arXiv subjects
Publications and source records attributed to Zs. Szep.
Using a large flavor number expansion and a gap equation for the pion mass the chiral quark-meson model is solved at the lowest order in the fermion contributions. In the chiral limit the tricritical point (TCP) is determined analytically. The softening of the sigma particle is verified at this point and is further investigated for a physical pion mass in the neighbourhood of the critical endpoint (CEP).
Using resummation in perturbation theories at finite temperature or in non-equilibrium is unavoidable to obtain consistent results. Resummation, however, is often in conflict with renormalization. In this talk we give two possible solutions to problems of this type in static resummation. One is the use of auxiliary finite temperature UV divergent counterterms. The other is to use a renormalization scheme fitted to the environment. We also suggest a way how one could proceed in case of dynamic resummations.
Using a gap equation for the pion mass a nonperturbative method is given for solving the chiral quark-meson model in the chiral limit at the lowest order in the fermion contributions encountered in a large N_f approximation. The location of the tricritical point is analytically determined. A mean field potential is constructed from which critical exponents can be obtained.
Resummation, ie. reorganization of perturbative series, can result in an inconsistent perturbation theory, unless the counterterms are reorganized in an appropriate way. In this paper two methods are presented for resummation of counterterms: one is a direct method where the necessary counterterms are constructed order by order; the other is a general one, based on renormalization group arguments. We demonstrate at one hand that, in mass independent schemes, mass resummation can be performed by gap equations renormalized prior to the substitution of the resummed mass for its argument. On the other hand it is shown that any (momentum-independent) form of mass and coupling constant resummation is compatible with renormalization, and one can explicitly construct the corresponding counterterms.
The chiral phase boundary of strong matter is determined in the T-μplane from the chiral quark model, applying a non-perturbatively renormalised treatment, involving chains of pion-bubbles and 1-loop fermion contributions. In the absence of explicit symmetry breaking the second order portion of the phase boundary and the location of the tricritical point (TCP) are determined analytically. Sensitivity of the results to the renormalisation scale is carefully investigated. The softening of the sigma-pole near the second order transitions is confirmed.
By assuming certain analytic properties of the propagator, it is shown that universal features of the spectral function including threshold enhancement arise if a pole describing a particle at high temperature approaches in the complex energy plane the threshold position of its two-body decay with the variation of T. The case is considered, when one can disregard any other decay processes. The quality of the proposed description is demonstrated by comparing it with the detailed large N solution of the linear sigma model around the pole-threshold coincidence.
The scalar-isoscalar propagator of the effective linear $σ$ model of meson dynamics is investigated with the help of an expansion in the number of the Goldstone bosons. A generic scenario is suggested for the temperature and density driven evolution of its pole in the second Riemann sheet. An extended temperature range, correlated with characteristic pole locations, is found where the phenomenon of threshold enhancement takes place in the corresponding spectral function.
The enhancement of the scalar-isoscalar spectral function near the two-pion threshold is studied in the framework of an effective linear $σ$ model, using a large N approximation in the number of the Goldstone bosons. The effect is rather insensitive to the detailed T=0 characteristics of the $σ$ pole, it is accounted by a pole moving with increasing $T$ along the real axis of the second Riemann sheet towards the threshold location from below.
The spectral function of the scalar-isoscalar channel of the O(N) symmetric linear $σ$ model is studied in the broken symmetry phase. The investigation is based on the leading order evaluation of the self-energy in the limit of large number of Goldstone bosons. We describe its temperature dependent variation in the whole low temperature phase. This variation closely reflects the trajectory of the scalar-isoscalar quasiparticle pole. In the model with no explicit chiral symmetry breaking we have studied near the critical point also the corresponding dynamical exponent.
The thermal evolution of the spectral densities derivable from the two-point functions of the elementary and the quadratic composite fields of the O(N) model is studied in the isosinglet channel and in the broken symmetry phase at infinite N. The results are applied with realistic parameter values to the N=4 case. They provide a reasonable description of the $σ$ meson at T=0. Threshold enhancement is observed around $T\sim 1.07m_π$. For higher temperatures the maximum of the spectral function in the single meson channel decreases and becomes increasingly rounded.
