arXiv subjects

Andrey Shkerin

Publications and source records attributed to Andrey Shkerin.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics of nucleation in thermal phase transitions

We study dynamical effects during nucleation in thermal first-order phase transitions in field theory. Focusing on the classical regime of the decay of a metastable state, we present the general formula for the thermal decay rate including the dynamical prefactor and give a recipe for its systematic evaluation. We describe the physical mechanism which reduces the actual thermal decay rate with respect to the statistical rate obtained in equilibrium theory. We also discuss the thermality conditions ensuring the existence of a steady-state thermal rate, in which case our formula is exact up to exponentially small corrections. We show that it reproduces the known results for the nucleation rate in stochastic mechanics and field theory, and allows us to unify and go beyond them. We illustrate this in real-time numerical simulations of simple field theory models. We observe significant non-perturbative contributions which can dominate the dynamical prefactor in weakly-coupled field theories at moderate exponential suppression of the decay rate. We explore the connection of these non-perturbative effects to oscillons. Notably, our numerical method requires exponentially less computing time than direct simulations of decays and is thus applicable to systems with arbitrarily strong exponential suppression. Finally, we discuss small or poorly thermalized systems when the thermality conditions are violated and the steady-state rate does not exist.

hep-th↗

arXiv2026

New Gauge Forces, Neutron Stars and Schwinger Neutrino Production

We investigate neutrino effects of new long-range forces arising from gauging $B-L$, $L_e-L_{\mu/\tau}$ or $L_{\mu}-L_{\tau}$ symmetries of the Standard Model. The leptonic potential generated by astronomical bodies, such as the Earth, the Sun or a neutron star, results in the Schwinger pair production of neutrinos charged under the new gauge symmetry. The oppositely charged particles accumulate in the potential well forming a degenerate Fermi gas, while equally charged particles fly away forming a steady flux of neutrinos. We find that, for the $B-L$ and $L_e-L_{\mu/\tau}$ forces, these effects are too weak to be observable. For the $L_{\mu}-L_{\tau}$ force these effects are significant in neutron stars if the gauge coupling is $g\gtrsim 10^{-18}$. The muonic force changes the element abundances of a neutron star in equilibrium and suppresses its $L_{\mu}-L_{\tau}$ charge. This invalidates the constraint on $g$ from neutron star mergers, at $g\gtrsim 10^{-17}$. Furthermore, for such values of $g$, the neutrino flux produced by the Schwinger effect could potentially be detected from a single young neutron star at a distance of $\simeq 100$ pc, with the typical neutrino energy $E_\nu\sim 100$ MeV. A dedicated search for such a signal will reassert the bound $g\lesssim 10^{-18}$.

hep-ph↗

arXiv2026

False vacuum decay catalyzed by black hole in a heat bath

We study false vacuum decay catalyzed by black holes. We consider a scalar field model with unstable potential in the background of a dilaton black hole in two dimensions. The model reproduces many features of the Schwarzschild black hole background in four dimensions, including the centrifugal barrier for linearized field perturbations. We study decays from the non-equilibrium state describing the evaporating black hole immersed in the thermal bath with a different temperature. We analytically construct the tunneling solution relevant at small field excitations and evaluate the decay suppression. We show how they reduce to those for the Hartle-Hawking (equilibrium) and Unruh states in the corresponding limits. For large field excitations the decay proceeds via stochastic activation; we find the relevant non-thermal sphaleron configuration in a certain region of parameters of the model and construct the semiclassical solution describing tunneling onto this sphaleron. Our results provide insights into the vacuum decay induced by small primordial black holes in the radiation-dominated era of the universe.

hep-th↗

arXiv2025

Holographic phase transitions via thermally-assisted tunneling

We construct the thermal bounce solution in holographic models that describes first-order phase transitions between the deconfined and confined phases in strongly-coupled gauge theories. This new, periodic Euclidean solution represents transitions that occur via thermally-assisted tunneling and interpolates between the $O(4)$-symmetric vacuum bubble at zero temperature and the high temperature $O(3)$-symmetric critical bubble associated with classical thermal fluctuations. The exact thermal bounce solution can be used to obtain the bounce action at low temperatures which allows for a more accurate determination of vacuum decay rates, significantly improving previous estimates in holographic models. In particular, provided the phase transition is sufficiently supercooled, new predictions are obtained for the gravitational wave signal strength for critical temperatures ranging from the TeV scale up to $10^{12}$ GeV, some of which are within reach of future gravitational wave detectors.

hep-ph↗

arXiv2025

Simple third order operator-splitting schemes for stochastic mechanics and field theory

We present a method for constructing numerical schemes with up to 3rd strong convergence order for solution of a class of stochastic differential equations, including equations of the Langevin type. The construction proceeds in two stages. In the first stage one approximates the stochastic equation by a differential equation with smooth coefficients randomly sampled at each time step. In the second stage the resulting regular equation is solved with the conventional operator-splitting techniques. This separation renders the approach flexible, allowing one to freely combine the numerical techniques most suitable to the problem at hand. The approach applies to ordinary and partial stochastic differential equations. In the latter case, it naturally gives rise to pseudo-spectral algorithms. We numerically test the strong convergence of several schemes obtained with this method in mechanical examples. Application to partial differential equations is illustrated by real-time simulations of a scalar field with quartic self-interaction coupled to a heat bath. The simulations accurately reproduce the thermodynamic properties of the field and are used to explore dynamics of thermal false vacuum decay in the case of negative quartic coupling.

