Pion polarizabilities at finite temperature

A first calculation of pion polarizabilities at finite temperature is presented. Compton scattering off a pion is examined in the Nambu-Jona-Lasinio model and an expansion in the inverse number of colors N _c is used. Quark loop, σ-pole and vector meson contributions to the invariant amplitude are considered. The calculated polarizabilities of the charged and neutral pions at T = 0 agree fairly well with experiment. For increasing temperature, the polarizabilities stay fairly constant until they diverge at the Mott temperature (T _M = 202.2 MeV), above which the pion can decay into two quarks.     

Pion propagation in real time field theory at finite temperature

We describe how the thermal counterpart of a vacuum two-point function may be obtained in the real time formalism in a simple way by using directly the 2×2 matrices that different elements acquire in this formalism. Using this procedure we calculate the analytic (single component) thermal amplitude for the pion pole term in the ensemble average of two axial-vector currents to two loops in chiral perturbation theory. The general expressions obtained for the effective mass and the decay constants of the pion are evaluated in the chiral and the non-relativistic limits. PACS 11.10.Wx; 12.38.Mh; 12.39.Fe     

Axion decay in a constant electromagnetic background field and at finite temperature using world-line methods

We investigate the radiative decay of the axion into two photons in an external electromagnetic field to one loop order. Our approach is based on the world-line formalism, which is very suitable to take into account the external field to all orders. Afterwards we discuss how the calculation could be generalized to finite temperature. Received: 18 June 1998 / Published online: 19 October 1998     

Pion scalar density and chiral symmetry restoration at finite temperature and density

This paper is devoted to the evaluation of the pionic scalar density at finite temperature and baryonic density. We express the latter effect in terms of the nuclear response evaluated in the random phase approximation. We discuss the density and temperature evolution of the pionic density which governs the quark condensate evolution. Numerical evaluations are performed. Received: 7 December 1999 / Accepted: 8 May 2000     

Pion condensation at finite temperature: (I). Mean field theory

The pion-condensed state of neutron-rich matter at finite temperature is calculated within the framework of a simple σ-model, treating the pion field as a mean field. At high densities the matter is condensed with a spatially non-uniform condensate. However, we find the unexpected result that as the density is lowered, at any finite temperature, pure neutron matter always makes a transition to a state with a spatially uniform condensate. Pure neutron matter, within mean field theory, is condensed at all non-zero temperatures and densities. Matter with a small proton fraction at zero temperature has a qualitatively similar phase diagram, except that it is normal when both the temperature and density are sufficiently low.     

Vacuum decay and the effective action at finite temperature

The problem of vacuum decay at finite temperature is discussed. It is shown that, when the unstable minimum in the potential arises via quantum effects, the corrections to the kinetic terms cannot be ignored since they are singular. This means that previous adaptations of the instanton method to realistic models are suspect. We also discuss alternative methods of calculations which may be able to avoid the problem.     

Pion decay site spectroscopy in copper

With the aim of obtaining direct information on the sites occupied by positive pions (π⁺), channelling experiments have been performed on the decay muons (μ⁺) from π⁺ implanted into high-purity copper crystals. At 100 K virtually exclusive occupancy of octahedral interstitial sites (0) is found in good accordance with earlier μSR results. At room temperature a significant fraction of the implanted pions decay on metastable tetrahedral interstitial sites (T). The data suggest strongly that above about 100 K both μ⁺ and π⁺ diffuse by phonon-assisted tunnelling between stable 0 sites and metastable T sites. The enthalpy difference between these two sites is estimated to be about 50 meV for μ⁺ and of the same order of magnitude for π⁺. These values are much smaller than the available theoretical predictions. The implications of low-lying metastable interstitial sites for the μ⁺ stopping sites of low temperatures are discussed.     

Pion condensation at finite temperature: (II). Simple models including thermal excitations of the pion field

We calculate, in several increasingly complex models, the threshold for charged pion condensation in neutron-rich matter at finite temperature, including effects of thermal excitations of the pion field. The thermal pion excitations have two important effects: first, to modify the phase diagram qualitatively from that predicted by mean field theory, and second, to make the phase transition to a nonuniform condensed state at finite temperature always first, rather than second, order.     

Pion properties at finite isospin chemical potential with isospin symmetry breaking

Pion properties at finite temperature, finite isospin and baryon chemical potentials are investigated within the SU(2) NJL model. In the mean field approximation for quarks and random phase approximation fpr mesons, we calculate the pion mass, the decay constant and the phase diagram with different quark masses for the u quark and d quark, related to QCD corrections, for the first time. Our results show an asymmetry between μI 0 and μI 0 in the phase diagram, and different values for the charged pion mass(or decay constant) and neutral pion mass(or decay constant) at finite temperature and finite isospin chemical potential. This is caused by the effect of isospin symmetry breaking, which is from different quark masses.     

Dynamical bag constant and surface tension of baryon droplets at finite temperature and density

From the non-topological soliton bag model a droplet model for baryons at finite temperature and density is constructed. The resulting surface tension and dynamical bag constant is calculated as a function of temperature and density.     

Merons at finite temperature

We prove the existence of solutions to the SU(2) Yang—Mills equations on IR⁴, periodic in time, with meron singularities along the time-axis.Supported in part by the Icelandic Science Foundation.