arXiv subjects

Sz. Borsanyi

Publications and source records attributed to Sz. Borsanyi.

29 records · Page 2Linked to original sources

arXiv2005

Nonequilibrium quantum fields from first principles

Calculations of nonequilibrium processes become increasingly feasable in quantum field theory from first principles. There has been important progress in our analytical understanding based on 2PI generating functionals. In addition, for the first time direct lattice simulations based on stochastic quantization techniques have been achieved. The quantitative descriptions of characteristic far-from-equilibrium time scales and thermal equilibration in quantum field theory point out new phenomena such as prethermalization. They determine the range of validity of standard transport or semi-classical approaches, on which most of our ideas about nonequilibrium dynamics were based so far. These are crucial ingredients to understand important topical phenomena in high-energy physics related to collision experiments of heavy nuclei, early universe cosmology and complex many-body systems.

hep-th↗

arXiv2005

Isotropization far from equilibrium

Isotropization occurs on time scales much shorter than the thermal equilibration time. This is a crucial ingredient for the understanding of collision experiments of heavy nuclei or other nonequilibrium phenomena in complex many body systems. We discuss in detail the limitations of estimates based on standard ``linear'' or relaxation-time approximations, where isotropization and thermal equilibration rates agree. For a weak-coupling $ϕ^4$-model the relaxation-time approximation underestimates the thermal equilibration time by orders of magnitude, in contrast to the isotropization time. The characteristic nonequilibrium isotropization rate can be enhanced as compared to the close-to-equilibrium value. Our results are obtained from the two-particle irreducible effective action, which includes off-shell and memory effects and does not involve a gradient expansion. This allows us to determine the range of validity of a description to lowest-order in gradients, which is typically employed in kinetic equations.

hep-ph↗

arXiv2005

Renormalized thermodynamics from the 2PI effective action

High-temperature resummed perturbation theory is plagued by poor convergence properties. The problem appears for theories with bosonic field content such as QCD, QED or scalar theories. We calculate the pressure as well as other thermodynamic quantities at high temperature for a scalar one-component field theory, solving a three-loop 2PI effective action numerically without further approximations. We present a detailed comparison with the two-loop approximation. One observes a strongly improved convergence behavior as compared to perturbative approaches. The renormalization employed in this work extends previous prescriptions, and is sufficient to determine all counterterms required for the theory in the symmetric as well as the spontaneously broken phase.

hep-ph↗

arXiv2003

Non-equilibrium Goldstone phenomenon in tachyonic preheating

The dominance of the direct production of elementary Goldstone waves is demonstrated in tachyonic preheating by numerically determining the evolution of the dispersion relation, the equation of state and the kinetic power spectra for the angular degree of freedom of the complex matter field. The importance of the domain structure in the order parameter distribution for the quantitative understanding of the excitation mechanism is emphasized. Evidence is presented for the very early decoupling of the low-momentum Goldstone modes.

hep-ph↗

arXiv2003

Thermalization of fermionic quantum fields

We solve the nonequilibrium dynamics of a 3+1 dimensional theory with Dirac fermions coupled to scalars via a chirally invariant Yukawa interaction. The results are obtained from a systematic coupling expansion of the 2PI effective action to lowest non-trivial order, which includes scattering as well as memory and off-shell effects. The dynamics is solved numerically without further approximation, for different far-from-equilibrium initial conditions. The late-time behavior is demonstrated to be insensitive to the details of the initial conditions and to be uniquely determined by the initial energy density. Moreover, we show that at late time the system is very well characterized by a thermal ensemble. In particular, we are able to observe the emergence of Fermi--Dirac and Bose--Einstein distributions from the nonequilibrium dynamics.

hep-ph↗

arXiv2003

Nonequilibrium Goldstone phenomenon in Hybrid Inflation

We study the onset of Goldstone phenomenon in a hybrid inflation scenario. The physically motivated range of parameters is analyzed in order to meet the cosmological constraints. Classical equations of motion are solved and the evolution through the spontaneous symmetry breaking is followed. We emphasize the role of topological defects that partially maintain the disordered phase well after the waterfall. We study the emergence of the Goldstone excitations and their role in the onset of the radiation dominated universe.

hep-ph↗

arXiv2002

Goldstone excitations from spinodal instability

The squared mass of a complex scalar field is turned dynamically into negative by its O(2)-invariant coupling to a real field slowly rolling down in a quadratic potential. The emergence of gapless excitations is studied in real time simulations after spinodal instability occurs. Careful tests demonstrate that the Goldstone modes appear almost instantly after the symmetry breaking is over, much before thermal equilibrium is established.

hep-ph↗

arXiv2001

Ising-like dynamical signatures and the end-point of the QCD transition line

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.

hep-ph↗

arXiv2001

Relaxation of 2+1 dimensional classical O(2) symmetric scalar fields

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.

hep-ph↗

arXiv2000

Fate of the classical false vacuum

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.

hep-th↗

arXiv1999

Dynamical Manifestation of the Goldstone Phenomenon at 1-loop

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.

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