Charm Quark Research Articles

Energy loss versus energy gain of heavy quarks in a hot medium

We study the energy loss and the energy gain of heavy quarks in a hot thermal medium. These include the study of the energy change due to the polarization and to the interaction with the thermal fluctuations of the medium. The dynamics of the heavy quarks with the medium is described by the Wong equations, that allow for the inclusion of both the backreaction on the heavy quarks due to the polarization of the medium, and of the interaction with the thermal fluctuations of the gluon field. Both the momentum as well as the temperature dependence of the energy loss and gain of charm and bottom quark are studied. We find that heavy quark energy gain dominate the energy loss at high-temperature domain achievable at the early stage of the high energy collisions. This finding supports the recently observed heavy quarks results in Glasma and will have a significant impact on heavy quark observables at RHIC and LHC energies.

Scale setting the Möbius domain wall fermion on gradient-flowed HISQ action using the omega baryon mass and the gradient-flow scales t0 and w0

We report on a subpercent scale determination using the omega baryon mass and gradient-flow methods. The calculations are performed on 22 ensembles of ${N}_{f}=2+1+1$ highly improved, rooted staggered sea-quark configurations generated by the MILC and CalLat Collaborations. The valence quark action used is M\"obius domain wall fermions solved on these configurations after a gradient-flow smearing is applied with a flowtime of ${t}_{\mathrm{gf}}=1$ in lattice units. The ensembles span four lattice spacings in the range $0.06\ensuremath{\lesssim}a\ensuremath{\lesssim}0.15\text{ }\text{ }\mathrm{fm}$, six pion masses in the range $130\ensuremath{\lesssim}{m}_{\ensuremath{\pi}}\ensuremath{\lesssim}400\text{ }\text{ }\mathrm{MeV}$ and multiple lattice volumes. On each ensemble, the gradient-flow scales ${t}_{0}/{a}^{2}$ and ${w}_{0}/a$ and the omega baryon mass $a{m}_{\mathrm{\ensuremath{\Omega}}}$ are computed. The dimensionless product of these quantities is then extrapolated to the continuum and infinite volume limits and interpolated to the physical light, strange and charm quark mass point in the isospin limit, resulting in the determination of $\sqrt{{t}_{0}}=0.1422(14)\text{ }\text{ }\mathrm{fm}$ and ${w}_{0}=0.1709(11)\text{ }\text{ }\mathrm{fm}$ with all sources of statistical and systematic uncertainty accounted for. The dominant uncertainty in both results is the stochastic uncertainty, though for $\sqrt{{t}_{0}}$ there are comparable continuum extrapolation uncertainties. For ${w}_{0}$, there is a clear path for a few-per-mille uncertainty just through improved stochastic precision, as recently obtained by the Budapest-Marseille-Wuppertal Collaboration.

Flavor Decomposition for the Proton Helicity Parton Distribution Functions.

We present, for the first time, an abinitio calculation of the individual up, down, and strange quark helicity parton distribution functions for the proton. The calculation is performed within the twisted mass clover-improved fermion formulation of lattice QCD. The analysis is performed using one ensemble of dynamical up, down, strange, and charm quarks with a pion mass of 260MeV. The lattice matrix elements are nonperturbatively renormalized and the final results are presented in the MS[over ¯] scheme at a scale of 2GeV. We give results for Δu^{+}(x), Δd^{+}(x) Δu^{-}(x), Δd^{-}(x), including disconnected quark loop contributions, as well as for Δs^{+}(x) and Δs^{-}(x). For the latter we achieve unprecedented precision compared to the phenomenological estimates.

New Tetraquark Spotted in Electron-Positron Collisions

The detection of a new particle containing both charm and strange quarks could offer new insights into how hadrons form.

Ballistic diffusion of heavy quarks in the early stage of relativistic heavy ion collisions at RHIC and the LHC

We study the diffusion of charm quarks in the early stage of high energy nuclear collisions at the RHIC and the LHC. The main novelty of the present study is the introduction of the color current carried by the heavy quarks that propagate in the evolving glasma (EG), which is responsible of the energy loss via polarization of the medium. We compute the transverse momentum broadening, ${\ensuremath{\sigma}}_{p}$, of charm in the prethermalization stage, and the impact of the diffusion on the nuclear modification factor in nucleus-nucleus collisions. The net effect of energy loss is marginal in the prethermalization stage. The study is completed by the calculation of coordinate spreading, ${\ensuremath{\sigma}}_{x}$, and by a comparison with Langevin dynamics. ${\ensuremath{\sigma}}_{p}$ in EG overshoots the result of standard Langevin dynamics at the end of the prehydro regime. We interpret this as a result of memory of the color force acting on the charm quarks that implies ${\ensuremath{\sigma}}_{p}\ensuremath{\propto}{t}^{2}$. Moreover, ${\ensuremath{\sigma}}_{x}\ensuremath{\propto}{t}^{2}$ in the prehydro stage shows that the charm quark in the EG is in the regime of ballistic diffusion.

