arXiv subjects

Andreas Ekstedt

Publications and source records attributed to Andreas Ekstedt.

At least 19 recordsLinked to original sources

arXiv2026

Scalar damping in cosmological phase transitions

We outline how to calculate the scalar damping term during a cosmological phase transition from kinetic theory. We determine the scalar damping rate from top quarks and weak gauge bosons in a Standard Model-like theory. We find that the convergence of the bosonic contributions hinges on how the soft modes are treated. We discuss the validity of the phenomenological friction term employed in hydrodynamical simulations. We find that for a Standard Model particle content, this approximation is (marginally) justified. We also test the hypothesis that the pressure from a runaway wall acts as an upper bound on the pressure from the local friction term. We find that next-to-leading order contributions in terms of velocity and mass are negative and that in the regime of validity, the local damping term indeed cannot surpass the pressure from runaway bubbles.

hep-ph↗

arXiv2026

Seeded bubble nucleation on the lattice

We provide the first non-perturbative lattice determination of the bubble nucleation rate as seeded by topological defects during a first order phase transition. Our case of study is the cubic anisotropy model, which can mimic the Higgs-plus-singlet setup for the electroweak theory, in $d=2+1$ spacetime dimensions, where bubbles are seeded by (line-like) domain walls. We compare the nucleation rate from the lattice with the semi-classical prediction based on the effective field theory living on the domain walls, including for the first time the fluctuation determinant away from spherical symmetry. Our results show very good agreement across all the considered parameter space.

hep-lat↗

arXiv2024

How fast does the WallGo? A package for computing wall velocities in first-order phase transitions

WallGo is an open source software for the computation of the bubble wall velocity in first-order cosmological phase transitions. It also computes the energy budget available for the generation of gravitational waves. The main part of WallGo, built in Python, determines the wall velocity by solving the scalar-field(s) equation of motion, the Boltzmann equations and energy-momentum conservation for the fluid velocity and temperature. WallGo also includes two auxiliary modules: WallGoMatrix, which computes matrix elements for out-of-equilibrium particles, and WallGoCollision, which performs higher-dimensional integrals for Boltzmann collision terms. Users can implement custom models by defining an effective potential and specifying a list of out-of-equilibrium particles and their interactions. As the first public software to compute the wall velocity including out-of-equilibrium contributions, WallGo improves the precision of the computation compared to common assumptions in earlier computations. It utilises a spectral method for the deviation from equilibrium and collision terms that provides exponential convergence in basis polynomials, and supports multiple out-of-equilibrium particles, allowing for Boltzmann mixing terms. WallGo is tailored for non-runaway wall scenarios where leading-order coupling effects dominate friction. While this work introduces the software and the underlying theory, a more detailed documentation can be found in https://wallgo.readthedocs.io.

hep-ph↗

arXiv2024

Bubble-wall speed with loop corrections

In this paper, we investigate the dynamics of the nucleating scalar field during the first-order phase transitions by incorporating one-loop corrections of classical fluctuations. We assume that a high-temperature expansion is valid\te where the mass of the scalar field is significantly smaller than the temperature\te so that we can treat the bubble-wall dynamics in a regime where quantum fluctuations can be integrated out. We present a systematic framework for calculating classical loop corrections to the wall speed; contrast our results with traditional methods based on the derivative expansion; show that the latent heat can differ from the effective-potential result; and discuss general hydrodynamic corrections. Finally, we show an application of the presented framework for a simple scalar field model, finding that the one-loop improvement decreases the wall speed and that an effective-potential approximation underestimates full one-loop corrections by about a factor of two.

hep-ph↗

arXiv2024

Radiative first-order phase transitions to next-to-next-to-leading order

We develop new perturbative tools to accurately study radiatively-induced first-order phase transitions. Previous perturbative methods have suffered internal inconsistencies and been unsuccessful in reproducing lattice data, which is often attributed to infrared divergences of massless modes (the Linde problem). We employ a consistent power counting scheme to perform calculations, and compare our results against lattice data. We conclude that the consistent expansion removes many previous issues, and indicates that the infamous Linde problem is not as big a factor in these calculations as previously thought.

hep-ph↗

arXiv2024

Cosmological phase transitions at three loops: the final verdict on perturbation theory

We complete the perturbative program for equilibrium thermodynamics of cosmological first-order phase transitions by determining the finite-temperature effective potential of gauge-Higgs theories at next-to-next-to-next-to-next-to-leading order (N$^4$LO). The computation of the three-loop effective potential required to reach this order is extended to generic models in dimensionally reduced effective theories in a companion article. Our N$^4$LO result is the last perturbative order before confinement renders electroweak gauge-Higgs theories non-perturbative at four loops. By contrasting our analysis with non-perturbative lattice results, we find a remarkable agreement. As a direct application for predictions of gravitational waves produced by a first-order transition, our computation provides the final fully perturbative results for the phase transition strength and speed of sound.

