Quarks and gluons in color fields

Production and collective motion ofq \(\bar q\) and gluon pairs in strong color fields is studied using boost-invariant version of collisionless Boltzmann-Vlasov transport equations. It is shown that, for realistic fields, the effective formation time for pair production according to Schwinger mechanism is a small fraction of 1 fermi. Collective oscillations of quarkgluon gas, although damped by pair production and expansion of the system, remain an important feature of the process. The considered mechanism produces quark and gluons in a state which is rather far from equilibrium and thus strong collision effects (collision time ?1 fm) are needed to achieve equilibrated distributions at formation time. Finally, it is observed that, if such equilibrated distribution is produced at later times, the expansion of the system modifies dramatically its properties, as is explicitly shown for small oscillations of quark-gluon plasma in a weak external field.     

Strange Quarks and Lattice QCD

The last few years have seen a dramatic improvement in our knowledge of the strange form factors of the nucleon. With regard to the vector from factors the level of agreement between theory and experiment gives us considerable confidence in our ability to calculate with non-perturbative QCD. The calculation of the strange scalar form factor has moved significantly in the last 2 years, with the application of new techniques which yield values considerably smaller than believed for the past 20 years. These new values turn out to have important consequences for the detection of neutralinos, a favourite dark matter candidate. Finally, very recent lattice studies have resurrected interest in the famed H-dibaryon, with modern chiral extrapolation of lattice data suggesting that it may be only slightly unbound. We review some of the major sources of uncertainty in that chiral extrapolation.     

Quarks and gluons in a model with induced geometry

It is shown that the QCD-Lagrangian describing interacting gluons and uncharged quarks can be consistently introduced in a classical field model involving induced geometry. A discussion of quark confinement is given.     

In-medium QCD and Cherenkov gluons

The equations of in-medium gluodynamics are proposed. Their classical lowest order solution is explicitly shown for a color charge moving with constant speed. For nuclear permittivity larger than 1 it describes emission of Cherenkov gluons resembling the results of classical electrodynamics. The choice of nuclear permittivity and Lorentz invariance of the problem are discussed. Effects induced by the transversely and longitudinally moving (relative to the collision axis) partons at LHC energies are described.     

Hadrons in dense QCD

The theoretical foundation of the in-medium hadron properties is discussed from the point of view of the QCD spectral functions. The dynamical models of in-medium hadrons proposed so far are also summarized.     

QCD in the nuclear medium and effects due to Cherenkov gluons

The equations of in-medium gluodynamics are proposed. Their classical lowest-order solution is explicitly shown for a color charge moving with constant speed. For chromopermittivity larger than 1 it describes emission of Cherenkov gluons resembling results of classical electrodynamics. The values of the real and imaginary parts of the chromopermittivity are obtained from the fits to experimental data on the double-humped structure around the away-side jet obtained at RHIC. The dispersion of the chromopermittivity is predicted by comparing the RHIC, SPS, and cosmic-ray data. This is important for LHC experiments. Cherenkov gluons may be responsible for the asymmetry of dilepton mass spectra near ρ meson observed in the SPS experiment with excess in the low-mass wing of the resonance. This feature is predicted to be common for all resonances. The “color rainbow” quantum effect might appear according to higher-order terms of in-medium QCD if the chromopermittivity depends on color.     

Deconfinement in dense two-color QCD

We study SU(2) lattice gauge theory with two flavors of Wilson fermion at non-zero chemical potential μ and low temperature on a 8³×16 system. We identify three régimes along the μ-axis. For μ≲m_π/2 the system remains in the vacuum phase and all physical observables considered remain essentially unchanged. The intermediate régime is characterised by a non-zero diquark condensate and an associated increase in the baryon density, consistent with what is expected for Bose–Einstein condensation of tightly bound diquarks. We also observe screening of the static quark potential here. In the high-density deconfined régime we find a non-zero Polyakov loop and a strong modification of the gluon propagator, including significant screening in the magnetic sector in the static limit, which must have a non-perturbative origin. The behaviour of thermodynamic observables and the superfluid order parameter are consistent with a Fermi surface disrupted by a BCS diquark condensate. The energy per baryon as a function of μ exhibits a minimum in the deconfined régime, implying that macroscopic objects such as stars formed in this theory are largely composed of quark matter.     

Kinetic equation for gluons in the background gauge of QCD

We derive the quantum kinetic equation for a pure gluon plasma, applying the background field and closed-time-path method. The derivation is more general and transparent than earlier works. A term in the equation is found which, as in the classical case, corresponds to the color charge precession for partons moving in the gauge field.     

An infrared singularity in the damping rate for longitudinal gluons in hot QCD

We calculate γ_l(0), the damping rate for longitudinal on-shell gluons with zero momentum in hot QCD using the hard-thermal-loop scheme, and find it to be divergent in the infrared. We argue that this result suggests that the scheme may be sensitive in this sector and thus may need to be improved upon. We discuss the validity of our result and emphasize the fact that it depends on the way we perform our calculation. In particular, we do not exclude the possibility that alternative approaches that we briefly comment on may in principle yield infrared-safe results.     

Meson screening masses at high temperature in quenched QCD with improved Wilson Quarks

We report on a lattice investigation of improved quenched Wilson fermions above and below the confinement-deconfinement phase transition. Results on meson screening masses as well as spatial wave functions are presented. Moreover, the meson dispersion relation is studied. Below the critical temperature we do not observe any significant temperature effect while above the data are consistent with a leading free quark behavior. Received: 2 April 2001 / Published online: 8 June 2001     

Curvature of Pseudocritical Transition Line for Two-Flavor QCD with Improved Kogut-Susskind Quarks

The results on the curvature of a pseudocritical transition line for two-flavor QCD through lattice simulations are presented. The simulations are carried out with Symanzik-improved gauge action and Asqtad fermion action on a lattice 12~3×4 at quark mass am = 0.010. At the imaginary chemical potentials aμ_I = 0.050, 0.150, 0.200,0.225 and 0.250, we investigate the chiral condensate φφ, plaquette variable P and imaginary part of Polyakov loop Im(L) and their susceptibilities. Analytic continuation from an imaginary chemical potential to a real one is used to obtain the expression for transition temperature as a function of the chemical potential. The eurvature is 0.0326(46).     

Neutral Larkin-Ovchinnikov-Fulde-Ferrell state and chromomagnetic instability in two-flavor dense QCD

In two-flavor dense quark matter, we describe the dynamics in the single plane wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state satisfying the color and electric neutrality conditions. We find that because the neutral LOFF state itself suffers from a chromomagnetic instability in the whole region where it coexists with the (gapped or gapless) two-flavor superconducting phases, it cannot cure this instability in those phases. This is unlike the recently revealed gluonic phase which seems to be able to resolve this problem.     

Fluctuations and correlations of soft gluons at the nonperturbative stage of evolution of QCD jets

At the nonperturbative stage of jet evolution, fluctuations of soft gluons are less than those for coherent states under specific conditions. This fact suggests that there can arise squeezed gluon states. The angular and rapidity dependences of the gluon correlation function are investigated at this stage of jet evolution. It is shown that these new states of soft gluons can display sub-Poissonian or super-Poissonian statistics corresponding to, respectively, antibunching and bunching of gluons, by analogy with squeezed photon states.