arXiv subjects

Anton Rebhan

Publications and source records attributed to Anton Rebhan.

At least 73 records · Page 4Linked to original sources

arXiv2009

Meson supercurrents and the Meissner effect in the Sakai-Sugimoto model

The Sakai-Sugimoto model provides a holographic description for chiral symmetry breaking. We use this model to investigate chirally broken phases in an external magnetic field at finite isospin and baryon chemical potentials. The equations of motion for the bulk gauge fields are solved analytically and the free energy is computed from the Yang-Mills and Chern-Simons contributions to the D8 brane action. In the case of a neutral pion condensate, a magnetic field is found to induce nonzero gradients of the Goldstone boson fields corresponding to meson supercurrents. A charged pion condensate, on the other hand, expels the magnetic field due to the Meissner effect. Upon comparing the Gibbs free energies of these two phases we find that the rotation of the chiral condensate into a charged pion condensate for finite isospin chemical potentials is partially undone by switching on a magnetic field, and we determine the critical magnetic field which removes the charged pion condensate in a first-order phase transition.

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arXiv2008

Nonabelian plasma instabilities in Bjorken expansion

Plasma instabilities are parametrically the dominant nonequilibrium dynamics of a weakly coupled quark-gluon plasma. In recent years the time evolution of the corresponding collective colour fields has been studied in stationary anisotropic situations. Here I report on recent numerical results on the time evolution of the most unstable modes in a longitudinally expanding plasma as they grow from small rapidity fluctuations to amplitudes where non-Abelian self-interactions become important.

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arXiv2008

Instabilities of an anisotropically expanding non-Abelian plasma: 1D+3V discretized hard-loop simulations

Non-Abelian plasma instabilities play a crucial role in the nonequilibrium dynamics of a weakly coupled quark-gluon plasma and they importantly modify the standard perturbative bottom-up thermalization scenario in heavy-ion collisions. Using the auxiliary-field formulation of the hard-loop effective theory, we study numerically the real time evolution of instabilities in an anisotropic collisionless Yang-Mills plasma expanding longitudinally in free streaming. In this first real-time lattice simulation we consider the most unstable modes, long-wavelength coherent color fields that are constant in transverse directions and which therefore are effectively 1+1-dimensional in spacetime, except for the auxiliary fields which also depend on discretized momentum rapidity and transverse velocity components. We reproduce the semi-analytical results obtained previously for the Abelian regime and we determine the nonlinear effects which occur when the instabilities have grown such that non-Abelian interactions become important.

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arXiv2007

Perturbative Quantum Corrections to the Supersymmetric CP^1 Kink with Twisted Mass

We present an explicit calculation of the one-loop quantum corrections to the mass and the two central charges of the kink solution of an N=(2,2) supersymmetric CP^1 model with twisted mass, using supersymmetry preserving dimensional regularization adapted to solitons. We find that the quantum corrections of the mass and one of the central charges are nontrivial (but saturate the BPS bound), while the other central charge receives no corrections. The nontrivial central charge correction corresponds to a quantum anomaly, which in our scheme appears as parity violation in the regulating extra dimension, and its magnitude is in agreement with exact results obtained by Dorey on the basis of a massive analog of mirror symmetry from a dual U(1) gauge theory, confirming also the recent work by Shifman, Vainshtein, and Zwicky.

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arXiv2006

Plasma Instabilities in an Anisotropically Expanding Geometry

We study (3+1)D kinetic (Boltzmann-Vlasov) equations for relativistic plasma particles in a one-dimensionally expanding geometry motivated by ultrarelativistic heavy-ion collisions. We set up local equations in terms of Yang-Mills potentials and auxiliary fields that allow simulations of hard-(expanding)-loop (HEL) dynamics on a lattice. We determine numerically the evolution of plasma instabilities in the linear (Abelian) regime and also derive their late-time behavior analytically, which is consistent with recent numerical results on the evolution of the so-called melting color-glass condensate. We also find a significant delay in the onset of growth of plasma instabilities which are triggered by small rapidity fluctuations, even when the initial state is highly anisotropic.