hep-ph↗

arXiv2024

BubbleDet: A Python package to compute functional determinants for bubble nucleation

We present a Python package, BubbleDet, for computing one-loop functional determinants around spherically symmetric background fields. This gives the next-to-leading order correction to both the vacuum decay rate, at zero temperature, and to the bubble nucleation rate in first-order phase transitions at finite temperature. For predictions of gravitational wave signals from cosmological phase transitions, this is expected to remove one of the leading sources of theoretical uncertainty. BubbleDet is applicable to arbitrary scalar potentials and in any dimension up to seven. It has methods for fluctuations of scalar fields, including Goldstone bosons, and for gauge fields, but is limited to cases where the determinant factorises into a product of separate determinants, one for each field degree of freedom. To our knowledge, BubbleDet is the first package dedicated to calculating functional determinants in spherically symmetric background

hep-ph↗

arXiv2023

Propagation of gauge fields in hot and dense plasmas at higher orders

Thermal field theory is indispensable for describing hot and dense systems. Yet perturbative calculations are often stymied by a host of energy scales, and tend to converge slowly. This means that precise results require the apt use of effective field theories. In this paper we refine the effective description of slowly varying gauge field known as hard thermal loops. We match this effective theory to the full theory to two-loops. Our results apply for any renormalizable model and fermion chemical potential. We also discuss how to consistently define asymptotic masses at higher orders; and how to treat spectral functions close to the lightcone. In particular, we demonstrate that the gluon mass is well-defined to next-to-leading order.

hep-ph↗

arXiv2023

Two-loop hard thermal loops for any model

Hard thermal loops describe how soft gauge fields are screened and damped in hot plasmas. As such they are used to calculate transport coefficients, Sphaleron rates, equations of state, and particle production. However, most calculations are done using one-loop self-energies. And two-loop contributions can be large. To that end this paper provides vector two-loop self-energies for generic models: Any scalar, fermion, or vector representation; and all possible renormalizable terms. Several examples are given to showcase the results. Two-loop results for higher-point functions are also given.

hep-ph↗

arXiv2022

Bubble Nucleation to All Orders

This paper extends classical results by Langer and Kramers and combines them with modern methods from high-temperature field theory. Assuming Langevin dynamics, the end-product is an all-orders description of bubble-nucleation at high temperatures. Specifically, it's shown that equilibrium and non-equilibrium effects factorize to all orders, and that the nucleation rate splits into a statistical and a dynamical prefactor. The derivation clarifies, and incorporates, higher-order corrections from zero-modes. The rate is also shown to be real to all orders in perturbation theory. The methods are applied to several models. As such, Feynman rules are given; the relevant power-counting is introduced; RG invariance is shown; the connection with the effective action is discussed, and an explicit construction of propagators in an inhomogeneous background is given. The formalism applies to both phase and Sphaleron transitions. While mainly focused on field theory, the methods are applicable to finite-dimensional systems. Finally, as this paper assumes an effective Langevin description, all results only hold within this framework.

hep-ph↗

arXiv2022

DRalgo: a package for effective field theory approach for thermal phase transitions

DRalgo is an algorithmic implementation that constructs an effective, dimensionally reduced, high-temperature field theory for generic models. The corresponding Mathematica package automatically performs the matching to next-to-leading order. This includes two-loop thermal corrections to scalar and Debye masses as well as one-loop thermal corrections to couplings. DRalgo also allows for integrating out additional heavy scalars. Along the way, the package provides leading-order beta functions for general gauge-charges and fermion-families; both in the fundamental and in the effective theory. Finally, the package computes the finite-temperature effective potential within the effective theory. The article explains the theory of the underlying algorithm while introducing the software on a pedagogical level.

hep-ph↗

arXiv2022

Convergence of the nucleation rate for first-order phase transitions

This paper investigates the importance of radiative corrections for first-order phase transitions, with particular focus on the bubble-nucleation rate. All calculations are done with a strict power-counting, and observables are consistently calculated at every order. This ensures that physical quantities are gauge and renormalization-scale invariant. Furthermore, to avoid large logarithms at high-temperatures, an effective three-dimensional theory is used. This effective theory automatically incorporates higher-order thermal masses. The results of this paper indicate that sub-leading corrections to the rate can be large. This is partly because radiative corrections are enhanced for large bubbles. To illustrate the calculations, three models are considered: a real-scalar model, a radiative-barrier model, and a model with an effective dimension $6$ operator. Relevant observables are calculated for each model, and the reliability of perturbation theory is discussed.

