arXiv subjects

A. Jakovac

Publications and source records attributed to A. Jakovac.

At least 55 records · Page 3Linked to original sources

arXiv2004

Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect

Recently the electron dephasing and energy relaxation due to different magnetic impurities have been extensively investigated experimentally in thin wires and in this Letter these quantities are theoretically studied. It was shown earlier that a magnetic impurity in a metallic host with strong spin-orbit interaction experiences a surface anisotropy of the form $H=K_d ({\bf n}{\bf S})^2$ which causes size effects for impurities with integer spin. Here we show that the dephasing and the energy relaxation are influenced by the surface anisotropy in very different ways for integer spin having a singlet ground state. That must result also in strong size effects and may resolve the puzzle between the concentrations estimated from the two kind of experiments.

cond-mat.str-el↗

arXiv2004

Non-Gibbs Particle Spectra from Thermal Equilibrium

We propose that transverse momentum spectra with power-law tails observed in ultrarelativistic heavy ion collisions can be interpreted as originating in a medium in thermal equilibrium. General conditions on the dynamical equations are formulated, leading in equilibrium to various energy distributions of particles. Starting with the linear Fokker-Planck equation we analyze conditions for Boltzmann-Gibbs, Tsallis, or other equilibrium distributions based upon the dependence of fluctuation and dissipation on the energy of the observed subsystem. RHIC neutral pion data definitely exclude Boltzmann-Gibbs distribution over the entire range of observed transverse momenta.

hep-ph↗

arXiv2004

The nature of the soft excitation at the critical end point of QCD

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).

hep-ph↗

arXiv2004

Renormalization and resummation in field theories

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.

hep-ph↗

arXiv2004

Analytic determination of the T-μphase diagram of the chiral quark model

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.

hep-ph↗

arXiv2004

Renormalization and resummation in finite temperature field theories

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.

hep-ph↗

arXiv2003

T-μphase diagram of the chiral quark model from a large flavor number expansion

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.

hep-ph↗

arXiv2003

Quantum corrections to Boltzmann equations

Because of IR (pinch) singularities a resummation is necessary in non-equilibrium field theories, that can be performed by using Kadanoff--Baym equations. Taking Landau prescription correctly into account, Kadanoff--Baym equations reduce to Boltzmann equations only in a restricted kinematical range; in other cases a new equation (the former constraint equation) has to be considered. In relaxation time approximation this new equation results in the shifting and smearing of multiparticle thresholds.

hep-ph↗

arXiv2002

Generalized Boltzmann equations for on-shell particle production in a hot plasma

A novel refinement of the conventional treatment of Kadanoff--Baym equations is suggested. Besides the Boltzmann equation another differential equation is used for calculating the evolution of the non-equilibrium two-point function. Although it was usually interpreted as a constraint on the solution of the Boltzmann equation, we argue that its dynamics is relevant to the determination and resummation of the particle production cut contributions. The differential equation for this new contribution is illustrated in the example of the cubic scalar model. The analogue of the relaxation time approximation is suggested. It results in the shift of the threshold location and in smearing out of the non-analytic threshold behaviour of the spectral function. Possible consequences for the dilepton production are discussed.

hep-ph↗

arXiv2001

Time evolution in linear response: Boltzmann equations and beyond

In this work a perturbative linear response analysis is performed for the time evolution of the quasi-conserved charge of a scalar field. One can find two regimes, one follows exponential damping, where the damping rate is shown to come from quantum Boltzmann equations. The other regime (coming from multiparticle cuts and products of them) decays as power law. The most important, non-oscillating contribution in our model comes from a 4-particle intermediate state and decays as 1/t^3. These results may have relevance for instance in the context of lepton number violation in the Early Universe.

hep-ph↗

arXiv2001

Four-dimensional lattice results on the MSSM electroweak phase transition

We present the results of our large scale 4-dimensional (4d) lattice simulations for the MSSM electroweak phase transition (EWPT). We carried out infinite volume and continuum limit extrapolations and found a transition whose strength agrees well with perturbation theory. We determined the properties of the bubble wall that are important for a successful baryogenesis.

hep-ph↗

arXiv2001

Effect of the scalar condensate on the linear gauge field response in the Abelian Higgs model

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.

hep-ph↗

arXiv2000

Resummation and damping in the O(N) model

In this talk I summarize the one loop and higher loop calculations of the effective equations of motion of the O(N) symmetric scalar model in the linear response approximation. At one loop one finds essential difference in long time behavior for the fields below and above a dynamically generated length scale. A partial resummation assuming quasi-particle propagation seems to cancel the relevance of this scale.

hep-ph↗

arXiv2000

Electroweak Phase Transition in the MSSM: 4-Dimensional Lattice Simulations

Recent lattice results have shown that there is no Standard Model (SM) electroweak phase transition (EWPT) for Higgs boson masses above \approx 72 GeV, which is below the present experimental limit. According to perturbation theory and 3-dimensional (3d) lattice simulations there could be an EWPT in the Minimal Supersymmetric Standard Model (MSSM) that is strong enough for baryogenesis up to m_h \approx 105 GeV. In this letter we present the results of our large scale 4-dimensional (4d) lattice simulations for the MSSM EWPT. We carried out infinite volume and continuum limits and found a transition whose strength agrees well with perturbation theory, allowing MSSM electroweak baryogenesis at least up to m_h = 103 \pm 4 GeV. We determined the properties of the bubble wall that are important for a successful baryogenesis.

hep-ph↗

arXiv1999

Dynamical resummation and damping in the O(N) model

A general real-time formalism is developed to resum the self-energy operator of broken symmetry scalar field theories in form of self-consistent gap equations for the spectral function. The solution of the equations is approximated with finite lifetime quasi-particles. In the Landau damping rates viscosity terms, analogous to gauge theories, appear, what leads to a finite damping rate for the long wavelength Goldstone modes.

hep-ph↗

arXiv1999

Effective theory for the soft fluctuation modes in the spontaneously broken phase of the N-component scalar field theory

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.

hep-ph↗

arXiv1998

Viscosity of the Scalar Fields from the Classical Theory

We show how the resummation for time dependent quantities at high temperature can be performed with an effective classical theory. As an application we demonstrate that the leading term in the shear viscosity, which is related to the $ρ_{Φ^2Φ^2}$ spectral function can be calculated classically, either using classical linear response theory or from the classical $Φ^2$ correlation function. The classical result depends explicitly on the cutoff, and the choice $Λ\sim T$ reproduces the known quantum result.

hep-ph↗

arXiv1997

Classical Limit for Scalar Fields at High Temperature

We study real-time correlation functions in scalar quantum field theories at temperature $T=1/β$. We show that the behaviour of soft, long wavelength modes is determined by classical statistical field theory. The loss of quantum coherence is due to interactions with the soft modes of the thermal bath. The soft modes are separated from the hard modes by an infrared cutoff $Ł\ll 1/(\hbarβ)$. Integrating out the hard modes yields an effective theory for the soft modes. The infrared cutoff $Ł$ controls corrections to the classical limit which are $\cO{\hbar\betaŁ}$. As an application, the plasmon damping rate is calculated.

hep-th↗