arXiv subjects

Arttu Rajantie

Publications and source records attributed to Arttu Rajantie.

68 records · Page 4Linked to original sources

arXiv2011

Soliton form factors from lattice simulations

The form factor provides a convenient way to describe properties of topological solitons in the full quantum theory, when semiclassical concepts are not applicable. It is demonstrated that the form factor can be calculated numerically using lattice Monte Carlo simulations. The approach is very general and can be applied to essentially any type of soliton. The technique is illustrated by calculating the kink form factor near the critical point in 1+1-dimensional scalar field theory. As expected from universality arguments, the result agrees with the exactly calculable scaling form factor of the two-dimensional Ising model.

hep-lat↗

arXiv2010

Non-Gaussianity from resonant curvaton decay

We calculate curvature perturbations in the scenario in which the curvaton field decays into another scalar field via parametric resonance. As a result of a nonlinear stage at the end of the resonance, standard perturbative calculation techniques fail in this case. Instead, we use lattice field theory simulations and the separate universe approximation to calculate the curvature perturbation as a nonlinear function of the curvaton field. For the parameters tested, the generated perturbations are highly non-Gaussian and not well approximated by the usual fNL parameterisation. Resonant decay plays an important role in the curvaton scenario and can have a substantial effect on the resulting perturbations.

astro-ph.CO↗

arXiv2010

Counting defects with the two-point correlator

We study how topological defects manifest themselves in the equal-time two-point field correlator. We consider a scalar field with Z_2 symmetry in 1, 2 and 3 spatial dimensions, allowing for kinks, domain lines and domain walls, respectively. Using numerical lattice simulations, we find that in any number of dimensions, the correlator in momentum space is to a very good approximation the product of two factors, one describing the spatial distribution of the defects and the other describing the defect shape. When the defects are produced by the Kibble mechanism, the former has a universal form as a function of k/n, which we determine numerically. This signature makes it possible to determine the kink density from the field correlator without having to resort to the Gaussian approximation. This is essential when studying field dynamics with methods relying only on correlators (Schwinger-Dyson, 2PI).

hep-ph↗

arXiv2009

Super-horizon cosmic string correlations

When gauged cosmic strings form in a symmetry-breaking phase transition, the gauge field configuration at the time becomes imprinted in the spatial string distribution by the flux trapping mechanism. It is shown that quantum and thermal gauge field fluctuations give rise to long-range superhorizon correlations in the string network. Numerical simulations in the Abelian Higgs model confirm this finding. In contrast, the Kibble-Zurek mechanism, which most cosmic string studies are based on, only gives rise to short-distance subhorizon correlations. The potential observable effects of the correlations are discussed.

astro-ph↗

arXiv2009

Quantum kink and its excitations

We show how detailed properties of a kink in quantum field theory can be extracted from field correlation functions. This makes it possible to study quantum kinks in a fully non-perturbative way using Monte Carlo simulations. We demonstrate this by calculating the kink mass as well as the spectrum and approximate wave functions of its excitations. This way of measuring the kink mass has clear advantages over the existing approaches based on creation and annihilation operators or the kink free energy. Our methods are straightforward to generalise to more realistic theories and other defect types.

hep-lat↗

arXiv2008

Non-Gaussianity from massless preheating

Preheating can convert superhorizon fluctuations of light scalar fields present at the end of inflation into observable density perturbations. We show in detail how lattice field theory simulations and the separate universes approximation can be used to calculate these perturbations and make predictions for the nonlinearity parameter fNL . We also present a simple approximation scheme that can reproduce these results analytically. Applying these methods to the massless preheating model, we determine the parameter values that are ruled out by too high levels of non-Gaussianity.

astro-ph↗

arXiv2008

Lattice calculation of non-Gaussianity from preheating

If light scalar fields are present at the end of inflation, their non-equilibrium dynamics such as parametric resonance or a phase transition can produce non-Gaussian density perturbations. We show how these perturbations can be calculated using non-linear lattice field theory simulations and the separate universe approximation. In the massless preheating model, we find that some parameter values are excluded while others lead to acceptable but observable levels of non-Gaussianity. This shows that preheating can be an important factor in assessing the viability of inflationary models.

