Quark Correlations Research Articles

T-odd quark–gluon–quark correlation function in the diquark model

We study the transverse momentum dependent quark–gluon–quark correlation function. Using a spectator diquark model, we calculate the eight time-reversal-odd interaction-dependent twist-3 quark distributions appearing in the decomposition of the transverse momentum dependent quark–gluon–quark correlator. In order to obtain finite results, we assume a dipole form factor for the nucleon–quark–diquark coupling, instead of a point-like coupling. The results are compared with the time-reversal-odd interaction-independent twist-3 TMDs calculated in the same model.

Diquarks in the nilpotency expansion of QCD and their role at finite chemical potential

We assume that the most important quark correlations are pairwise at all baryon densities. We introduce correlated pairs by means of Bogoliubov transformations which are functions of time and spatial gauge fields, in the formalism of the transfer matrix with lattice regularization. The dependence on time and gauge fields allows us to enforce gauge invariance and other symmetries term by term in the transformed quantities. The resulting action should be suitable for the description of multiquark mesons and baryons as states of a quark and a diquark. We derive the quark contribution to the free energy at finite chemical potential in a certain approximation. Its expression cannot be evaluated analytically, but it has a definite sign.

Are there hadronic bound states above the QCD transition temperature?

Recent lattice QCD calculations, at physical pion masses and small lattice spacings that approach the continuum limit, have revealed that non-diagonal quark correlators above the critical temperature are finite up to about 2 $T_c$. Since the transition from hadronic to free partonic degrees of freedom is merely an analytic cross-over, it is likely that, in the temperature regime between 1-2 $T_c$, quark and gluon quasiparticles and pre-hadronic bound states can coexist. The correlator values, in comparison to PNJL model calculations beyond mean-field, indicate that at least part of the mixed phase resides in color-neutral bound states. A similar effect was postulated for the in-medium fragmentation process, i.e. for partons which do not thermalize with the system and thus constitute the non-equilibrium component of the particle emission spectrum from a deconfined plasma phase. Here, for the first time we investigate the likelihood of forming bound states also in the equilibrated, parton dominated phase above $T_c$ which is described by lattice QCD.

Correlations of heavy quarks produced at the Large Hadron Collider

We study the correlations of heavy quarks produced in relativistic heavy-ion collisions and find them to be quite sensitive to the effects of the medium and the production mechanisms. In order to put this on a quantitative footing, as a first step, we analyze the azimuthal, transverse momentum, and rapidity correlations of heavy quark–antiquark () pairs in pp collisions at (α3s). This sets the stage for the identification and study of medium modification of similar correlations in the relativistic collision of heavy nuclei at the Large Hadron Collider. Next we study the additional production of charm quarks in heavy ion collisions due to multiple scatterings, namely jet–jet collisions, jet–thermal collisions, and thermal interactions. We find that these give rise to azimuthal correlations which are quite different from those arising from the prompt initial production at leading order and at next to leading order.Communicated by Professor Steffen Bass.

Distributions and correlations of constituent quarks in jets

In the frame of the quark recombination model, we study the momentum distributions and correlations of constituent quarks in jets by analyzing the final state hadrons generated by PYTHIA for the hard parton fragmentation processes in vacuum. Parameterizations for the distributions are tabulated.

Top quark spin correlations at the Tevatron and the LHC

Spin correlations of top quarks produced in hadron collisions have not been observed experimentally with large significance. In this Letter, we propose a new variable that may enable demonstration of the existence of spin correlations with 3–4 σ significance using just a few hundred dilepton events both at the Tevatron and the LHC. Such number of dilepton events has been observed at the Tevatron. At the LHC, it will become available once integrated luminosity of a few hundred inverse picobarns is collected.

Transverse spin asymmetries for W-production in proton–proton collisions

We study parity-even and parity-odd polarization observables for the process pp→l±X, where the lepton comes from the decay of a W-boson. By using the collinear twist-3 factorization approach, we consider the case when one proton is transversely polarized, while the other is either unpolarized or longitudinally polarized. These observables give access to two particular quark–gluon–quark correlation functions, which have a direct relation to transverse momentum dependent parton distributions. We present numerical estimates for RHIC kinematics. Measuring, for instance, the parity-even transverse single spin correlation would provide a crucial test of our current understanding of single spin asymmetries in the framework of QCD.

