Quark Hadron Research Articles

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.

Observation of the dead cone effect in charm and bottom quark jets and its QCD explanation

The production of a heavy quark is accompanied by gluon bremsstrahlung which is suppressed at small angles $\Theta\lesssim M_Q/E$ for mass $M_Q$ and high energy $E$ according to perturbative Quantum Chromo Dynamics (QCD) (``dead cone effect''). As particles at small angles typically have large momenta, the heavy quark mass also causes a suppression of high momentum particles. In this paper, we studied this effect in c- and b-quark events using data from Z boson decays in $e^+e^-$ annihilation. The heavy quark fragmentation function for charged particles is reconstructed in the momentum fraction variable $x$ or $\xi=\ln(1/x)$ by removing the decays of the heavy quark hadrons. Indeed, we find an increasing suppression of particles with rising $x$ down to a fraction of $\lesssim 1/10$ for particles with $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparison to light quark fragmentation. The sensitivity to the dead cone effect in the present momentum analysis is considerably increased in comparison to the recently presented angular analysis. This amount of suppression and the differences between c- and b-quark fragmentation are in good quantitative agreement with the expectations based on perturbative QCD within the Modified Leading Logarithmic Approximation (MLLA) in the central kinematic region. The data also support a two parameter description in the MLLA of these phenomena (``Limiting Spectrum''). The sensitivity of these measurements to the heavy quark mass is investigated.

Parton decomposition of nucleon spin and momentum: Gluons from dressed quarks

The lowest two Mellin moments of hadronic Generalized Parton Distributions are explored within a model that allows investigation of the inter-related quark and gluon contributions. For light quarks their dynamical connection is strong due to quark dressing. Our principal focus is the angular momentum J of the nucleon. This work employs and extends dynamical insights obtained from our recent model results for quark and gluon momentum fractions 〈x〉q/g in both pion and nucleon. The employed model is based on the Rainbow-Ladder truncation of the Dyson-Schwinger equations of QCD. The special case of a 1-loop treatment of a single hadronic quark is used to motivate several insights and obtain initial estimates such as the Wilson line correction to the established Landau gauge model (−7% for both Jq and 〈x〉q), and the “binding gluon” contribution to 〈x〉g (≤10%). We obtain the proton J within 1% after inclusion of the pion cloud mechanism to produce the sea. The gluon second Mellin moments of the proton reflect similar dynamics; (〈x〉g, Jg) are (26%, 24%) at model scale, and (40%, 38%) at 2 GeV. We also find that Jtot is shared almost equally between the total orbital and total intrinsic spin contributions.

Resumming quark's longitudinal momentum logarithms in LaMET expansion of lattice PDFs

In the large-momentum expansion for parton distribution functions (PDFs), the natural physics scale is the longitudinal momentum (pz) of the quarks (or gluons) in a large-momentum hadron. We show how to expose this scale dependence through resumming logarithms of the type lnn⁡pz/μ in the matching coefficient, where μ is a fixed renormalization scale. The result enhances the accuracy of the expansion at moderate pz>1 GeV, and at the same time, clearly shows that the partons cannot be approximated from quarks with pz∼ΛQCD which are not predominantly collinear with the parent hadron momentum, consistent with power counting of the large-momentum effective theory. The same physics mechanism constrains the coordinate space expansion at large distances z, the conjugate of pz, as illustrated in the example of fitting the moments of the PDFs.

Entanglement between valence and sea quarks in hadrons of 1+1 dimensional QCD

The conceptual interpretation of valence- and sea-quark separation, which is a key aspect of the parton model and of an intuitive picture of hadron structure, becomes obscured by quantum effects in QCD. This suggests that there may be measures of entanglement between quark degrees of freedom that are present in QCD, but absent in the intuitive picture with a clear valence-sea (VS) separation. In this paper, we define the first rigorous measure of VS entanglement in QCD in an attempt to bring conceptual clarity to this issue, and, potentially, to find a measure of the applicability of the parton model to QCD bound states. This VS entanglement vanishes in the large-Nc limit, and it remains low when finite-Nc states resemble their large-Nc counterparts. We perform a numerical study of VS entanglement in 1 + 1 dimensional discrete light-cone quantized QCD, and in the process develop a method for building the color-singlet basis of 1 + 1d QCD that is manifestly complete and orthogonal by construction. We calculate this VS entanglement entropy for the first time and find that it is relatively low for the first few excited states of both mesons and baryons compared to all other states in the spectrum, with the VS entropy of ground state hadrons providing a minimum. We also see that for ground state mesons the entropy is well described in the 1/Nc approximation. These results suggest that low energy hadrons may be the only QCD bound states for which the large-Nc expansion, and perhaps the parton model, provide an accurate description. This work also provides the first evidence that the VS entanglement entropy of QCD in 3 + 1d, which would likely serve as an order parameter for the transition between quark and hadron degrees of freedom, may be perturbatively accessible through a large-Nc expansion.

