Quark Contributions Research Articles

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.

Studies of Quark and Gluon Contributions to Jets using Jet Charge Measurements in pp and PbPb Collisions

Jet charge, defined as the momentum-weighted sum of the electric charges of particles inside a jet is sensitive to the electric charge of the initiating parton and can be used to study the color charge dependence of the parton energy loss in the QGP. In this paper, the first measurements of jet charge in heavy-ion collisions are presented using lead-lead (PbPb) collision data and compared to results from proton-proton (pp) data at the same collision energy. The measurements are unfolded for detector and background effects and are studied differentially in pT and additionally as a function of collision centrality in PbPb collisions. We also present a template fitting technique for estimating the fractions of quark- and gluon-initiated jets in pp and PbPb collisions based on Monte Carlo templates. This analysis uses pp and PbPb collision data collected by the CMS experiment at sNN=5.02TeV.

Heavy and light flavor jet quenching in heavy-ion collisions in a perturbative QCD approach

We study the flavor dependence of jet quenching in heavy-ion collisions. A next-to-leading-order perturbative QCD framework is applied to account for both quark and gluon contributions to light and heavy flavor hadron productions. A linear Boltzmann transport model coupled to hydrodynamic simulation is utilized to study the nuclear modification of heavy and light flavor jets in the dynamically evolving quark-gluon plasma. Within our jet quenching framework, we obtain a nice description of the nuclear modification factors of charged hadrons, D mesons, B mesons and B–decayed D mesons over a wide range of transverse momentum (8-300 GeV). Our result shows that perturbative QCD is sufficient to explain the color, mass and energy dependence of parton energy loss and jet quenching in heavy-ion collisions.

X0,1(2900) and (D−K+) invariant mass from QCD Laplace sum rules at NLO

We revisit, improve and complete some recent estimates of the 0+ and 1− open charm (c¯d¯)(us) tetraquarks and the corresponding molecules masses and decay constants from QCD spectral sum rules (QSSR) by using QCD Laplace sum rule (LSR) within stability criteria where the factorised perturbative NLO corrections and the contributions of quark and gluon condensates up to dimension-6 in the OPE are included. We confront our results with the D−K+ invariant mass recently reported by LHCb from B+→D+(D−K+) decays. We expect that the bump near the D−K+ threshold can be originated from the 0++(D−K+) molecule and/or D−K+ scattering. The prominent X0 (2900) scalar peak and the bump XJ(3150) (if J=0) can emerge from a minimal mixing model, with a tiny mixing angle θ0≃(5.2±1.9)0, between a scalar Tetramole (TM0) (superposition of nearly degenerated hypothetical molecules and compact tetraquarks states with the same quantum numbers) having a mass MTM0 = 2743(18) MeV and the first radial excitation of the D−K+ molecule with mass M(DK)1=3678(310) MeV. In an analogous way, the X1 (2900) and the XJ(3350) (if J=1) could be a mixture between the vector Tetramole(TM1) with a mass MTM1=2656(20) MeV and its first radial excitation having a mass M(TM1)1=4592(141) MeV with an angle θ1≃(9.1±0.6)0. A (non)-confirmation of the previous minimal mixing models requires an experimental identification of the quantum numbers of the bumps at 3150 and 3350 MeV.

CP violation in the extension of SM with a complex singlet scalar and vector quarks

We consider the simplest extension of the SM with a complex singlet and a pair of heavy doublet vector quarks, the so-called cSMCS model. In this model, the CP violation can emerge spontaneously as a consequence of the time-dependent phase of the complex singlet vacuum expectation value and the mass mixing of the SM and heavy vector quarks. In our model, the CP-violating time-dependent phase depends on the Higgs field within the bubble-wall via the Higgs-singlet coupling that can directly explain the observed baryon-to-entropy ratio ∼9×10−11. Additionally, as a result of the mixing between vector quarks and SM quarks, the tree level Flavour Changing Neutral Currents and the charged-current decay channels t′→Wb,Zt,hit and b′→Wt,Zb,hib would arise that are well constrained by experimental data. We investigate the implications of these constraints on the total cross-section for the production of heavy vector quark pairs via pp→t′t′¯ and pp→b′b′¯ channels and obtain the bounds on the heavy quark masses for this model. Accordingly, we show the contribution of these heavy quarks in the Higgs bosons signal strength, Rγγ, as well as corrections to the gauge boson propagators.

Renormalization scale setting for D-meson mixing

A naive application of the heavy quark expansion (HQE) yields theory estimates for the decay rate of neutral $D$ mesons that are four orders of magnitude below the experimental determination. It is well known that this huge suppression results from severe GIM cancellations. We find that this mismatch can be solved by individually choosing the renormalisation scale of the different internal quark contributions. For $b$ and $c$ hadron lifetimes, as well as for the decay rate difference of neutral $B$ mesons the effect of our scale setting procedure lies within the previously quoted theory uncertainties, while we get enlarged theory uncertainties for the semi-leptonic CP asymmetries in the $B$ system.