Nucleon axial and pseudoscalar form factors from lattice QCD at the physical point

We compute the nucleon axial and induced pseudoscalar form factors using three ensembles of gauge configurations, generated with dynamical light quarks with mass tuned to approximately their physical value. One of the ensembles also includes the strange and charm quarks with their mass close to physical. The latter ensemble has large statistics and finer lattice spacing and it is used to obtain final results, while the other two are used for assessing volume effects. The pseudoscalar form factor is also computed using these ensembles. We examine the momentum dependence of these form factors as well as relations based on pion pole dominance and the partially conserved axial-vector current hypothesis.

Investigation of quark distributions in a family of pentaquarks using the Thomas–Fermi quark model

Using the Thomas–Fermi quark model, a collective, spherically symmetric density of states is created to represent a gas of interacting fermions with various degeneracies at zero temperature. Over a family of pentaquarks, , color interaction probabilities were obtained after averaging over all the possible configurations. Three different functions are developed for light, charm, and anti-charm quarks and are assumed to be linearly related by some proportionality constants. Interesting patterns of quark distributions are observed while analyzing the quark function consistency conditions for such constants.

Investigation of multi-quark systems

In this study, the researcher decided to study Tetraquark and Pentaquark systems. We introduce the Pentaquark system as a meson-baryon system and Tetraquark as a diquark-antidiquark system. A diquark consists of two quarks. The interaction potential between these particles is considered as a combination of two color charge potentials and the confinement potential is considered as harmonic and linear sentences. Solve the Schrodinger equation by considering the effects of spin-spin, isospin- isospin, spin-isospin, and then obtain the mass of the diquark and the heavy tetraquuarks with charm quarks and bottom. These calculations are calculated using the approach of diquark - antidiquark within the framework of the non-relativistic quark model. In the following, we calculate the mass of Pentaquark systems in the same way.

Fermion spectrum and g-2 anomalies in a low scale 3-3-1 model

We propose a renormalizable theory based on the SU(3)_Ctimes SU(3)_Ltimes U(1)_X gauge symmetry, supplemented by the spontaneously broken U(1)_{L_g} global lepton number symmetry and the S_3 times Z_2 discrete group, which successfully describes the observed SM fermion mass and mixing hierarchy. In our model the top and exotic quarks get tree level masses, whereas the bottom, charm and strange quarks as well as the tau and muon leptons obtain their masses from a tree level Universal seesaw mechanism thanks to their mixing with charged exotic vector like fermions. The masses for the first generation SM charged fermions are generated from a radiative seesaw mechanism at one loop level. The light active neutrino masses are produced from a loop level radiative seesaw mechanism. Our model successfully accommodates the experimental values for electron and muon anomalous magnetic dipole moments.

Excited and exotic bottomonium spectroscopy from lattice QCD

We explore the spectrum of excited and exotic bottomonia using lattice QCD. Highly excited states are identified with masses up to 11,000 MeV, many of which can be grouped into supermultiplets matching those of the quark model while exotic spin-parity-charge-conjugation quantum numbers JPC = 0+−, 1−+, 2+− that cannot be formed from overline{q}q alone are also identified. Single-meson operator constructions are used that have good JPC in the continuum, these are found to overlap well onto heavy quark states up to J ≤ 4. A continuum JPC is assigned to each level, based on the distribution amongst lattice irreps and dominant operator overlaps. States with a dominant gluonic component are identified and form a hybrid supermultiplet with JPC = (0, 1, 2)−+, 1−−, approximately 1500 MeV above the ground-state ηb, similar to previous computations with light, strange and charm quark systems.

The search for charmed states of matter.

With the discovery of the Higgs boson in 2012, the Standard Model of particle physics was completed. This description encompasses all known subatomic particles and their interactions. Much of the public interest in high-energy physics now focuses on experimental searches for ‘new physics’ beyond the Standard Model. Yet it would be a mistake to imagine that the Standard Model is now fully understood. Many questions remain about the ways in which known fundamental particles interact and unite, especially at the very high energies needed to produce the most exotic varieties, such as heavy quarks (quarks are the constituents of hadrons, which include the ordinary nuclear particles protons and neutrons) and heavy leptons (leptons are members of the family that includes electrons).The Beijing Electron-Positron Collider (BEPC), operated by the Chinese Academy of Sciences' Institute of High Energy Physics, is one of the installations that are probing these questions. It has been running since 1988, using a detector called the Beijing Spectrometer (BES). (The site also houses the Beijing Synchrotron Radiation Facility for conducting studies in condensed matter using intense X-rays.) Since 2008, these two instruments have been operating in upgraded form: the BESIII detector and BEPCII accelerator. The facility is now one of the key international centers for investigating the properties and behavior of new exotic hadrons, in particular those that include the charm quark. Italian physicist Luciano Maiani, Director of the European particle physics center CERN in Switzerland from 1999 to 2003, is one of the world leaders in this area of high-energy physics, and played a central role in the identification of the charm quark itself. NSR spoke to him about the aims of the latest work at the BEPCII, and the prospects for new discoveries.