astro-ph↗

arXiv2007

Numerical experiments with p F- and q D-strings: the formation of (p,q) bound states

We investigate the behaviour of (p,q) string networks, focusing on two aspects: (1) modelling more realistic (p,q) string networks than the Z_N networks used so far and (2) investigating the effect of long-range interactions on the evolution of the network. We model the network with no long-range interactions using two sets of fields, complex scalars coupled to gauge fields, with a potential chosen such that the two types of strings will form bound states. This way we can model junctions of 3 strings with different tension; in Z_N models used so far in simulations all the strings have identical tensions. In order to introduce long-range interactions we also study a network in which one of the scalars forms global strings. We observe that in the absence of long-range interactions the formation of bound states has a significant influence on the evolution of the network. When long-range interactions are turned on the bound states are short-lived and have a minimal effect on the network evolution.

hep-th↗

arXiv2006

Looking for defects in the 2PI correlator

Truncations of the 2PI effective action are seen as a promising way of studying non-equilibrium dynamics in quantum field theories. We probe their applicability in the non-perturbative setting of topological defect formation in a symmetry-breaking phase transition, by comparing full classical lattice field simulations and the 2PI formulation for classical fields in an O($N$) symmetric scalar field theory. At next-to-leading order in 1/N, the 2PI formalism fails to reproduce any signals of defects in the two-point function. This suggests that one should be careful when applying the 2PI formalism for symmetry breaking phase transitions.

hep-ph↗

arXiv2002

The inverted XY universality of the superconductivity phase transition

It has been conjectured that the phase transition in the Ginzburg-Landau theory is dual to the XY model transition. We study numerically a particular limit of the GL theory where this duality becomes exact, clarifying some of the problems encountered in standard GL theory simulations. This may also explain the failure of the superconductor experiments to observe the XY model scaling.

hep-lat↗

arXiv2001

Baryogenesis at the end of hybrid inflation

The baryon asymmetry of the universe may originate in the phase transition at the end of hybrid inflation, provided that the reheat temperature is low enough. I show that if the field that triggers the end of inflation is the electroweak Higgs field and CP is violated, the transition leads to baryon asymmetry even if no preheating or non-thermal symmetry restoration takes place. I estimate the strength of this effect and the constraints it imposes on the inflationary model.

hep-ph↗

arXiv2001

Hybrid Inflation and Baryogenesis at the TeV Scale

We consider the construction of inverted hybrid inflation models in which the vacuum energy during inflation is at the TeV scale, and the inflaton couples to the Higgs field. Such models are of interest in the context of some recently proposed models of electroweak baryogenesis. We demonstrate how constraints on these models arise from quantum corrections, and how self-consistent examples may be constructed, albeit at the expense of fine-tuning. We discuss two possible ways in which the baryon asymmetry of the universe may be produced in these models. One of them is based on preheating and a consequent non-thermal electroweak symmetry restoration and the other on the formation of Higgs winding configurations by the Kibble mechanism at the end of inflation.

hep-ph↗

arXiv1997

SU(5)+Adjoint Higgs at Finite Temperature

I present a three-dimensional effective theory that describes the high-temperature equilibrium behavior of the SU(5) theory accurately, but is much easier to study using non-perturbative methods than the original four-dimensional one. The effective theory is obtained by perturbatively integrating out the super-heavy Matsubara modes as well as the heavy temporal component of the gauge field. Regardless of the particle spectrum of the original theory, the resulting three-dimensional theory contains only a gauge field and an adjoint Higgs field. The phase diagram of the theory is analysed perturbatively and it is shown to have a cellular structure in three-dimensional parameter space.

hep-ph↗

arXiv1997

SU(5)+Adjoint Higgs Model at Finite Temperature

A three-dimensional effective theory of the finite-temperature SU(5)+adjoint Higgs model is constructed using the method of dimensional reduction. The resulting theory can be used for computer simulations of the GUT phase transition of the early universe. In this paper the transition is analyzed at two-loop level in perturbation theory. The structure of the effective theory does not essentially depend on the matter contents of the original four-dimensional one. Therefore the analysis of this paper applies also to more realistic theories.

hep-ph↗