arXiv subjects

A. Rajantie

Publications and source records attributed to A. Rajantie.

At least 37 records · Page 2Linked to original sources

arXiv2004

Magnetic monopoles from gauge theory phase transitions

Thermal fluctuations of the gauge field lead to monopole formation at the grand unified phase transition in the early Universe, even if the transition is merely a smooth crossover. The dependence of the produced monopole density on various parameters is qualitatively different from theories with global symmetries, and the monopoles have a positive correlation at short distances. The number density of monopoles may be suppressed if the grand unified symmetry is only restored for a short time by, for instance, nonthermal symmetry restoration after preheating.

hep-ph↗

arXiv2003

Phase Transitions in the Early Universe and Defect Formation

In the first one of these two lectures, I give an introductory review of phase transitions in finite temperature field theories. I highlight the differences between theories with global and local symmetries, and the similarities between cosmological phase transitions and phase transitions in superconductors. In the second one, I review the theory of defect formation in finite temperature phase transitions, emphasising the differences between systems with broken global and local symmetries. These same principles apply to relativistic and condensed matter systems alike.

hep-ph↗

arXiv2003

Estimation of vortex density after superconducting film quench

This paper addresses the problem of vortex formation during a rapid quench in a superconducting film. It builds on previous work showing that in a local gauge theory there are two distinct mechanisms of defect formation, based on fluctuations of the scalar and gauge fields, respectively. We show how vortex formation in a thin film differs from the fully two-dimensional case, on which most theoretical studies have focused. We discuss ways of testing theoretical predictions in superconductor experiments and analyse the results of recent experiments in this light.

cond-mat↗

arXiv2003

Formation of Magnetic Monopoles in Hot Gauge Theories

In this talk, I discuss the formation of magnetic monopoles in a phase transition from the confining SU(2) phase to the Coulomb phase in a hot Georgi-Glashow model. I argue that monopoles are formed from long-wavelength thermal fluctuations, which freeze out after the phase transition.

hep-ph↗

arXiv2002

The monopole mass in the three-dimensional Georgi-Glashow model

We study the three-dimensional Georgi-Glashow model to demonstrate how magnetic monopoles can be studied fully non-perturbatively in lattice Monte Carlo simulations, without any assumptions about the smoothness of the field configurations. We examine the apparent contradiction between the conjectured analytic connection of the `broken' and `symmetric' phases, and the interpretation of the mass (i.e., the free energy) of the fully quantised 't Hooft-Polyakov monopole as an order parameter to distinguish the phases. We use Monte Carlo simulations to measure the monopole free energy and its first derivative with respect to the scalar mass. On small volumes we compare this to semi-classical predictions for the monopole. On large volumes we show that the free energy is screened to zero, signalling the formation of a confining monopole condensate. This screening does not allow the monopole mass to be interpreted as an order parameter, resolving the paradox.

hep-lat↗

arXiv2002

Dynamics of tachyonic preheating after hybrid inflation

We study the instability of a scalar field at the end of hybrid inflation, using both analytical techniques and numerical simulations. We improve previous studies by taking the inflaton field fully into account, and show that the range of unstable modes depends sensitively on the velocity of the inflaton field, and thereby on the Hubble rate, at the end of inflation. If topological defects are formed, their number density is determined by the shortest unstable wavelength. Finally, we show that the oscillations of the inflaton field amplify the inhomogeneities in the energy density, leading to local symmetry restoration and faster thermalization. We believe this explains why tachyonic preheating is so effective in transferring energy away from the inflaton zero mode.

hep-ph↗

arXiv2001

Formation of topological defects in gauge field theories

When a symmetry gets spontaneously broken in a phase transition, topological defects are typically formed. The theoretical picture of how this happens in a breakdown of a global symmetry, the Kibble-Zurek mechanism, is well established and has been tested in various condensed matter experiments. However, from the viewpoint of particle physics and cosmology, gauge field theories are more relevant than global theories. In recent years, there have been significant advances in the theory of defect formation in gauge field theories, which make precise predictions possible, and in experimental techniques that can be used to test these predictions in superconductor experiments. This opens up the possibility of carrying out relatively simple and controlled experiments, in which the non-equilibrium phase transition dynamics of gauge field theories can be studied. This will have a significant impact on our understanding of phase transitions in the early universe and in heavy ion collider experiments. In this paper, I review the current status of the theory and the experiments in which it can be tested.

hep-ph↗

arXiv2001

Critical behaviour of the Ginzburg-Landau model in the type II region

We study the critical behaviour of the three-dimensional U(1) gauge+Higgs theory (Ginzburg-Landau model) at large scalar self-coupling λ(``type II region'') by measuring various correlation lengths as well as the Abrikosov-Nielsen-Olesen vortex tension. We identify different scaling regions as the transition is approached from below, and carry out detailed comparisons with the criticality of the 3d O(2) symmetric scalar theory.