Mixing angles of [formula omitted] in heavy quark effective theory

The Heavy Quark Effective Theory provides a systematic method to estimate a mixing angles of hadron states in a heavy quark, such as charm quark (c) and bottom quark (b). Using this method, we can estimate the mixing angles of the baryons ΞQ−ΞQ′. The mixing angles between ΞQ and ΞQ′ are given by |θb|=4.51∘±0.79∘ for the Q=b case and |θc|=8.12∘±0.80∘ for the Q=c case.

Mellin moments ⟨x⟩ and ⟨x2⟩ for the pion and kaon from lattice QCD

We present a calculation of the pion quark momentum fraction, $\langle x \rangle$, and its third Mellin moment $\langle x^2 \rangle$. We also obtain directly, for the first time, $\langle x \rangle$ and $\langle x^2 \rangle$ for the kaon using local operators. We use an ensemble of two degenerate light, a strange and a charm quark ($N_f=2+1+1$) of maximally twisted mass fermions with clover improvement. The quark masses are chosen so that they reproduce a pion mass of about 260 MeV, and a kaon mass of 530 MeV. The lattice spacing of the ensemble is 0.093 fm and the lattice has a spatial extent of 3 fm. We analyze several values of the source-sink time separation within the range of $1.12-2.23$ fm to study and eliminate excited-states contributions. The necessary renormalization functions are calculated non-perturbatively in the RI$'$ scheme, and are converted to the $\overline{\rm MS}$ scheme at a scale of 2 GeV. The final values for the momentum fraction are $\langle x \rangle^\pi_{u^+}=0.261(3)_{\rm stat}(6)_{\rm syst}$, $\langle x \rangle^K_{u^+}=0.246(2)_{\rm stat}(2)_{\rm syst}$, and $\langle x \rangle^K_{s^+}=0.317(2)_{\rm stat}(1)_{\rm syst}$. For the third Mellin moments we find $\langle x^2 \rangle^\pi_{u^+}=0.082(21)_{\rm stat}(17)_{\rm syst}$, $\langle x^2 \rangle^K_{u^+}=0.093(5)_{\rm stat}(3)_{\rm syst}$, and $\langle x^2 \rangle^K_{s^+}=0.134(5)_{\rm stat}(2)_{\rm syst}$. The reported systematic uncertainties are due to excited-state contamination. We also give the ratio $\langle x^2 \rangle/\langle x \rangle$ which is an indication of how quickly the PDFs lose support at large $x$.

Relation between the MS¯ and the kinetic mass of heavy quarks

We compute the relation between the pole mass and the kinetic mass of a heavy quark to three loops. Using the known relation between the pole and the $\overline{\rm MS}$ mass we obtain precise conversion relations between the $\overline{\rm MS}$ and kinetic masses. The kinetic mass is defined via the moments of the spectral function for the scattering involving a heavy quark close to threshold. This requires the computation of the imaginary part of a forward scattering amplitude up to three-loop order. We discuss in detail the expansion procedure and the reduction to master integrals. For the latter analytic results are provided. We apply our result both to charm and bottom quark masses. In the latter case we compute and include finite charm quark mass effects. Furthermore, we determine the large-$\beta_0$ result for the conversion formula at four-loop order. For the bottom quark we estimate the uncertainty in the conversion between the $\overline{\rm MS}$ and kinetic masses to about 15 MeV which is an improvement by a factor two to three as compared to the two-loop formula. The improved precision is crucial for the extraction of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$ at Belle II.

Deciphering the Nature of X(3872) in Heavy Ion Collisions.

Exploring the nature of exotic multiquark candidates such as the X(3872) plays a pivotal role in understanding quantum chromodynamics (QCD). Despite significant efforts, consensus on their internal structures is still lacking. As a prime example, it remains a pressing open question to decipher the X(3872) state between two popular exotic configurations: a loose hadronic molecule or a compact tetraquark. We demonstrate a novel approach to help address this problem by studying the X(3872) production in heavy ion collisions, where a hot fireball with ample light as well as charm (anti-)quarks is available for producing the exotics. Adopting a multiphase transport model (AMPT) for describing such collisions and implementing appropriate production mechanism of either molecule or tetraquark picture, we compute and compare a series of observables for X(3872) in Pb-Pb collisions at the Large Hadron Collider. We find the fireball volume plays a crucial role, leading to a 2-order-of-magnitude difference in the X(3872) yield and a markedly different centrality dependence between hadronic molecules and compact tetraquarks, thus offering a unique opportunity for distinguishing the two scenarios. We also make the first prediction of X(3872) elliptic flow coefficient to be tested by future experimental measurements.