Open heavy flavour production in small systems with ALICE

Charm and beauty production measurements in proton–proton (pp) collisions are an important test of perturbative QCD calculations, based on the factorisation approach, which assumes universal fragmentation functions among different collision systems. Recent measurements of charm baryon production at midrapidity in small systems show a baryon-to-meson ratio significanlty higher than that in e+e− and e−p collisions: this observation suggests that charm fragmentation is not universal across different collision systems. Therefore, measurements of charm baryon production are fundamental to characterise the charm quark hadronisation in pp and proton–lead (p–Pb) collisions at the LHC energies. In these proceedings, the latest measurements of D0, D+ and Ds+ mesons, the new measurements of Λc+, Ξc0,+, Σc0,++, the first measurement of Ωc0 baryons and of the non-prompt Λc+, performed with the ALICE detector at midrapidity in pp collisions at s=5.02 and s=13TeV are presented. Furthermore, the recent Λc+/D0 yield ratio, measured down to pT=0 in pp and p–Pb collisions will be discussed together with the measurements of the charm fragmentation fractions into hadrons in pp and p–Pb collisions at sNN=5.02TeV. This rich set of experimental results challenges existing theoretical calculations.

Perturbative and non-perturbative interactions between heavy quarks and quark-gluon plasma within a unified approach

While perturbative QCD is sufficient for understanding the color, mass and energy dependences of parton energy loss and jet quenching at large transverse momentum in heavy-ion collisions, a simultaneous description of heavy flavor nuclear modification factor RAA and elliptic flow coefficient v2 at low and intermediate pT still remains a challenge due to the effects from non-perturbative interactions. In this work, we extend the linear Boltzmann transport model by implementing a generalized Cornell-type potential that incorporates both short-range Yukawa interaction and long-range color confining interaction between heavy quarks and the QGP medium. Combining our new approach for heavy-quark-QGP interaction with a (3+1)-dimensional hydrodynamic model CLVisc for the QGP evolution and a hybrid fragmentation-coalescence model for heavy quark hadronization, we obtain a satisfactory description of heavy meson RAA and v2 from low to intermediate to high pT observed at both RHIC and the LHC. By model-data-comparison, we obtain for the first time the in-medium heavy quark potential from open heavy flavor measurements; the result is in agreement with the lattice QCD calculation. Our study indicates that while jet quenching at high pT is dominated by perturbative QCD interactions, non-perturbative interactions are indispensible for understanding heavy flavor quenching and flow at low and intermediate pT.

Density-Induced Hadron–Quark Crossover via the Formation of Cooper Triples

We discuss the hadron–quark crossover accompanied by the formation of Cooper triples (three-body counterpart of Cooper pairs) by analogy with the Bose–Einstein condensate to Bardeen–Cooper–Schrieffer crossover in two-component fermionic systems. Such a crossover is different from a phase transition, which often involves symmetry breaking. We calculate the in-medium three-body energy from the three-body T-matrix with a phenomenological three-body force characterizing a bound hadronic state in vacuum. With increasing density, the hadronic bound-state pole smoothly undergoes a crossover toward the Cooper triple phase where the in-medium three-body clusters coexist with the quark Fermi sea. The relation to the quarkyonic matter model can also be found in a natural manner.