hep-lat↗

arXiv2001

Electroweak preheating on a lattice

In many inflationary models, a large amount of energy is transferred rapidly to the long-wavelength matter fields during a period of preheating after inflation. We study how this changes the dynamics of the electroweak phase transition if inflation ends at the electroweak scale. We simulate a classical SU(2)xU(1)+Higgs model with initial conditions in which the energy is concentrated in the long-wavelength Higgs modes. With a suitable initial energy density, the electroweak symmetry is restored non-thermally but broken again when the fields thermalize. During this symmetry restoration, baryon number is violated, and we measure its time evolution, pointing out that it is highly non-Brownian. This makes it difficult to estimate the generated baryon asymmetry.

hep-ph↗

arXiv2001

Phase transition dynamics in the hot Abelian Higgs model

We present a detailed numerical study of the equilibrium and non-equilibrium dynamics of the phase transition in the finite-temperature Abelian Higgs model. Our simulations use classical equations of motion both with and without hard-thermal-loop corrections, which take into account the leading quantum effects. From the equilibrium real-time correlators, we determine the Landau damping rate, the plasmon frequency and the plasmon damping rate. We also find that, close to the phase transition, the static magnetic field correlator shows power-law magnetic screening at long distances. The information about the damping rates allows us to derive a quantitative prediction for the number density of topological defects formed in a phase transition. We test this prediction in a non-equilibrium simulation and show that the relevant time scale for defect formation is given by the Landau damping rate.

hep-ph↗

arXiv2001

Local gauge invariance and formation of topological defects

In superconductors, and in other systems with a local U(1) gauge invariance, there are two mechanisms that form topological defects in phase transitions. In addition to the standard Kibble mechanism, thermal fluctuations of the magnetic field also lead to defect formation. This mechanism is specific to local gauge theories, predicts a qualitatively different spatial defect distribution and is the dominant source for topological defects in slow transitions. I review the arguments that lead to these conclusions and discuss possibilities of testing the scenario in superconductor experiments.

cond-mat↗

arXiv2000

Topological defects in lattice gauge theories

We present a non-perturbative formalism for measuring defect free energies (monopole mass or vortex tension) in three-dimensional SU(2)+adjoint Higgs models. Starting from twisted, translation invariant boundary conditions, we perform a change of variables that allows us to express the defect free energies in terms of 't Hooft loops. We propose that the defect free energies can be used to distinguish between phases in this model, and also more generally in other gauge field theories where no local order parameters exist. In the case of monopoles, our construction can also be used in four-dimensional pure-gauge SU(2) theory, where it gives the monopole mass in the maximally Abelian gauge without the need of actually fixing the gauge in the simulation. Moreover, the expression is manifestly independent of the choice of the Abelian projection as long as it is compatible with the classical 't Hooft-Polyakov solution.

hep-lat↗

arXiv2000

Numerical simulations of electroweak baryogenesis at preheating

It has recently been suggested that the baryon washout problem of standard electroweak baryogenesis could be avoided if inflation ends at a low enough energy density and a parametric resonance transfers its energy repidly into the standard model fields. We present preliminary results of numerical simulations in a SU(2)xU(1) gauge-Higgs model in which this process was studied.

hep-ph↗

arXiv2000

Preheating and phase transitions in gauge theories

It has recently been suggested that the baryon washout problem of the standard electroweak baryogenesis scenario could be avoided if inflation ends with a period of parametric resonance at a low enough energy density. I present results of numerical simulations in which this process was studied in the Abelian Higgs model. Our results show that because of the masslessness of the gauge field, the parametric resonance takes place naturally, and that the system reaches a quasi-equilibrium state in which the long-wavelength part of the spectrum has a high effective temperature. This enhances baryon number violation and makes baryogenesis more efficient.

hep-ph↗

arXiv2000

Defect formation and local gauge invariance

We propose a new mechanism for formation of topological defects in a U(1) model with a local gauge symmetry. This mechanism leads to definite predictions, which are qualitatively different from those of the Kibble-Zurek mechanism of global theories. We confirm these predictions in numerical simulations, and they can also be tested in superconductor experiments. We believe that the mechanism generalizes to more complicated theories.

cond-mat.supr-con↗

arXiv2000

Phase transitions from preheating in gauge theories

We show by studying the Abelian Higgs model with numerical lattice simulations that non-thermal phase transitions arising out of preheating after inflation are possible in gauge-Higgs models under rather general circumstances. This may lead to the formation of gauged topological defects and, if the scale at which inflation ends is low enough, to electroweak baryogenesis after preheating.

hep-ph↗

arXiv2000

O(2) symmetry breaking vs. vortex loop percolation

We study with lattice Monte Carlo simulations the relation of global O(2) symmetry breaking in three dimensions to the properties of a geometrically defined vortex loop network. We find that different definitions of constructing a network lead to different results even in the thermodynamic limit, and that with typical definitions the percolation transition does not coincide with the thermodynamic phase transition. These results show that geometrically defined percolation observables need not display universal properties related to the critical behaviour of the system, and do not in general survive in the field theory limit.

hep-lat↗

arXiv2000

Non-equilibrium dynamics of hot Abelian Higgs model

The real-time dynamics of finite-temperature gauge theories can be approximated, to leading-order accuracy in the coupling constants, by a classical field theory with the hard thermal loop Lagrangian. I show how this approach can be used in numerical lattice simulations to study dynamics of the Abelian Higgs model in or slightly out of equilibrium.

hep-ph↗