minimum bias pp collisions toward the small value in e+e− collisions as the system size gradually reduces. We address the multiplicity dependence of b-baryon production in the canonical statistical hadronization model with input b-hadron spectrum augmented with many hitherto unobserved states from quark model predictions. We demonstrate that the decreasing trend of the Λb/B toward low multiplicities can be quantitatively understood from the canonical suppression on the yield of Λb, as caused by the requirement of strict conservation of baryon number in sufficiently small systems. We have therefore proposed a plausible scenario for understanding the origin of the nonuniversality of heavy quark fragmentation in elementary collisions. Published by the American Physical Society 2024

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

Probing hadron–quark phase transition in twin stars using f-modes

ABSTRACT Although it is conjectured that a phase transitions from hadronic to deconfined quark matter in the ultrahigh-density environment of neutron stars (NS), the nature of phase transition remains an unresolved mystery. Furthermore, recent efforts reveal that the finite surface tension effects can lead to a mixed phase with different geometric shapes (so-called ‘pasta’ phases), leading to a smooth phase transition from hadronic to quark matter in the NS interior. Depending on whether there is a strong or a pasta-induced smooth first-order phase transition, one may expect a third family of stable, compact stars or ‘twin stars’ to appear, with the same mass but different radii compared to NSs. The possibility of identifying twin stars using astrophysical observations has been a subject of interest. This study investigates the potential of probing the nature of the hadron–quark phase transition through future gravitational wave (GW) detections from fundamental (f-) mode oscillations in NSs. Using a newly developed model that parametrizes the hadron–quark phase transition with ‘pasta phases’, we calculate f-mode characteristics within a full general relativistic framework. We then use universal relations in GW asteroseismology to derive stellar properties from the detected mode parameters. Our findings suggest that detecting GWs from f modes with third-generation GW detectors offers a promising scenario for the existence of twin stars. However, we also estimate various uncertainties in determining the mode parameters and conclude that these uncertainties make it more challenging to identify the nature of the hadron–quark phase transition.

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Analysis of Neutron Star f-mode Oscillations in General Relativity with Spectral Representation of Nuclear Equations of State

We study quasi-normal f-mode oscillations in neutron star (NS) interiors within a linearized general relativistic formalism. We utilize approximately 9000 nuclear equations of state (EOSs) using spectral representation techniques, incorporating constraints on nuclear saturation properties, chiral effective field theory for pure neutron matter, and perturbative quantum chromodynamics for densities pertinent to NS cores. The median values of the f-mode frequency, ν f (damping time, τ f ) for NSs with masses ranging from 1.4 to 2.0 M ⊙ lie between 1.80 and 2.20 kHz (0.13–0.22 s) for our entire EOS set. Our study reveals a weak correlation between f-mode frequencies and individual nuclear saturation properties, prompting the necessity for more intricate methodologies to unveil multiparameter relationships. We observe a robust linear relationship between the radii and f-mode frequencies for different NS masses. Leveraging this correlation alongside NICER observations of PSR J0740+6620 and PSR J0030+0451, we establish constraints that exhibit partial and minimal overlap for observational data from Riley et al. and Miller et al., respectively, with our nucleonic EOS data set. Moreover, NICER data align closely with the radius and frequency values for a few hadron–quark hybrid EOS models. This indicates the need to consider additional exotic particles such as deconfined quarks at suprasaturation densities. We conclude that future observations of the radius or f-mode frequency for more than one NS mass, particularly at the extremes of the viable NS mass scale, would either rule out nucleon-only EOSs or provide definitive evidence in its favor.

Quenching and flow of charm and bottom quarks via semi-leptonic decay of D and B mesons in Pb+Pb collisions at the LHC* *Supported in part by the National Natural Science Foundation of China (12225503, 11935007, 11890710, 11890711, 12175122, 2021-867). W.-J. X. is supported in part by China Postdoctoral Science Foundation (2023M742099). Some of the calculations were performed in the Nuclear

Heavy flavor particles provide important probes of the microscopic structure and thermodynamic properties of the quark-gluon plasma (QGP) produced in high-energy nucleus-nucleus collisions. We studied the energy loss and flow of charm and bottom quarks inside the QGP via the nuclear modification factor ( ) and elliptic flow coefficient ( ) of their decayed leptons in heavy-ion collisions at the LHC. The dynamical evolution of the QGP was performed using the CLVisc (3+1)-dimensional viscous hydrodynamics model; the evolution of heavy quarks inside the QGP was simulated with our improved Langevin model that considers both collisional and radiative energy loss of heavy quarks; the hadronization of heavy quarks was simulated via our hybrid coalescence-fragmentation model; and the semi-leptonic decay of D and B mesons was simulated via PYTHIA. Using the same spatial diffusion coefficient for charm and bottom quarks, we obtained smaller and larger of charm decayed leptons than bottom decayed leptons, indicating stronger energy loss of charm quarks than bottom quarks inside the QGP within our current model setup.