Spectral representation for baryon to meson transition distribution amplitudes

We construct a spectral representation for the baron to meson transition distribution amplitudes (TDAs), i.e. matrix elements involving three quark correlators which arise in the description of baryon to meson transitions within the factorization approach to hard exclusive reactions. We generalize for these quantities the notion of double distributions introduced in the context of generalized parton distributions. We propose the generalization of A.Radyushkin's factorized Ansatz for the case of baryon to meson TDAs. Our construction opens the way to modeling of baryon to meson TDAs in their complete domain of definition and quantitative estimates of cross-sections for various hard exclusive reactions.

Heavy-quark production from relativistic heavy-ion collisions

We study the production of heavy quarks, charm at BNL-RHIC ( = 200 GeV/nucleon) and CERN-LHC ( = 5.5 TeV/nucleon) and bottom at CERN-LHC, from heavy ions colliding at relativistic energies. We consider initial fusion of gluons (and quark–anti-quark annihilation), pre-thermal parton interactions and interactions in a thermalized quark–gluon plasma. We also consider free-streaming partons as another extreme and compare the results with those from a thermalized plasma of partons. The pre-thermal contribution is calculated by considering the interaction among partons having large transverse momenta (jet–partons) after the initial interaction and from the passage of these partons through a thermalized quark–gluon plasma. Charm production from pre-thermal processes is found to be comparable to that from prompt (initial) interactions at LHC. It is suggested that this may have important implications for the study of the nuclear modification factor, RAA, as well as for the back-to-back correlation of heavy quarks and the production of dileptons having a large mass.

A path integral for heavy quarks in a hot plasma

We propose a model for the propagation of a heavy quark in a hot plasma, to be viewed as a first step towards a full description of the dynamics of heavy quark systems in a quark–gluon plasma, including bound state formation. The heavy quark is treated as a non-relativistic particle interacting with a fluctuating field, whose correlator is determined by a hard thermal loop approximation. This approximation, which concerns only the medium in which the heavy quark propagates, is the only one that is made, and it can be improved. The dynamics of the heavy quark is given exactly by a quantum mechanical path integral that is calculated in this paper in the Euclidean space–time using numerical Monte Carlo techniques. The spectral function of the heavy quark in the medium is then reconstructed using a Maximum Entropy Method. The path integral is also evaluated exactly in the case where the mass of the heavy quark is infinite; one then recovers known results concerning the complex optical potential that controls the long time behavior of the heavy quark. The heavy quark correlator and its spectral function is also calculated semi-analytically at the one-loop order, which allows for a detailed description of the coupling between the heavy quark and the plasma collective modes.

A fundamental test of the Higgs Yukawa coupling at RHIC in A+A collisions

Searches for the intermediate boson, W±, the heavy quantum of the Weak Interaction, via its semi-leptonic decay, W → e+v, in the 1970's instead discovered unexpectedly large hadron production at high pT, notably π0, which provided a huge background of e± from internal and external conversions. Methods developed at the CERN ISR which led to the discovery of direct-single-e± in 1974, later determined to be from the semi-leptonic decay of charm which had not yet been discovered, were used by PHENIX at RHIC to make precision measurements of heavy quark production in p-p and Au+Au collisions, leading to the puzzle of apparent equal suppression of light and heavy quarks in the QGP. If the Higgs mechanism gives mass to gauge bosons but not to fermions, then a proposal that all 6 quarks are nearly massless in a QGP, which would resolve the puzzle, can not be excluded. This proposal can be tested with future measurements of heavy quark correlations in A+A collisions

Coulomb gauge confinement in the heavy quark limit

The relationship between the nonperturbative Green's functions of Yang-Mills theory and the confinement potential is investigated. By rewriting the generating functional of quantum chromodynamics in terms of a heavy quark mass expansion in Coulomb gauge, restricting to leading order in this expansion and considering only the two-point functions of the Yang-Mills sector, the rainbow-ladder approximation to the gap and Bethe-Salpeter equations is shown to be exact in this case and an analytic, nonperturbative solution is presented. It is found that there is a direct connection between the string tension and the temporal gluon propagator. Further, it is shown that for the 4-point quark correlation functions, only confined bound states of color-singlet quark-antiquark (meson) and quark-quark (baryon) pairs exist.

Next-to-leading order QCD corrections to heavy quark correlations in longitudinally polarized hadron-hadron collisions

We present a comprehensive phenomenological study of heavy flavor distributions and correlations in longitudinally polarized proton-proton collisions at BNL-RHIC. All results are obtained with a flexible parton-level Monte Carlo program at next-to-leading order accuracy and include the fragmentation into heavy mesons, their subsequent semileptonic decays, and experimental cuts. Next-to-leading order QCD corrections are found to be significant for both cross sections and double-spin asymmetries. The sensitivity of heavy flavor measurements at BNL-RHIC to the gluon polarization of the nucleon is assessed. Electron-muon and muon-muon correlations turn out to be the most promising observables. Theoretical uncertainties are estimated by varying renormalization and factorization scales, heavy quark masses, and fragmentation parameters.