Deconfinement and Hadron Resonance Gas for Heavy Quarks

I discuss the deconfinement transition in 2+1 flavor QCD in terms of Polyakov loops as well as the hadron resonance gas for hadrons containing static quarks and charm quarks.

Energy loss of heavy quarks in the isotropic collisional hot QCD medium at a finite chemical potential

The present article is the followup of Eur. Phys. J. C 79, 761 (2019), where we studied the energy loss of the heavy quarks traversing through the isotropic collisional hot QCD medium. As the QCD phase diagram at finite baryon density and the moderate temperature is expected to be traced by the upcoming experimental facilities such as Anti-proton and Ion Research (FAIR) and Nuclotron-based Ion Collider fAcility (NICA), the theoretical research needs the inclusion of finite chemical potential to study the hot QCD medium. Therefore, the aim here is to develop a formalism that investigates the collisional energy loss of heavy quarks moving in the hot QCD medium having small but a finite quark chemical potential. The calculation uses the extended effective fugacity quasi-particle model [1–3] and the effective kinetic theory with Bhatnagar-Gross-Krook (BGK) collisional kernel. The change in the energy of charm and bottom quarks with respect to their corresponding momenta has been investigated at different values of collisions frequency and chemical potential. We find that the bottom quark loses less energy than the charm quark for a fixed momentum, collisional frequency and chemical potential. The energy loss values are found to decrease with increasing chemical potential.

Elliptic flow of hadrons in equal-velocity quark combination mechanism in relativistic heavy-ion collisions

We apply a quark combination model with equal-velocity combination (EVC) approximation to study the elliptic flow (v_{2}) of hadrons in heavy-ion collisions in a wide collision energy range (sqrt{s_{NN}}= 27–5020 GeV). Utilizing the simple relationship between v_{2} of hadrons and those of quarks under EVC, we find that v_{2} of up/down quarks obtained by experimental data of proton is consistent with that obtained by data of varLambda and varXi . v_{2} of strange quarks obtained by data of varOmega is consistent with that obtained by data of varLambda and varXi , and at RHIC energies it is also consistent with that obtained by data of phi . This means that v_{2} of these hadrons have a common quark-level source. Using data of D^0, we obtain v_{2} of charm quarks with p_Tlesssim 6 GeV/c. We find that under EVC charm quark dominates v_{2} of D mesons at low p_{T} but light-flavor quarks significantly contribute to v_{2} of D mesons in the range 3lesssim p_{T}lesssim 8 GeV/c. We predict v_{2} of charmed baryons varLambda _{c}^{+} and varXi _{c}^{0} which show a significant enhancement at intermediate p_{T} due to the double contribution of light-flavor quarks. The properties of the obtained quark v_{2} under EVC are studied and a regularity for v_{2} of quarks as the function of p_{T}/m is found.

Hadron Yields in Central Nucleus–Nucleus Collisions, the Statistical Hadronization Model and the QCD Phase Diagram

The description of hadron production in relativistic heavy-ion collisions in the statistical hadronization model is very good, over a broad range of collision energy. We outline this both for the light (u, d, s) and heavy (charm) quarks and discuss the connection it brings to the phase diagram of QCD.

ΛcN interaction in leading-order covariant chiral effective field theory

We study the $\Lambda_c N$ interaction in the covariant chiral effective field theory (ChEFT) at leading order. All the relevant low-energy constants are determined by fitting to the lattice QCD simulations from the HAL QCD Collaboration. Extrapolating the results to the physical point, we show that the $\Lambda_c N$ interaction is weakly attractive in the $^1S_0$ channel, but in the $^3S_1$ channel, it is only attractive at extremely low energies and soon turns repulsive for larger laboratory energy. Furthermore, we show that the neglect of the $^3S_1-{}^3D_1$ coupling provided by the leading order covariant ChEFT would result in an attractive interaction in the $^3S_1$ channel at the physical point, which coincides with the previous non-relatistic ChEFT study. As a byproduct, we predict the $^3D_1$ phase shifts and the mixing angel $\varepsilon_1$, which can be checked by future lattice QCD simulations. In addition, we compare the $\Lambda_c N$ interaction with the $\Lambda N$ and $NN$ interactions to study how the baryon-nucleon ($BN$) interactions evolve as a function of the baryon mass with the replacement of a light quark by a strange or charm quark in the baryon ($B$).