Beauty production in heavy–ion collisions with ALICE at the LHC

In this contribution, the measurement of the centrality dependence of the nuclear modification factor (RAA) of non-prompt D0 meson in Pb-Pb collisions at √SNN = 5.02 TeV is presented. This measurement provides important constraints to the in-medium mass dependence of energy loss and hadroniza-tion of the beauty quark. In addition, the first measurements of non-prompt Ds+ meson production in central and semi-central Pb-Pb collisions at √SNN = 5.02 TeV are discussed. The non-prompt Ds+ meson measurements provide further information on the production and hadronization of Bs0 mesons. Finally, the first measurement of non-prompt D-meson elliptic flow in Pb-Pb collisions at √SNN = 5.02 TeV is shown. This measurement can help to investigate the degree of thermalization of beauty quarks in the hot and dense QCD medium.

Rescattering effect on the measurement of K*0 spin alignment in heavy-ion collisions

Spin alignment of vector mesons in noncentral relativistic heavy-ion collisions provides a novel probe of the global quark polarization and hadronization mechanism. In this paper, we discuss the spin alignment of a short-lived vector meson, namely ${K}^{*0}$, arising from the hadronic rescattering process using the UrQMD model. This spin alignment is not related to global quark polarization but cannot be distinguished from those arising from global quark polarization in experiment. The spin alignment parameter ${\ensuremath{\rho}}_{00}$ is found to deviate from 1/3 by up to $\ensuremath{-}0.008$ (0.03) with respect to the reaction (production) plane. These deviations are much larger than the expected signal from all the theoretical models implementing the conventional global quark polarization mechanism as well as the current experimental precision, and should be considered seriously when comparing measurements with theoretical calculations.

Simulation of multiple hadron production and transition to quark-gluon plasmas based on nonlinear dynamics

A dynamics of quark-gluon plasmas has been defined by the evolution parameter, which at high energies depends on collision energy of atomic nuclei for appropriate multiplicities of secondary hadrons. The paper is based on the solution of evolution equation for patrons momentum distribution. A transition from regular propagation to irregular chaotic is an indicator of the quark-gluon plasma emerging. Nonlinear renormalization group equation has been solved by the method of Poincaré mappings. The equation is a model of the evolution of the momentum distribution of partons due to competing processes of their creation and fusion. As the energy increases, sequential bifurcation (doubling) of phase trajectories occurs, and scale-invariant fractal structures are to create. At sufficiently high energies of quarks and gluons, a dynamically determined quark-gluon system, corresponding to QGP, arises in space. There are also hadron-like structures. Quantum coherence effects follow by dynamic chaos. As a result, quarks and gluons merge into stable attractor structures with their subsequent decay into hadrons. The nonlinear equation introduced for quark-gluon cascade, which comprises dynamical chaos, has include effects of parton recombination. The chaotic dynamics has been connected specifically to the transverse momenta of partons. Clearly, the formation of dynamically determined stable structures should be unknown before, attractor mechanism of quark hadronization.

Search for fractality and phase transition in p–p collisions at LHC energy

In this paper, a detailed study of the multiplicities of charged particles produced in [Formula: see text]–[Formula: see text] collisions, at [Formula: see text][Formula: see text]TeV, has been carried out. Analysis of UrQMD generated events along pseudo–rapidity ([Formula: see text]), azimuthal angle ([Formula: see text]) and [Formula: see text]-[Formula: see text] phase spaces is made with the help of scaled factorial moment method. The results have also been compared with the UrQMD events with finite impact parameter ([Formula: see text]) and minimum bias (MB) conditions. From the intermittency exponent ([Formula: see text]), the anomalous fractal dimension [Formula: see text] is derived and the variations of [Formula: see text] with order [Formula: see text] are investigated. The observed intermittent fluctuations are manifested by various parameters such as anomalous fractal dimension [Formula: see text], degree of multifractality ([Formula: see text]), critical exponent ([Formula: see text]), Levy index ([Formula: see text]) and multifractal specific heat. In the framework of Ginzburg–Landau theory, the second-order phase transition in the light of scaled factorial moment method has also been carried out to search for the quark-hadron phase transition. It is observed that the intermittent type of fluctuations are strong enough in [Formula: see text]-[Formula: see text] space compared to the one-dimensional [Formula: see text] space and [Formula: see text] space. The data reflects the signature of multifractality in [Formula: see text] space and [Formula: see text]-[Formula: see text] space. Whereas in case of [Formula: see text] space, it shows the evidence of monofractality. It is also interesting to note that the values of critical exponents for all impact parameters indicate the apparent existence of quark hadron phase transition except for MB events. However, no such behaviour is observed in [Formula: see text] space. From the knowledge of generalized fractal dimension [Formula: see text], multifractal specific heat is derived from scale factorial moment analysis in case of one-dimensional [Formula: see text] space, [Formula: see text] space and two-dimensional [Formula: see text]-[Formula: see text] space.