hep-lat↗

arXiv2024

Thermal false vacuum decay in (1+1)-dimensions: Evidence for non-equilibrium dynamics

We numerically study the evolution of a classical real scalar field in ${(1+1)}$ dimensions with initial conditions describing thermal fluctuations around a metastable vacuum. We track false vacuum decay in real time and compare several observables to the predictions of the standard Euclidean formalism. We find agreement for the shape of the critical bubble and the exponential suppression of the decay rate. However, the decay rate prefactor is almost an order of magnitude lower than the predicted value. We argue that this signals a breakdown of thermal equilibrium during the bubble nucleation. In addition, the inefficient thermalization in the system biases the properties of the statistical ensemble and leads to further decrease of the decay rate with time. We substantiate our interpretation with a suite of stochastic field simulations with controlled thermalization time. Varying this time we find that the predictions of the standard equilibrium formalism are recovered when it is sufficiently short. We propose an upper bound on the thermalization time that must be satisfied in order to ensure the applicability of the Euclidean rate calculation. We discuss that this bound is unavoidably violated in common single-field models, irrespective of the number of spacetime dimensions, implying that deviations from equilibrium in these models cannot be neglected. In theories with multiple fields, the bound may or may not hold, depending on the setup details. We investigate one more signature of non-equilibrium dynamics -- coherent oscillonic precursors to the critical bubble nucleation. We show that they get suppressed in the stochastic dynamical simulations when the thermalization time is reduced.

hep-th↗

arXiv2024

Thermal False Vacuum Decay Is More Than It Seems

We study the decay of a thermally excited metastable vacuum in classical field theory using real-time numerical simulations. We find a significantly lower decay rate than predicted by standard thermal theory at moderate temperatures, $E_b/T\sim 10$, where $E_b$ is the critical bubble energy. The discrepancy is due to the violation of thermal equilibrium during the critical bubble nucleation and is reduced if thermalization is enhanced by introduction of dissipation and thermal noise. We formulate a condition for the system to remain in equilibrium during the nucleation process and show that it is generally violated in weakly coupled field theories. Nevertheless, we argue that the violation of thermal equilibrium becomes irrelevant for the false vacuum decay rate at sufficiently low temperatures and the standard thermal rate is recovered.

hep-th↗

arXiv2024

Nontopological Electromagnetic Hedgehogs

We study classical localised configurations - solitons - in a theory of self-interacting complex Proca field with the global $U(1)$ symmetry. We focus on spherically-symmetric solitons near the nonrelativistic limit, which are supported by the quartic interactions of the neutral Proca field. Such solitons can source the radial electric (magnetic) field if one introduces a parity-even (parity-odd) coupling of the Proca field to the electromagnetic field tensor. We discuss the conditions of existence of such nontopological ''electromagnetic hedgehogs'' and their properties.

hep-th↗

arXiv2023

Probing the Local Dark Matter Halo with Neutrino Oscillations

Dark matter particles can form halos gravitationally bound to massive astrophysical objects. The Earth could have such a halo where depending on the particle mass, the halo either extends beyond the surface or is confined to the Earth's interior. We consider the possibility that if dark matter particles are coupled to neutrinos, then neutrino oscillations can be used to probe the Earth's dark matter halo. In particular, atmospheric neutrinos traversing the Earth can be sensitive to a small size, interior halo, inaccessible by other means. Depending on the halo mass and neutrino energy, constraints on the dark matter-neutrino couplings are obtained from the halo corrections to the neutrino oscillations.

hep-ph↗

arXiv2023

Out of this world neutrino oscillations

We study how vacuum neutrino oscillations can be affected by a causal, nonlinear and state-dependent modification of quantum field theory that may be interpreted using the many-worlds formulation of quantum mechanics. The effect is induced by a Higgs-neutrino Yukawa interaction that causes a nonlinear interference between the neutrino mass eigenstates. This leads to a tiny change in the oscillation pattern of light, active neutrinos without altering the oscillation frequencies. At large baselines where the oscillations disappear, the nonlinear effect is also suppressed and does not source correlations between the mass eigenstates once they are entangled with the environment. Our example provides a way to compute effects of nonlinear quantum mechanics and field theory that may probe the possible physical reality of many worlds.

hep-th↗

arXiv2023

Non-perturbative production of fermionic dark matter from fast preheating

We investigate non-perturbative production of fermionic dark matter in the early universe. We study analytically the gravitational production mechanism accompanied by the coupling of fermions to the background inflaton field. The latter leads to the variation of effective fermion mass during preheating and makes the resulting spectrum and abundance sensitive to its parameters. Assuming fast preheating that completes in less than the inflationary Hubble time and no oscillations of the inflaton field after inflation, we find an abundant production of particles with energies ranging from the inflationary Hubble rate to the inverse duration of preheating. The produced fermions can account for all observed dark matter in a broad range of parameters. As an application of our analysis, we study non-perturbative production of heavy Majorana neutrino in the model of Palatini Higgs inflation.