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arXiv2006

Quantum Mass and Central Charge of Supersymmetric Monopoles - Anomalies, current renormalization, and surface terms

We calculate the one-loop quantum corrections to the mass and central charge of N=2 and N=4 supersymmetric monopoles in 3+1 dimensions. The corrections to the N=2 central charge are finite and due to an anomaly in the conformal central charge current, but they cancel for the N=4 monopole. For the quantum corrections to the mass we start with the integral over the expectation value of the Hamiltonian density, which we show to consist of a bulk contribution which is given by the familiar sum over zero-point energies, as well as surface terms which contribute nontrivially in the monopole sector. The bulk contribution is evaluated through index theorems and found to be nonvanishing only in the N=2 case. The contributions from the surface terms in the Hamiltonian are cancelled by infinite composite operator counterterms in the N=4 case, forming a multiplet of improvement terms. These counterterms are also needed for the renormalization of the central charge. However, in the N=2 case they cancel, and both the improved and the unimproved current multiplet are finite.

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arXiv2005

Thermodynamics of QCD at large quark chemical potential

We review the existing weak-coupling results on the thermodynamic potential of deconfined QCD at small and large quark chemical potential and compare with results from lattice gauge theory as well as the exactly solvable case of large-N_f QCD. We also discuss the new analytical results on non-Fermi-liquid effects in entropy and specific heat as well as in dispersion laws of quark quasiparticles at large quark chemical potential.

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arXiv2005

The entropy of hot QCD at large N_f: Successfully testing weak coupling techniques

It has been known for some time that the entropy of hot QCD is well reproduced by weak coupling techniques. These do not identify with perturbation theory, known to have poor convergence properties, but involve resumming the physics of hard thermal loops in the non-perturbative Phi-derivable two-loop approximation. We test this approximation scheme in the limit of large flavor number (N_f) where the exact result can be calculated for a wide range of couplings for which the influence of the Landau pole is negligible. Using full momentum-dependent fermionic self-energies instead of the previously only known weighted average value to order g^3, the exact result for the entropy can be remarkably well reproduced by the HTL resummed theory for a natural choice of the renormalization scale, and this up to large values of the coupling. This gives confidence in the reliability of weak coupling techniques when applied to the thermodynamics of QCD, at least for temperatures T > 3 T_c.

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arXiv2005

Dynamics of Quark-Gluon-Plasma Instabilities in Discretized Hard-Loop Approximation

Non-Abelian plasma instabilities have been proposed as a possible explanation for fast isotropization of the quark-gluon plasma produced in relativistic heavy-ion collisions. We study the real-time evolution of these instabilities in non-Abelian plasmas with a momentum-space anisotropy using a hard-loop effective theory that is discretized in the velocities of hard particles. We extend our previous results on the evolution of the most unstable modes, which are constant in directions transverse to the direction of anisotropy, from gauge group SU(2) to SU(3). We also present first full 3+1-dimensional simulation results based on velocity-discretized hard loops. In contrast to the effectively 1+1-dimensional transversely constant modes we find subexponential behaviour at late times.

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arXiv2005

Asymptotic thermal quark masses and the entropy of QCD in the large-N_f limit

We study the thermodynamics of QCD in the limit of large flavor number (N_f) and test the proposal to resum the physics of hard thermal loops (HTL) through a nonperturbative expression for the entropy obtained from a Phi-derivable two-loop approximation. The fermionic contribution to the entropy involves a full next-to-leading order evaluation of the asymptotic thermal quark mass, which is non-local, and for which only a weighted average value was known previously. For a natural choice of renormalization scale we find remarkably good agreement of the next-to-leading-order HTL results for the fermion self energy and in turn for the entropy with the respective exact large-N_f results even at very large coupling.

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arXiv2005

Study of the gluon propagator in the large-N_f limit at finite temperature and chemical potential for weak and strong couplings

At finite temperature and chemical potential, the leading-order (hard-thermal-loop) contributions to the gauge-boson propagator lead to momentum-dependent thermal masses for propagating quasiparticles as well as dynamical screening and Landau damping effects. We compare the hard-thermal-loop propagator with the complete large-N_f gluon propagator, for which the usually subleading contributions, such as a finite width of quasiparticles, can be studied at nonperturbatively large effective coupling. We also study quantitatively the effect of Friedel oscillations in low-temperature electrostatic screening.

