Narrow Results

We make a first crude direct estimate of the net gluon polarization in the proton, ΔG, combining data on the asymmetries in high-p_T hadron production from the HERMES, SMC and COMPASS collaborations. Although these data sample a restricted range of x, they provide no hint that ΔG is large. Fixing the normalizations of different theoretical parametrizations using the hadron asymmetry data, we find typical central values of ΔG ~ 0.5, with uncertainties of similar magnitude. Values of ΔG≥2 are disfavored by Δχ² ~ 9 to 20, depending on the parametrization used.

In this paper we calculate the thermodynamic quantities and bulk viscosity in a simple confining Dyson–Schwinger equation model of two-flavor QCD. Our results show that the velocity of sound shows a sharp increase around and at the phase transition temperature the derivative of the velocity with respect to T has a distinct change. It is also found that the trace anomaly increases sharply below the phase transition point and the bulk viscosity has a peak located at which is almost the same as the peak of the trace anomaly. A comparison of our results with those in the previous literature is given.

In this paper, based on Kubo's formula and the QCD low energy theorem, we propose a direct formula for calculating the bulk viscosity of QCD at finite chemical potential μ and zero temperature. According to this formula, the bulk viscosity at finite μ is totally determined by the dressed quark propagator at finite μ. We then use a dynamical, confining Dyson–Schwinger equation model of QCD to calculate the bulk viscosity at finite μ. It is found that no sharp peak behavior of the bulk viscosity at finite μ is observed, which is quite different from that of the bulk viscosity at finite temperature.

We have explicitly shown that Quantum Chromodynamics is a color gauge invariant theory with non-zero mass gap, which has been defined as the value of the regularized full gluon self-energy at a finite scale point. The mass gap itself is mainly generated by the nonlinear interaction of massless gluon modes. All this allows one to establish the structure of the full gluon propagator in the explicit presence of the mass gap. In this case, the two independent general types of formal solutions for the full gluon propagator as a function of the regularized mass gap have been found: (i) The nonlinear iteration solution at which the gluons remain massless is explicitly present. (ii) Existence of the solution with an effective gluon mass is also demonstrated.

We have explicitly shown that Quantum Chromodynamics is a color gauge invariant theory with non-zero mass gap, which has been defined as the value of the regularized full gluon self-energy at a finite scale point. The mass gap itself is mainly generated by the nonlinear interaction of massless gluon modes. All this allows one to establish the structure of the full gluon propagator in the explicit presence of the mass gap. In this case, the two independent general types of formal solutions for the full gluon propagator as a function of the regularized mass gap have been found: (i) The nonlinear iteration solution at which the gluons remain massless is explicitly present. (ii) Existence of the solution with an effective gluon mass is also demonstrated.