In-medium hadronization of heavy quarks and its effect on charmed meson and baryon distributions in heavy-ion collisions

We present a new model for the description of heavy-quark hadronization in relativistic heavy-ion collisions in the presence of a reservoir of lighter thermal particles with which recombination can occur leading to the formation of color-singlet clusters. Color neutralization is assumed to occur locally, within the same fluid cell occupied by the heavy quark and it proceeds via the recombination of the latter with light antiquarks (quarks) or diquarks (anti-diquarks), which are assumed to be present in the medium with thermal abundance as effective degrees of freedom around the QCD crossover temperature T_c. Typically the resulting color-singlet clusters have quite low invariant mass, in most of the cases below 4 GeV, and in this case they are assumed to undergo an isotropic 2-body decay in their local rest-frame. Heavier clusters are instead fragmented as Lund strings. The possibility of recombination with light diquarks enhances the yields of charmed baryons, in qualitative agreement with recent measurements. The assumption of local color neutralization leads to a strong space-momentum correlation, which provides a substantial enhancement of the collective flow of the final-state charmed hadrons, affecting both their momentum and their angular distributions.

Implications of NICER for Neutron Star Matter: The QHC21 Equation of State

The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation of radii from 1.4 to 2.1 M ⊙, reveal key features of the equation of state of neutron star matter. The pressure rises rapidly in the regime of baryon density n ∼ 2–4 times nuclear saturation density, n 0—the region where we expect hadronic matter to be undergoing transformation into quark matter—and the pressure in the nuclear regime is greater than predicted by microscopic many-body variational calculations of nuclear matter. To incorporate these insights into the microscopic physics from the nuclear to the quark matter regimes, we construct an equation of state, QHC21, within the framework of quark–hadron crossover. We include nuclear matter results primarily based on the state-of-the-art chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces. We employ explicit nuclear degrees of freedom only up to n ∼ 1.5 n 0, in order to explore the possibility of further physical degrees of freedom than nucleonic here. The resulting QHC21, which has a peak in sound velocity in ∼2–4 n 0, is stiffer than the earlier QHC19 below 2 n 0, predicting larger radii in substantial agreement with the NICER data.

Strange hadron spectroscopy with the KLong facility at Jefferson lab

The strange quark hadrons sit at an important crossroads between the light and heavy quark hadrons, but their spectrum is comparatively poorly known. The KLF experiment was recently approved to run in Hall D of Jefferson Lab, and will use an intense secondary beam of KL mesons with the existing GlueX spectrometer to collect data several orders of magnitude larger than existing dataset. In this talk, I will discuss the expected physics reach of this experiment and the status of its preparations

Measurement of the inclusive and differential t overline{t} γ cross sections in the dilepton channel and effective field theory interpretation in proton-proton collisions at sqrt{s} = 13 TeV

The production cross section of a top quark pair in association with a photon is measured in proton-proton collisions in the decay channel with two oppositely charged leptons (e±μ∓, e+e−, or μ+μ−). The measurement is performed using 138 fb−1 of proton-proton collision data recorded by the CMS experiment at sqrt{s} = 13 TeV during the 2016–2018 data-taking period of the CERN LHC. A fiducial phase space is defined such that photons radiated by initial-state particles, top quarks, or any of their decay products are included. An inclusive cross section of 175.2 ± 2.5(stat) ± 6.3(syst) fb is measured in a signal region with at least one jet coming from the hadronization of a bottom quark and exactly one photon with transverse momentum above 20 GeV. Differential cross sections are measured as functions of several kinematic observables of the photon, leptons, and jets, and compared to standard model predictions. The measurements are also interpreted in the standard model effective field theory framework, and limits are found on the relevant Wilson coefficients from these results alone and in combination with a previous CMS measurement of the t overline{t} γ production process using the lepton+jets final state.