Deciphering the mechanism of near-threshold J/psi photoproduction

The photoproduction of the J/psi off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a J/psi which rescatters with the proton to give the J/psi p final state. In this paper, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the varLambda _c bar{D}^{(*)} intermediate states. In particular, there must be cusp structures at the varLambda _c bar{D}^{(*)} thresholds in the energy dependence of the J/psi photoproduction cross section. The same mechanism also implies the J/psi -nucleon scattering lengths of order 1 mfm. Given this, one expects only a minor contribution of charm quarks to the nucleon mass.

Charge symmetry violation in the determination of strangeness form factors

The strange quark contributions to the electromagnetic form factors of the proton are ideal quantities to study the role of hidden flavor in the properties of the proton. This has motivated intense experimental measurements of these form factors. A major remaining source of systematic uncertainty in these determinations is the assumption that charge symmetry violation (CSV) is negligible. We use recent theoretical determinations of the CSV form factors and reanalyse the available parity-violating electron scattering data, up to $Q^2$ $\sim$ 1 GeV$^2$. Our analysis considers systematic expansions of the strangeness electric and magnetic form factors of the proton. The results provide an update to the determination of strangeness over a range of $Q^2$ where, under certain assumptions about the effective axial form factor, an emergence of non-zero strangeness is revealed in the vicinity of $Q^2$ $\sim$ 0.6 GeV$^2$. Given the recent theoretical calculations, it is found that the current limits on CSV do not have a significant impact on the interpretation of the measurements and hence suggests an opportunity for a next generation of parity-violating measurements to more precisely map the distribution of strange quarks.

Constraints on charm-anticharm asymmetry in the nucleon from lattice QCD

We present the first lattice QCD calculation of the charm quark contribution to the nucleon electromagnetic form factors GE,Mc(Q2) in the momentum transfer range 0≤Q2≤1.4GeV2. The quark mass dependence, finite lattice spacing and volume corrections are taken into account simultaneously based on the calculation on three gauge ensembles including one at the physical pion mass. The nonzero value of the charm magnetic moment μMc=−0.00127(38)stat(5)sys, as well as the Pauli form factor, reflects a nontrivial role of the charm sea in the nucleon spin structure. The nonzero GEc(Q2) indicates the existence of a nonvanishing asymmetric charm-anticharm sea in the nucleon. Performing a nonperturbative analysis based on holographic QCD and the generalized Veneziano model, we study the constraints on the [c(x)−c¯(x)] distribution from the lattice QCD results presented here. Our results provide complementary information and motivation for more detailed studies of physical observables that are sensitive to intrinsic charm and for future global analyses of parton distributions including asymmetric charm-anticharm distribution.

Light quark mediated Higgs boson threshold production in the next-to-leading logarithmic approximation

We study the amplitude of the Higgs boson production in gluon fusion mediated by a light quark loop and evaluate the logarithmically enhanced radiative corrections to the next-to-leading logarithmic approximation which sums up the terms of the form {alpha}_s^n{ln}^{2n-1}left({m}_H/{m}_qright) to all orders in the strong coupling constant. This result is used for the calculation of the process cross section near the production threshold and gives a quantitative estimate of the three and four-loop bottom quark contribution to the Higgs boson production at the Large Hadron Collider.

Covariant model for the Dalitz decay of the N(1535) resonance

We develop a covariant model for the $\gamma^\ast N \to N(1535)$ transition in the timelike kinematical region, the region where the square momentum transfer $q^2$ is positive. Our starting point is the covariant spectator quark model constrained by data in the spacelike kinematical region ($Q^2 = -q^2 >0$). The model is used to estimate the contributions of valence quarks to the transition form factors, and one obtains a fair description of the Dirac form factor at intermediate and large $Q^2$. For the Pauli form factor there is evidence that beyond the quark-core contributions there are also significant contributions of meson cloud effects. Combining the quark-core model with an effective description of the meson cloud effects, we derive a parametrization of the spacelike data that can be extended covariantly to the timelike region. This extension enabled us to estimate the Dalitz decay widths of the $N(1535)$ resonance, among other observables. Our calculations can help in the interpretation of the present experiments at HADES ($pp$ collisions and others).