hep-ph↗

arXiv2022

Higher-Order Corrections to the Bubble-Nucleation Rate at Finite Temperature

In this paper I discuss how to consistently incorporate higher-order corrections to the bubble-nucleation rate at finite temperature. Doing so I examine the merits of different approaches, with the goal of reducing uncertainties for gravitational-wave calculations. To be specific, the region of applicability and accuracy of the derivative expansion is discussed. The derivative expansion is then compared to a numerical implementation of the Gelfand-Yaglom theorem. Both methods are applied to popular first-order phase transition models, like a loop-induced barrier and a SM-EFT tree-level barrier. The results of these calculations are presented in easy-to-use parametrizations that can directly be used in gravitational-wave calculations. In addition, higher-order corrections for models with multiple scalar fields, such as singlet/triplet extensions, are studied. Lastly, the convergence and uncertainty of all calculations are discussed in detail. And I argue that leading-order results for the Standard Model with a tree-level barrier might be inaccurate.

hep-ph↗

arXiv2021

A natural mechanism for approximate Higgs alignment in the 2HDM

The 2HDM possesses a neutral scalar interaction eigenstate whose tree-level properties coincide with the Standard Model (SM) Higgs boson. In light of the LHC Higgs data which suggests that the observed Higgs boson is SM-like, it follows that the mixing of the SM Higgs interaction eigenstate with the other neutral scalar interaction eigenstates of the 2HDM should be suppressed, corresponding to the so-called Higgs alignment limit. The exact Higgs alignment limit can arise naturally due to a global symmetry of the scalar potential. If this symmetry is softly broken, then the Higgs alignment limit becomes approximate (although still potentially consistent with the current LHC Higgs data). In this paper, we obtain the approximate Higgs alignment suggested by the LHC Higgs data as a consequence of a softly broken global symmetry of the Higgs Lagrangian. However, this can only be accomplished if the Yukawa sector of the theory is extended. We propose an extended 2HDM with vector-like top quark partners, where explicit mass terms in the top sector provide the source of the soft symmetry breaking of a generalized CP symmetry. In this way, we can realize approximate Higgs alignment without a significant fine-tuning of the model parameters. We then explore the implications of the current LHC bounds on vector-like top quark partners for the success of our proposed scenario.

hep-ph↗

arXiv2020

The High-Temperature Expansion of the Thermal Sunset

We give a prescription for calculating the high-temperature expansion of the thermal sunset integral to arbitrary order. We derive all terms odd in $T$, and rederive previous results up to $\mathcal{O}(T^0) $ for both bosonic and fermionic thermal sunsets in dimensional regularisation. We perform analytical and numerical cross-checks. Intermediate steps involve integrals over three Bessel functions.

hep-th↗

arXiv2020

Flavorgenesis

We present a model where all fermions are contained in a single irreducible representation of an SU(19) gauge symmetry group. If there is only one scalar field, Yukawa interactions are controlled by a single number rather than by one or more $3\times3$ matrices of couplings. The low-energy concept of flavor emerges entirely from the scalar-sector parameters; more specifically, entries of the Standard Model Yukawa matrices are controlled by several vacuum expectation values.

hep-ph↗

arXiv2020

A Critical Look at the Electroweak Phase Transition

The electroweak phase transition broke the electroweak symmetry. Perturbative methods used to calculate observables related to this phase transition suffer from severe problems such as gauge dependence, infrared divergences, and a breakdown of perturbation theory. In this paper we develop robust perturbative tools for dealing with phase transitions. We argue that gauge and infrared problems are absent in a consistent power-counting. We calculate the finite temperature effective potential to two loops for general gauge-fixing parameters in a generic model. We demonstrate gauge invariance, and perform numerical calculations for the Standard Model in Fermi gauge.

hep-ph↗

arXiv2019

Towards solving the proton spin puzzle

The fact that the spins of the quarks in the proton, as measured in deep inelastic lepton scattering, only add up to about 30$\%$ of the spin of the proton is still not understood after 30 years. We show that our newly developed model for the quark and gluon momentum distributions in the proton, based on quantum fluctuations of the proton into baryon-meson pairs convoluted with Gaussian momentum distributions of partons in hadrons, can essentially reproduce the data on the proton spin structure function $g_1^P(x)$ and the associated spin asymmetry. A further improved description of the data is achieved by also including the relativistic correction of the Melosh transformation to the light-front formalism used in deep inelastic scattering. However, this does not fully resolve the spin puzzle, including also the neutron spin structure and the spin sum rules. These aspects can also be accounted for by our few-parameter model if the conventional SU(6) flavor-spin symmetry is broken, giving new information on the non-perturbative bound-state nucleon.

hep-ph↗