Hisao Nakkagawa

Temperature dependence of the non-abelian gauge couplings at finite temperature

Abstract The temperature dependence of the non-abelian gauge couplings at finite temperature is calculated using the momentum-space subtraction method. The inverse of the coupling, a ( μ , ξ ≡ T / μ ) −1 ∼ b ln[ μ / Λ ( ξ )], shows essentially a power-like ξ (≡ T / μ ) dependence in sharp contrast with the usual logarithmic dependence on the renormalization scale μ. The strong vertex dependence of the coupling is clarified.

Vanishing thermal mass in the strongly coupled QCD/QED medium

In this paper we perform a nonperturbative analysis of a thermal quasifermion in thermal QCD/QED by studying its self-energy function through the Dyson-Schwinger equation with the hard-thermal-loop resummed improved ladder kernel. Our analysis reveals several interesting results, some of which may force us to change the image of the thermal quasifermion: (1) The thermal mass of a quasifermion begins to decrease as the coupling gets stronger and finally disappears in the strong coupling region,(2) the imaginary part of the chiral invariant mass function (i.e., the decay width of the quasifermion) persists to have

Leading T3-behavior and vertex-configuration dependence of real-time thermal QCD coupling

O(g^2 T \log (1/g))

How to resolve the factorization- and the renormalization-scheme ambiguities simultaneously?☆

behavior. Present results suggest that in the recently produced strongly coupled quark-gluon-plasma, the thermal mass of a quasifermion should vanish. We also briefly comment on evidence of the existence of a massless, or an ultrasoft mode.

Structure of a thermal quasifermion in the QCD/QED Medium

Abstract The properties of effective coupling in real-time thermal QCD are investigated at one-loop order in the large-T regime, focusing on its strong dependence on the momentum configuration of a vertex. A striking observation is that the leading large-T behavior in a general vertex-configuration is cubic in T, a−1 ∼ T3, which is solely controlled by those contributions that have been overlooked so far. The relation with the coupling defined in the imaginary time formalism is also discussed.

N-Point Vertex Functions, Ward-Takahashi Identities and Dyson-Schwinger Equations in Thermal QCD/QED in the Real Time Hard-Thermal-Loop Approximation

Abstract A combined investigation of both the factorization- and renormalization-scheme dependences of perturbative QCD calculations is reported. Applyong Stevensons optimization method, we get a remarkable result, which forces us to exponentiate “everything” with uncorrected subprocess cross sections.

Non-abelian gauge couplings in thermal gauge field theories☆

In this paper we carried out a nonperturbative analysis of a thermal quasifermion in the chiral symmetric thermal QCD/QED medium by studying its self-energy function through the Dyson-Schwinger equation with the hard-thermal-loop resummed improved ladder kernel. Our analysis reveals several interesting results, two in some of which may force us to change the image of thermal quasifermions: (1) The thermal mass of a quasifermion begins to decrease as the strength of the coupling gets stronger and finally disappears in the strong coupling region, thus showing a property of a massless particle, and (2) its imaginary part (i.e., the decay width) persists to have

RG IMPROVEMENT OF THE EFFECTIVE POTENTIAL AT FINITE TEMPERATURE

O(g^2T \log(1/g))

Chiral Phase Transitions in QED at Finite Temperature Dyson-Schwinger Equation Analysis in the Real Time Hard-Thermal-Loop Approximation

behavior. These results suggest that in the recently produced strongly coupled quark-gluon plasma, the thermal mass of a quasiquark should vanish. Taking into account the largeness of the imaginary part, it seems very hard for a quark to exist as a qausiparticle in the strongly coupled quark-gluon plasma phase. Other important findings are as follows: (3) The collective plasmino mode disappears also in the strongly coupled system, and (4) there exists an ultrasoft third peak in the quasifermion spectral density at least in the weakly coupled QED/QCD plasma, indicating the existence of the ultrasoft fermionic mode.

Further Ambiguity in Perturbative Quantum Chromodynamics

In this paper we calculate the n-point hard-thermal-loop (HTL) vertex functions in QCD/QED for n= 2, 3 and 4 in the physical representation using the real time formalism (RTF). The result reveals that the n-point HTL vertex functions can be classified into two groups, a) those with odd numbers of external retarded indices, and b) those with even numbers of external retarded indices. The n-point HTL vertex functions with one retarded index, which obviously belong to group a), are identical to the HTL vertex functions that appear in the imaginary time formalism (ITF). All the HTL vertex functions belonging to group a) are O(g 2 T 2 ) and satisfy among them the simple QED-type Ward-Takahashi identities, as in the ITF. Those vertex functions belonging to group b) never appear in the ITF, their existence being characteristic of the RTF, and their HTLs exhibit high temperature behavior that is O(g 2 T 3 ), one power of T higher than usual. Despite this difference, we were able to verify that those HTL vertex functions belonging to group b) also satisfy among themselves the QED-type Ward-Takahashi identities, thus guaranteeing the gauge invariance of the HTLs in the real time thermal QCD/QED. As an application, we explicitly derive the HTL resummed Dyson-Schwinger equations for the physical fermion mass function and for the gauge boson polarization tensor in thermal QCD/QED.