Complete flavor decomposition of the spin and momentum fraction of the proton using lattice QCD simulations at physical pion mass

We evaluate the gluon and quark contributions to the spin of the proton using an ensemble of gauge configuration generated at physical pion mass. We compute all valence and sea quark contributions to high accuracy. We perform a non-perturbative renormalization for both quark and gluon matrix elements. We find that the contribution of the up, down, strange and charm quarks to the proton intrinsic spin is $\frac{1}{2}\sum_{q=u,d,s,c}\Delta\Sigma^{q^+}=0.191(15)$ and to the total spin $\sum_{q=u,d,s,c}J^{q^+}=0.285(45)$. The gluon contribution to the spin is $J^g=0.187(46)$ yielding $J=J^q+J^g=0.473(71)$ confirming the spin sum. The momentum fraction carried by quarks in the proton is found to be $0.618(60)$ and by gluons $0.427(92)$, the sum of which gives $1.045(118)$ confirming the momentum sum rule. All scale and scheme dependent quantities are given in the $\mathrm{ \overline{MS}}$ scheme at 2 GeV.

Multiplicity dependence of J/$$\psi $$ production and QCD dynamics in $$p+p$$ collisions at $$\sqrt{s}$$ = 13 TeV

In inelastic $p+p$ collisions, the interacting objects are quarks and gluons (partons). It is believed that there are multiple interactions between the partons in a single $p+p$ event. Recent studies of multiplicity dependence of particle production in $p+p$ collisions have gathered considerable interest in the scientific community. According to several theoretical calculations, multiple gluon participation in hadronic collisions is the cause of high-multiplicity events. If the interaction is hard enough (large $p_{\rm T}$ transfer), the semi-hard processes of multiple interactions of partons might also lead to production of heavy particles like J/$\psi$. At the LHC, an approximately linear increase of the relative J/$\psi$ yield with charged particle multiplicity is observed in $p+p$ collisions. In the present work, we have studied the contribution of quarks and gluons to the multiplicity dependence of J/$\psi$ production using pQCD inspired event generator, PYTHIA8 tune 4C, in $p+p$ collisions at $\sqrt{s} =$13 TeV by investigating relative J/$\psi$ yield and relative $\langle p_{\rm T} \rangle$ of J/$\psi$ as a function of charged particle multiplicity for different hard-QCD processes. We have estimated a newly defined ratio, $r_{pp} = {\langle p_{\rm T}^{2} \rangle}_{i}/{\langle p_{\rm T}^{2} \rangle}_{\rm MB}$, to understand J/$\psi$ production in high-multiplicity $p+p$ collisions. For the first time we attempt to study the nuclear modification factor like observables ($R_{\rm pp}$ and $R_{\rm cp}$) to understand the QCD medium formed in high-multiplicity $p+p$ collisions.

Quark and Polyakov-loop correlations in effective models at zero and nonvanishing density

The aim of this work is to shed light on some lesser known aspects of Polyakov-loop--extended chiral models (namely the Polyakov-loop extended Nambu--Jona-Lasinio and Quark-Meson models), especially on the correlation of the quark sector with the Polyakov loop. We show that the ordering of chiral and Polyakov-loop transitions and their difference in temperature as seen in lattice QCD calculations could be realized with a critical scale of the Polyakov-loop potential that is larger than the one in pure gauge theory. The comparison of the results for the Polyakov-loop susceptibility obtained using the self-consistent medium-dependent quark mass with those obtained while keeping these masses at a fixed value allows to disentangle chiral-symmetry restoration and center-symmetry breaking effects. Furthermore, a confined chirally restored phase is identified by a plateau in the quark contribution to thermodynamics and by sigma and pion spectral functions that coincide but have a small width. We also discuss that, for some large chemical potential values, the explicit center-symmetry breaking is so strong that statistical deconfinement is realized at infinitely small temperatures. Both the missing sensitivity of the Polyakov loop to the quark mass, except at close to the chiral transition, and the Polyakov loop being zero at zero temperature at all chemical potentials, can be interpreted as indications of a missing mechanism which accounts for the quark back-reaction on the Polyakov loop.

Flavor hierarchy of jet quenching in relativistic heavy-ion collisions

Relativistic heavy-ion experiments have observed similar quenching effects for (prompt) D mesons compared to charged hadrons for transverse momenta larger than 6-8 GeV, which remains a mystery since heavy quarks typically lose less energies in quark-gluon plasma than light quarks and gluons. Recent measurements of the nuclear modification factors of B mesons and B-decayed D mesons by the CMS Collaboration provide a unique opportunity to study the flavor hierarchy of jet quenching. Using a linear Boltzmann transport model combined with hydrodynamics simulation, we study the energy loss and nuclear modification for heavy and light flavor jets in high-energy nuclear collisions. By consistently taking into account both quark and gluon contributions to light and heavy flavor hadron productions within a next-to-leading order perturbative QCD framework, we obtain, for the first time, a satisfactory description of the experimental data on the nuclear modification factors for charged hadrons, D mesons, B mesons and B-decayed D mesons simultaneously over a wide range of transverse momenta (8-300 GeV). This presents a solid solution to the flavor puzzle of jet quenching and constitutes a significant step towards the precision study of jet-medium interaction. Our study predicts that at transverse momenta larger than 30-40 GeV, B mesons also exhibit similar suppression effects to charged hadrons and D mesons, which may be tested by future measurements.