The dynamic hadronization of charm quarks in heavy-ion collisions

The Pythia8/Angantyr model for heavy ion collisions was recently updated with a mechanism for global colour reconnection. The colour reconnection model used is QCD colour algebra inspired and enhances baryon production due to the formation of string junctions. In this paper, we present updates to the junction formation and string fragmentation mechanisms, connected to heavy quark fragmentation. This allows for the simulation of heavy quark fragmentation, using junction formation, in heavy ion collisions. The framework is validated for proton collisions, and we show results for charm baryon production in proton-lead collisions.

Recent Findings from Heavy-Flavor Angular Correlation Measurements in Hadronic Collisions

The study of angular correlations of heavy-flavor particles in hadronic collisions can provide crucial insight into the heavy quark production, showering, and hadronization processes. The comparison with model predictions allows us to discriminate among different approaches for heavy quark production and hadronization, as well as different treatments of the underlying event employed by the models to reproduce correlation observables. In ultra-relativistic heavy-ion collisions, where a deconfined state of matter, the quark–gluon plasma (QGP), is created, heavy-flavor correlations can shed light on the modification of the heavy quark fragmentation due to the interaction between charm and beauty quarks with the QGP constituents, as well as characterize their energy loss processes while traversing the medium. Insight into the possible emergence of collective-like mechanisms in smaller systems, resembling those observed in heavy-ion collisions, can also be obtained by performing correlation studies in high-multiplicity proton–proton and proton–nucleus collisions. In this review, the most recent and relevant measurements of heavy-flavor correlations performed in all collision systems at the LHC and RHIC will be presented, and the new understandings that they provide will be discussed.

Measurements of charm quark production and hadronization at CMS

The study of charm quark hadrons is an important probe into the hadronization of heavy quarks. More specifically, we present results on the production of Λc baryon, the nuclear modification factors (RAA), and the Λc/D0 yield ratios at √SNN = 5.02 TeV in proton-proton (pp) collisions and in different centrality regions in lead-lead (PbPb) collisions, using data recorded with the CMS detector in 2017 and 2018, respectively. The reported RAA for Λc provides useful information regarding the energy loss mechanism and the hadronization processes of charm quark in the quark-gluon plasma. The transverse momentum (pT) dependence of the RAA is similar to that of other charm and beauty hadrons but with its minimum shifted towards higher pT. Comparing the Λc/D0 production ratio in pp and PbPb collisions suggests that coalescence as a hadronization process is not significant for pT > 10 GeV/c. The ratio becomes comparable to the measurements in e+e− collisions for pT > 30 GeV/c. We also present results of the Λc baryon and D0 meson production and their ratios in proton-lead (pPb) collisions at √SNN = 8.16 TeV as a function of pT and final-state multiplicity using the data recorded by the CMS experiment in 2016. We do not observe significant multiplicity dependence for the baryon over meson ratio for charm hadrons. Based on a previous study, the difference between the results from charm quarks and those from light quarks suggests coalescence processes for heavy quarks do not increase further with multiplicity, unlike light quarks.

Bottom-hadron production in high-energy pp and heavy-ion collisions

The hadro-chemistry of bottom quarks produced in hadronic collisions encodes valuable information on the mechanism of color-neutralization in these reactions. We first compute the chemistry of bottom-hadrons in high-energy pp collisions employing statistical hadronization with a largely augmented set of states beyond the currently measured spectrum. This enables a comprehensive prediction of fragmentation fractions of weakly decaying bottom hadrons for the first time and a satisfactory explanation of the existing measurements in pp collisions at the LHC. Utilizing the bottom hadro-chemistry thus obtained as the baseline, we then perform transport simulations of bottom quarks in the hot QCD matter created in PbPb collisions at the LHC energy and calculate the pertinent bottom-hadron observables. The transverse momentum (pT) dependent modifications of the bottom baryon-to-meson ratio (Λb0/B-) relative to their pp counterparts are highlighted as a result of bottom quark diffusion and hadronization in the Quark-Gluon Plasma (QGP). We finally summarize the heavy quark (charm vs bottom) diffusion coefficients as extracted from transport simulations and compare them to result from recent full lattice QCD computations.

