arXiv subjects

A. Niegawa

Publications and source records attributed to A. Niegawa.

9 recordsLinked to original sources

arXiv2006

Renormalization in Coulomb-gauge QCD within the Lagrangian formalism

We study renormalization of Coulomb-gauge QCD within the Lagrangian second-order formalism. We derive a Ward identity and the Zinn-Justin equation, and, with the help of the latter, we give a proof of algebraic renormalizability of the theory. Through diagrammatic analyses, we show that, in the strict Coulomb gauge, g^2D^{00} is invariant under renormalization. (D^{00} is the time-time component of the gluon propagator.)

hep-th↗

arXiv2006

Thermal Operator Representation of Finite-Temperature Amplitudes in the Presence of Chemical Potential

In a recent paper [Phys. Rev. D {\bf 72}, 085006 (2005)], Brandt {\em et al}. deduced the thermal operator representation for a thermal $N$-point amplitude, both in the imaginary-time and real-time formalisms. In the case when a chemical potential present, however, the representation is not as simple as in the case with vanishing chemical potential. We propose a much simpler and transparent representation for the case of non-zero chemical potential.

hep-ph↗

arXiv2006

Quantum Field Theories in Nonextensive Tsallis Statistics

Within the framework of Tsallis statistics with q ~ 1, we construct a perturbation theory for treating relativistic quantum field systems. We find that there appear initial correlations, which do not exist in the Boltzmann-Gibbs statistics. Applying this framework to a quark-gluon plasma, we find that the so-called thermal masses of quarks and gluons are smaller than in the case of Boltzmann-Gibbs statistics.

hep-th↗

arXiv2006

Improvement of the hot QCD pressure by the minimal sensitivity criterion

The principles of minimal sensitivity (PMS) criterion is applied to the perturbative free energy density, or pressure, of hot QCD, which include the $\sim g_s^6 \ln g_s$ and part of the $\sim g_s^6$ terms. Applications are made separately to the short- and long-distance parts of the pressure. Comparison with the lattice results, at low temperatures, shows that the resultant `` optimal'' approximants are substantially improved when compared to the $\bar{MS}$ results. In particular, for the realistic case of three quark flavors, the `` optimal'' approximants are comparable with the lattice results.

hep-ph↗

arXiv2004

Self-energy-part resummed quark and gluon propagators in a spin-polarized quark matter and generalized Boltzmann equations

We construct perturbative frameworks for studying nonequilibrium spin-polarized quark matter. We employ the closed-time-path formalism and use the gradient approximation in the derivative expansion. After constructing self-energy-part resummed quark and gluon propagators, we formulate two kind of mutually equivalent perturbative frameworks: The first one is formulated on the basis of the initial-particle distribution function, and the second one is formulated on the basis of `` physical''-particle distribution function. In the course of construction of the second framework, the generalized Boltzmann equations and their relatives {\em directly} come out, which describe the evolution of the system. The frameworks are relevant to the study of a magnetic character of quark matters, e.g., possible quark stars.

hep-ph↗

arXiv2004

Ferromagnetism in quark matter

We investigate the magnetic property of the cold quark matter employing the magnetic moment $\vecμ'$ as the order parameter. Through analysis of the effective potential $V (\vecμ^{' 2})$, we find that, at relatively high densities $ρ> ρ_c$, the quark matter is in the normal phase. At $ρ= ρ_c$, the magnetic phase transition takes place and, in the low-density region $ρ< ρ_c$, the quark matter is in the ferromagnetic phase. For up, down and strange quarks, $ρ_c = 1.62 ρ_0$, $1.47 ρ_0$ and $2.20 ρ_0$ ($ρ_0$ the nuclear density), respectively. We also find that the leptons ($e$ and $μ$) inside the quark matter are in the normal phase.

hep-ph↗

arXiv1999

Reaction-rate formula in out of equilibrium quantum field theory

A complete derivation, from first principles, of the reaction-rate formula for a generic reaction taking place in an out of equilibrium quantum-field system is given. It is shown that the formula involves no finite-volume correction. Each term of the reaction-rate formula represents a set of physical processes that contribute to the reaction under consideration.

hep-th↗

arXiv1999

A Slavnov-Taylor identity and equality of damping rates for static transverse and longitudinal gluons in hot QCD

A Slavnov-Taylor identity is derived for the gluon polarization tensor in hot QCD. We evaluate its implications for damping of gluonic modes in the plasma. Applying the identity to next to the leading order in hard-thermal-loop resummed perturbation theory, we derive the expected equality of damping rates for static transverse and longitudinal (soft) gluons. This is of interest also in view of deviating recent reports of $γ_t(p=0)\neqγ_l(p=0)$ based on a direct calculation of $γ_l(p=0)$.

hep-ph↗

arXiv1993

Analytic Continuation of Thermal $N$-Point Functions from Imaginary- to Real-Energies

We consider thermal $n$-point Green functions in the framework of quantum field theory at finite temperature. We show how analytic continuations from imaginary to real energies relate these functions originally defined in the imaginary-time formalism to retarded and advanced real-time ones. The described method is valid to all orders of perturbation theory. It has the further advantage that it is independent of approximations often applied in actual finite-order calculations.

hep-ph↗