An increase in the size of coherent domains in the one component $Φ^4$ field theory under the influence of a uniformly changing external magnetic field near the critical end-point $T_Φ=T_c, h_Φ=0$ was proposed recently as an estimate also for the variation of the chiral correlation length of QCD near its respective hypothetical end point in the $T_{QCD}-μ_{QCD}$ plane. The present detailed numerical investigation of the effective model suggests that passing by the critical QCD end point with realistic rate of temperature change will trigger large amplitude oscillations in the temporal variation of the chiral correlation length. A simple mechanism for producing this phenomenon is suggested.
Real time thermalization and relaxation phenomena are studied in the low energy density phase of the 2+1 dimensional classical O(2) symmetric scalar theory by solving numerically its dynamics. The near-equilibrium decay rate of on-shell waves and the power law governing the large time asymptotics of the off-shell relaxation agree with the analytic results based on linear response theory. The realisation of the Mermin-Wagner theorem is also studied in the final equilibrium ensemble.
The effective equations of motion for low-frequency mean gauge fields in the Abelian Higgs model are investigated in the presence of a scalar condensate, near the high temperature equilibrium. We determine the current induced by an inhomogeneous background gauge field in the linear response approximation up to order $e^4$, assuming adiabatic variation of the scalar fields. The physical degrees of freedom are found and a physical gauge choice for the numerical study of the combined Higgs+gauge evolution is proposed.
Thermalisation of configurations with initial white noise power spectrum is studied in numerical simulations of a classical one-component $Φ^4$ theory in 2+1 dimensions, coupled to a small amplitude homogenous external field. The study is performed for energy densities corresponding to the broken symmetry phase of the system in equilibrium. The effective equation of the order parameter motion is reconstructed from its trajectory which starts from an initial value near the metastable point and ends in the stable ground state. This phenomenological theory quantitatively accounts for the decay of the false vacuum. The large amplitude transition of the order parameter between the two minima displays characteristics reflecting dynamical aspects of the Maxwell construction.
The complete set of static screening masses is determined for the SU(2) Higgs model from one-loop coupled gap equations. Results from the version, containing scalar fields both in the fundamental and adjoint representations are compared with the model arising when the integration over the adjoint scalar field is performed. A non-perturbative and non-linear mapping between the couplings of the two models is proposed, which exhibits perfect decoupling of the heavy adjoint scalar field. Also the alternative of a gauge invariant mass resummation is investigated in the high temperature phase.
The effective dynamics of the low-frequency modes is derived for the O(N) symmetric scalar field theory in the broken symmetry phase. The effect of the high-frequency fluctuations is taken into account at one-loop level exactly. A new length scale is shown to govern the long-time asymptotics of the linear response function of the Goldstone modes. The large time asymptotic decay of an arbitrary fluctuation is determined in the linear regime. We propose a set of local equations for the numerical solution of the effective non-linear dynamics. The applicability of the usual gradient expansion is carefully assessed.
We have calculated the damping rate $Γ(|{\bf k}|)$ for classical on-shell Goldstone modes of the O(2) symmetric scalar fields propagating in a thermal medium of the broken symmetry phase taking into account the effect of the explicit symmetry breaking. The result of the one-loop analysis can be expanded around $Γ(0)$, which depends non-analytically on the parameter of the explicit symmetry breaking, h. $Γ(0)$ vanishes when $h\to 0$, demonstrating in this way the absence of the restoring force, when the equilibrium direction of the symmetry breaking is modulated homogeneously.
The effective theory of low frequency fluctuations of selfinteracting scalar fields is constructed in the broken symmetry phase. The theory resulting from integrating fluctuations with frequencies much above the spontanously generated mass scale $(p_0>>M)$ is found to be local. Non-local dynamics, especially Landau damping emerges under the effect of fluctuations in the $p_0 \sim M$ region. A kinetic theory of relativistic scalar gas particles interacting via their locally variable mass with the low frequency scalar field is shown to be equivalent to this effective field theory for scales below the characteristic mass, that is beyond the accuracy of the Hard Thermal Loop (HTL) approximation.