Improved pion mean fields and masses of singly heavy baryons

Abstract A singly heavy baryon can be viewed as $N_c-1$ ($N_c$ being the number of colors) light valence quarks bound by the pion mean fields that are created by the presence of the $N_c-1$ valence quarks self-consistently, while the heavy quark inside a singly heavy baryon is regarded as a static color source. We investigate how the pion mean fields are created by the presence of $N_c$, $N_c-1$, and $N_c-2$ light valence quarks, which correspond to the systems of light baryons, singly heavy baryons, and doubly heavy baryons. As the number of colors decreases from $N_c$ to $N_c-1$, the pion mean fields undergo changes. As a result, the valence quark contributions to the moments of inertia of the soliton become larger than for $N_c$ valence quarks, whereas the sea quark contributions decrease systematically. On the other hand, the presence of the $N_c-2$ valence quarks is not enough to produce the strong pion mean fields, which leads to the classical soliton not being formed. This indicates that the pion mean-field approach is not suitable to describe doubly heavy baryons. We show that the mass spectra of the singly heavy baryons are better described by the improved pion mean fields, compared with the previous work in which the pion mean fields are assumed to be intact with $N_c$ varied.

Three loop QCD corrections to heavy quark form factors

Higher order calculations in perturbative Quantum Field Theories often produce coupled linear systems of differential equations which factorize to first order. Here we present an algorithm to solve such systems in terms of iterated integrals over an alphabet the structure of which is implied by the coefficient matrix of the given system. We apply this method to calculate the master integrals in the color–planar and complete light quark contributions to the three-loop massive form factors.

Sea quark contributions to nucleon electromagnetic form factors with the nonlocal chiral effective Lagrangian * * Supported by the National Natural Sciences Foundations of China (11475186), the Sino-German CRC 110 “Symmetries and the Emergence of Structure in QCD” project by NSFC (11621131001)

The sea quark contributions to the nucleon electromagnetic form factors of the up, down and strange quarks are studied with the nonlocal chiral effective Lagrangian. Both octet and decuplet intermediate states are included in the one loop calculations. Compared with the strange quark form factors, although their signs are the same, the absolute value of the light quark form factors are much larger. For both the electric and magnetic form factors, the contribution of the d quark is larger than of the u quark. The current lattice simulations of the light sea quark form factors are in between our results for the u and dquarks.

Alternative renormalizable SO(10) GUTs and data fitting

The alternative renormalizable minimal SO(10) model is composed of the Yukawa couplings with 10⊕120 Higgs fields, whereas the right-handed (RH) neutrino Majorana masses are generated via the Witten mechanism. The gauge coupling unification is achieved together with a unique pattern of the fermion masses and mixing at the grand unification scale due to additional contributions of vector-like quarks to the standard model renormalization group equations. We perform the fitting of the model to the experimental data of charged fermion masses and the CKM matrix. The best fit point is obtained with large pulls for mc, ms, mb, and mτ. For the modifications to the minimal model by adding either 10′ or 120′, a large deviation for the tau mass rules out all these models. In the case with the bottom and vector-like quark mixing, the mass matrices are well fitted the charged fermions but the bound on the light neutrino mass scale excludes this scenario. To ameliorate this deficit, we consider the two-step symmetry breaking scenario, SO(10)→SU(5)→SU(3)C×SU(2)L×U(1)Y, with the SO(10) breaking at the Planck scale leading to the radiatively generated RH neutrino Majorana masses being at the ordinary seesaw scale. For this case, we find the best fit point with χ2=7.8 consistent with experimental results including the neutrino sector. The largest deviation is 2.3σ corresponding to the strange quark mass. Hence, a more precise determination of the strange quark mass can test this model. For the best fit point, we find the effective Majorana neutrino mass of mββ=0.22 meV and the sum of light neutrino masses as Σ=0.078 eV, which are consistent with the current constraints from the search for the neutrinoless double beta decay and the CMB anisotropy measurement.

Inclusion of the excited states quark propagator to the nucleon electromagnetic form factors in the perturbative chiral quark model

Electromagnetic form factors of the nucleon are studied within the framework of the Perturbative Chiral Quark Model (PCQM). In a previous analysis of the nucleon electromagnetic form factors, the quark propagator used in the calculation was truncated to quark in the ground state only. The attempt to include the excited states quark propagator contribution to the neutron was performed but not on the proton. Therefore, in this work, we try to be consistent in the nucleonic sector by including the excited states quark propagator up to the second excited states to study the electromagnetic form factors of the nucleon. Our results are in good agreement with experimental data and show the significance of the contribution of the quark excited states propagator to the electromagnetic properties of the nucleon.