THE SPIN STRUCTURE OF THE NUCLEON IN LIGHT-CONE QUARK MODELS

The quark spin densities related to generalized parton distributions in impact-parameter space and to transverse-momentum dependent parton distributions are reviewed within a light-cone quark model, with focus on the role of the different spin-spin and spin-orbit correlations of quarks. Results for azimuthal spin asymmetries in semi-inclusive deep inelastic scattering due to T -even transverse-momentum dependent parton distributions are also discussed.

LONGITUDINALLY POLARIZED HADRO- AND PHOTOPRODUCTION OF HEAVY QUARKS

Measurements of double-spin asymmetries for single-inclusive heavy flavor production and for heavy quark correlations are expected to provide valuable information about the spin structure of the nucleon, in particular, the elusive gluon polarization. We discuss progress towards a versatile parton-level Monte-Carlo code for the production and decay of heavy quarks in longitudinally polarized hadron-hadron and photon-hadron collisions at next-to-leading order accuracy of QCD. Phenomenological studies are presented for BNL-RHIC and COMPASS.

Spectral Properties of Quarks at Finite Temperature in Lattice QCD

We analyze the quark spectral function above and below the critical temperature for deconfinement and at finite momentum in quenched lattice QCD. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. The bare quark mass and momentum dependences of the spectral function are analyzed with this ansatz. In the chiral limit we find that even near the critical temperature the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature ( T) limit. The pole mass of these modes at zero momentum, which should be identified to be the thermal mass of the quark, is approximately proportional to T in a rather wide range of T in the deconfined phase.

Quark propagator at finite temperature and finite momentum in quenched lattice QCD

We present an analysis of the quark spectral function above and below the critical temperature for deconfinement performed at zero and non-zero momentum in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. This indicates that excitation modes of the quark field have small decay rates. The bare quark mass and momentum dependence of the spectral function is analyzed with this ansatz. In the chiral limit we find that the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature limit. Over a rather wide temperature range in the deconfined phase the pole mass of these modes at zero momentum, which corresponds to the thermal mass of the quark, is approximately proportional to temperature. With increasing bare quark masses the plasmino mode gradually disappears and the spectral function is dominated by a single pole. We also discuss quasi-particle properties of heavy quarks in the deconfined phase. In the confined phase, it is found that the pole ansatz for the spectral function fails completely.

The light-front Nambu–Jona–Lasinio model at finite temperature and density

In recent years, light-front quantization has been extended to allow for a consistent treatment of systems at finite temperature and density. This is in particular interesting for an investigation of the processes in nuclear matter under extreme condition as occurring, e.g. during a heavy-ion collision. Utilizing a Dyson expansion to the N-point Green's functions at finite temperature and density, we investigate the properties of the nucleon in a medium of quark matter. Due to the Fock-space properties in light-front quantization these can be calculated within a properly modified in-medium Faddeev equation. To connect these results to those accessible by the instant form approach, we consider also two-body correlations, e.g. the occurrence of pionic and scalar diquark dynamics in quark matter and their Mott dissociation using a separable t-matrix approach. For the scalar quark–quark correlation, we determine the critical temperature of color superconductivity using the Thouless criterion.

On the existence of exotic and non-exotic multiquark meson states

To obtain an exact solution of a four-body system containing two quarks and two antiquarks interacting through two-body terms is a cumbersome task that has been tackled with more or less success during the last decades. We present an exact method for the study of four-quark systems based on the hyperspherical harmonics formalism that allows us to solve it without resorting to further approximations, like for instance the existence of diquark components. We apply it to systems containing two heavy and two light quarks using different quark-quark potentials. While $QQ\bar n \bar n$ states may be stable in nature, the stability of $Q\bar Qn \bar n$ states would imply the existence of quark correlations not taken into account by simple quark dynamical models.

A new molecular dynamics calculation and its application to the spectra of light and strange baryons

A new approach based on antisymmetrized molecular dynamics is proposed to correctly take account of the many-body correlations. We applied it to the spectra of low-lying, light and strange baryons. The inclusion of the quark–quark correlation is important to predict the precise spectra, and the semi- relativistic kinematics is also vital to correct the level ordering. The baryon spectra calculated by the present method is as precise as the Faddeev calculation.