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arXiv2005

Weak-coupling techniques for the thermodynamics of the quark-gluon plasma

We describe some of the recent progress in the calculation of thermodynamic quantities in QCD at high temperatures and densities by weak-coupling techniques and extrapolation to realistic coupling strength. We argue that a (mostly) weakly coupled quark-gluon plasma at temperatures only a few times the transition temperature is not yet ruled out by the observed fast apparent thermalization at RHIC, as nonabelian plasma instabilities might provide an efficient mechanism for fast isotropization even in a collisionless plasma.

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arXiv2005

Hard-Loop Dynamics of Non-Abelian Plasma Instabilities

Non-Abelian plasma instabilities may be responsible for the fast apparent quark-gluon thermalization in relativistic heavy-ion collisions if their exponential growth is not hindered by nonlinearities. We study the real-time evolution of instabilities in an anisotropic non-Abelian plasma with an SU(2) gauge group in the hard-loop approximation. We find exponential growth of non-Abelian plasma instabilities both in the linear and in the strongly nonlinear regime, with only a brief phase of subexponential behavior in between.

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arXiv2004

Hard-Loop Effective Action for Anisotropic Plasmas

We generalize the hard-thermal-loop effective action of the equilibrium quark-gluon plasma to a non-equilibrium system which is space-time homogeneous but for which the parton momentum distribution is anisotropic. We show that the manifestly gauge-invariant Braaten-Pisarski form of the effective action can be straightforwardly generalized and we verify that it then generates all n-point functions following from collisionless gauge-covariant transport theory for a homogeneous anisotropic plasma. On the other hand, the Taylor-Wong form of the hard-thermal-loop effective action has a more complicated generalization to the anisotropic case. Already in the simplest case of anisotropic distribution functions, it involves an additional term that is gauge invariant by itself, but nontrivial also in the static limit.

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arXiv2004

New developments in the quantization of supersymmetric solitons (kinks, vortices and monopoles)

We discuss the one-loop quantum corrections to the mass M and central charge Z of supersymmetric solitons: the kink, the vortex and the monopole. Contrary to previous expectations and published results, in each of these cases there are nonvanishing quantum corrections to the mass. For the N=1 kink and the N=2 monopole a new anomaly in Z rescues BPS saturation (M=Z); for the N=2 vortex, BPS saturation is rescued for two reasons: (i) the quantum fluctuations of the Higgs field acquire a nontrivial phase due to the winding of the classical solution, and (ii) a fermionic zero mode used in the literature is shown not to be normalizable.

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arXiv2004

Quantum corrections to mass and central charge of supersymmetric solitons

We review some recent developments in the subject of quantum corrections to soliton mass and central charge. We consider in particular approaches which use local densities for these corrections, as first discussed by Hidenaga Yamagishi. We then consider dimensional regularization of the supersymmetric kink in 1+1 dimensions and an extension of this method to a 2+1-dimensional gauge theory with supersymmetric abelian Higgs vortices as the solitons.

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arXiv2004

Advances in perturbative thermal field theory

The progress of the last decade in perturbative quantum field theory at high temperature and density made possible by the use of effective field theories and hard-thermal/dense-loop resummations in ultrarelativistic gauge theories is reviewed. The relevant methods are discussed in field theoretical models from simple scalar theories to non-Abelian gauge theories including gravity. In the simpler models, the aim is to give a pedagogical account of some of the relevant problems and their resolution, while in the more complicated but also more interesting models such as quantum chromodynamics, a summary of the results obtained so far are given together with references to a few most recent developments and open problems.

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arXiv2003

On the nature of the anomalies in the supersymmetric kink

We discuss the possibility to absorb all anomalies in the supersymmetry algebra of the N=(1,1) Wess-Zumino model in d=1+1 by a local counter term. This counter term corresponds to the change of the vacuum parameter $v_0^2$ in the model and the transition to an unconventional but admissible renormalization scheme. It does not modify the physical consequences such as BPS saturation, and thus the situation is rather different from gauge theory where local counter terms are required to absorb spurious gauge anomalies.

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