Insights on strange quark hadronization in small collision system with ALICE: Multiple strange hadrons and Σ± baryons

Among the most iconic results of Run-1 and Run-2 of the LHC is the observation of enhanced production of (multi-)strange to non-strange particles, gradually rising from low-multiplicity to high-multiplicity pp or p–Pb collisions and reaching values close to those measured in peripheral Pb–Pb collisions [1]. The observed behaviour cannot be quantitatively reproduced by any of the available QCD-inspired MC generators. In this contribution two extensions of this study are presented: the measurement of Σ baryons and the first measurement of the full Probability Density Function (PDF) for KS0, Λ, Ξ− and Ω−, therefore extending the study of strangeness production beyond the average of the distribution. This novel method represents a unique opportunity to test the connection between charged and strange particle multiplicity production.

Constraining the equation of state with heavy quarks in the quasi-particle model of QCD matter

In a quasi-particle model of QCD matter at finite temperature with thermal masses for quarks and gluons from hard thermal loops, the equation of state (EOS) can be described by an effective temperature dependence of the strong coupling g(T). Assuming the same effective coupling between the exchanged gluon and thermal partons, the EOS can also be related to parton energy loss. Based on the quasi-particle linear Boltzmann transport (QLBT) model coupled to a (3+1)-dimensional viscous hydrodynamic model of the quark-gluon plasma (QGP) evolution and a hybrid fragmentation-coalescence model for heavy quark hadronization, we perform a Bayesian analysis of the experimental data on D meson suppression RAA and anisotropy v2 at RHIC and the LHC. We achieve a simultaneous constraint on the QGP EOS and the heavy quark transport coefficient, both consistent with the lattice QCD results.

Stability and instability of strange dwarfs

Aims. More than 20 years ago, the existence of stable white dwarfs with a core of strange quark matter was proposed. More recently, via the study of radial modes, it has been concluded instead that such objects are unstable. We aim to clarify this issue. Methods. We investigated the stability of these objects by looking at their radial oscillations while incorporating boundary conditions at the quark–hadron interface, which correspond to either a rapid or a slow conversion of hadrons into quarks. Results. Our analysis shows that objects of this type are stable if the star is not strongly perturbed and ordinary matter cannot transform into strange quark matter because of the Coulomb barrier separating the two components. On the other hand, ordinary matter can be transformed into strange quark matter if the star undergoes a violent process, as in the preliminary stages of a type Ia supernova, and this causes the system to become unstable and collapse into a strange quark star. In this way, the accretion-induced collapse of strange dwarfs can be facilitated, and kilometre-sized objects with sub-solar masses can be produced.

Azimuthal correlations of heavy-flavor hadron decay electrons with charged particles in pp and p–Pb collisions at pmb {sqrt{s_{mathrm{{NN}}}}} = 5.02 TeV

The azimuthal (Delta varphi ) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at sqrt{s_{mathrm{{NN}}}} = 5.02 TeV. Results are reported for electrons with transverse momentum 4<p_{textrm{T}}<16textrm{GeV}/c and pseudorapidity |eta |<0.6. The associated charged particles are selected with transverse momentum 1<p_{textrm{T}}<7textrm{GeV}/c, and relative pseudorapidity separation with the leading electron |Delta eta | < 1. The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The Delta varphi distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators.

Masses and magnetic moments of exotic fully heavy pentaquarks

Inspired by the observation of a resonant state X(6600) of fully charm tetraquark by the CMS experiment of LHCb Collaboration in double J/psi decay channel, we perform a systematical study of all configurations of fully heavy pentaquarks P_{Q_{1}Q_{2}Q_{3}Q_{{4}}bar{Q_{5}}} (Q_{i}=c,b, i=1,2,3,4,5) in their ground states in unified framework of MIT bag model. The color-spin wavefunctions of pentaquarks, classified via Young tableau and presented in terms of the Young–Yamanouchi bases, are used to compute masses and magnetic moments of fully heavy pentaquarks via numerical variational method, predicting a set of masses ranging from 8.229 GeV for the P_{ccccbar{c}} to 24.770 GeV for the P_{bbbbbar{b}}. Combining with computed masses of fully heavy mesons and baryons, we find that masses of fully heavy hadrons(mesons, baryons, tetraquarks and pentaquarks) with identical flavor rise almost linearly with the number of valence quarks in hadrons, being consistent with the heavy quark symmetry in the heavy-quark limit.

Measurement of the cross section of top quark-antiquark pair production in association with a W boson in proton-proton collisions at sqrt{s} = 13 TeV

The production of a top quark-antiquark pair in association with a W boson ( textrm{t}overline{textrm{t}}textrm{W} ) is measured in proton-proton collisions at a center-of-mass energy of 13 TeV. The analyzed data was recorded by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb−1. Events with two or three leptons (electrons and muons) and additional jets are selected. In events with two leptons, a multiclass neural network is used to distinguish between the signal and background processes. Events with three leptons are categorized based on the number of jets and of jets originating from b quark hadronization, and the lepton charges. The inclusive textrm{t}overline{textrm{t}}textrm{W} production cross section in the full phase space is measured to be 868 ± 40(stat) ± 51(syst) fb. The textrm{t}overline{textrm{t}}textrm{W} + and textrm{t}overline{textrm{t}}textrm{W} − cross sections are also measured as 553 ± 30(stat) ± 30(syst) and 343 ± 26(stat) ± 25(syst) fb, respectively, and the corresponding ratio of the two cross sections is found to be 1.61pm 0.15{left(textrm{stat}right)}_{-0.05}^{+0.07}left(textrm{syst}right) . The measured cross sections are larger than but consistent with the standard model predictions within two standard deviations, and represent the most precise measurement of these cross sections to date.

Core States of Neutron Stars from Anatomizing Their Scaled Structure Equations

Given an Equation of State (EOS) for neutron star (NS) matter, there is a unique mass–radius sequence characterized by a maximum mass at radius R max. We first show analytically that the and R max scale linearly with two different combinations of the NS central pressure P c and energy density ε c, by dissecting perturbatively the dimensionless Tolman–Oppenheimer–Volkoff (TOV) equations governing NS internal variables. The scaling relations are then verified via 87 widely used and rather diverse phenomenological as well as 17 microscopic NS EOSs with/without considering hadron–quark phase transitions and hyperons, by solving numerically the original TOV equations. The EOS of the densest NS matter allowed before it collapses into a black hole is then obtained. Using the universal and R max scalings and Neutron Star Interior Composition Explorer and XMM-Newton mass–radius observational data for PSR J0740+6620, a very narrow constraining band on the NS central EOS is extracted directly from the data for the first time, without using any specific input EOS model.

Kinked tracks from Σ+ baryons as a probe of light quark polarizations

Jets initiated by energetic quarks carry in an obvious way approximate information about the original quark energy and momentum. In a less obvious way, they also carry information about the quark polarization. Previous works have shown how the polarization information can be extracted by ATLAS and CMS using events in which the quark hadronizes to an energetic baryon — Λb for b quarks, Λc for c quarks and Λ for s quarks. In this paper we extend these proposals to Σ+ baryons, which can provide sensitivity to polarizations of s and u quarks. We analyze the various aspects of how the kinked track signature of the Σ+→ pπ0 decay can be used for this purpose. We evaluate the feasibility of such measurements in toverline{t} samples.

Crustal Failure as a Tool to Probe Hybrid Stars

It is currently unknown if neutron stars (NSs) are composed of nucleons only or are hybrid stars, i.e., in addition to nucleonic crusts and outer cores, they also possess quark cores. Quantum chromodynamics allows for such a possibility, but accurate calculations relevant for compact stars are still elusive. Here we investigate some crust-breaking aspects of hybrid stars. We show that the crust-breaking frequency and maximum fiducial ellipticity are sensitive to the quark–hadron density jump and equation of state stiffness. Remarkably, the crust-breaking frequency related to static tides scales linearly with the mass of the star (for a given companion mass), and its slope encompasses information about the microphysics of the star. However, for precise crust-breaking frequency predictions, relativistic corrections to Kepler’s third law and the Newtonian tidal field should not be ignored. When a liquid quark core touches an elastic hadronic phase (the result of a significant energy density jump), the maximum ellipticity can increase by around an order of magnitude when compared to a liquid quark core touching a liquid hadronic phase. That is relevant because it would increase the odds of detecting continuous gravitational waves from NSs. Our order-of-magnitude analysis also suggests that a given upper limit to the ellipticity (crust-breaking frequency) could have representatives in stars with either small or intermediate (large) energy density jumps. Therefore, when upper limits to the ellipticity for isolated stars are better constrained or electromagnetic radiation (e.g., gamma-ray precursors) is detected along with gravitational waves in inspiraling binary systems, they may help constrain some aspects of phase transitions in NSs.