gr-qc↗

arXiv2022

On thermal false vacuum decay around black holes

In flat space and at finite temperature, there are two regimes of false vacuum decay in quantum field theory. At low temperature, the decay proceeds through thermally-assisted tunneling described by periodic Euclidean solutions -- bounces -- with non-trivial time dependence. On the other hand, at high temperature the bounces are time-independent and describe thermal jumps of the field over the potential barrier. We argue that only solutions of the second type are relevant for false vacuum decay catalyzed by a black hole in equilibrium with thermal bath. The argument applies to a wide class of spherical black holes, including $d$-dimensional AdS/dS-Schwarzschild black holes and Reissner-Nordström black holes sufficiently far from criticality. It does not rely on the thin-wall approximation and applies to multi-field scalar theories.

gr-qc↗

arXiv2022

Preheating in Palatini Higgs inflation on the lattice

We study preheating following Higgs inflation in the Palatini formulation of gravity. We numerically evolve perturbations of the radial mode of the Higgs field and that of three scalars modeling the gauge bosons. We compare the two non-perturbative mechanisms of growth of excitations -- parametric resonance and tachyonic instability -- and confirm that the latter plays the dominant role. Our results provide further evidence that preheating in Palatini Higgs inflation happens within a single oscillation of the Higgs field about the bottom of its potential, consistent with the approximation of an instantaneous preheating.

hep-ph↗

arXiv2022

Black hole induced false vacuum decay: The role of greybody factors

We study false vacuum decay catalyzed by black holes. We consider a toy two-dimensional model of a scalar field with an unstable potential in the background of a dilaton black hole. A realistic black hole in four dimensions possesses the potential barrier for linear field perturbations. We model this barrier -- the greybody factor -- for spherically-symmetric perturbations in the toy model by adding a coupling between the scalar field and dilaton. We compute analytically the decay rate for the black hole in thermal equilibrium (Hartle-Hawking state) and for the radiating black hole in empty space (Unruh state). Our results show that, contrary to the Hartle-Hawking vacuum, the decay probability of the Unruh vacuum remains exponentially suppressed at all black hole temperatures. We argue that this result holds also in four dimensions.

hep-th↗

arXiv2022

Gravitational Portals with Non-Minimal Couplings

We consider the effects of non-minimal couplings to curvature of the form $ξ_S S^2 R$, for three types of scalars: the Higgs boson, the inflaton, and a scalar dark matter candidate. We compute the abundance of dark matter produced by these non-minimal couplings to gravity and compare to similar results with minimal couplings. We also compute the contribution to the radiation bath during reheating. The main effect is a potential augmentation of the maximum temperature during reheating. A model independent limit of $\mathcal{O}(10^{12})$ GeV is obtained. For couplings $ξ_S \gtrsim \mathcal{O}(1)$, these dominate over minimal gravitational interactions.

hep-ph↗

arXiv2021

Scale and Weyl Invariance in Einstein-Cartan Gravity

We show how Einstein-Cartan gravity can accommodate both global scale and local scale (Weyl) invariance. To this end, we construct a wide class of models with nonpropagaing torsion and a nonminimally coupled scalar field. In phenomenological applications the scalar field is associated with the Higgs boson. For global scale invariance, an additional field -- dilaton -- is needed to make the theory phenomenologically viable. In the case of the Weyl symmetry, the dilaton is spurious and the theory reduces to a sub-class of one-field models. In both scenarios of scale invariance, we derive an equivalent metric theory and discuss possible implications for phenomenology.

hep-th↗

arXiv2021

Black hole induced false vacuum decay from first principles

We provide a method to calculate the rate of false vacuum decay induced by a black hole. The method uses complex tunneling solutions and consistently takes into account the structure of different quantum vacua in the black hole metric via boundary conditions. The latter are connected to the asymptotic behavior of the time-ordered Green's function in the corresponding vacua. We illustrate the technique on a two-dimensional toy model of a scalar field with inverted Liouville potential in an external background of a dilaton black hole. We analytically derive the exponential suppression of tunneling from the Boulware, Hartle-Hawking and Unruh vacua and show that they are parametrically different. The Unruh vacuum decay rate is exponentially smaller than the decay rate of the Hartle-Hawking state, though both rates become unsuppressed at high enough black hole temperature. We interpret the vanishing suppression of the Unruh vacuum decay at high temperature as an artifact of the two-dimensional model and discuss why this result can be modified in the realistic case of black holes in four dimensions.

hep-th↗

arXiv2021

Einstein-Cartan Portal to Dark Matter

It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Poincaré group, which puts gravity on the same footing as the Standard Model fields. The resulting theory -- Einstein-Cartan gravity -- inevitably contains four-fermion and scalar-fermion interactions that originate from torsion associated with spin degrees of freedom. We show that these interactions lead to a novel mechanism for producing singlet fermions in the Early Universe. These fermions can play the role of dark matter particles. The mechanism is operative in a large range of dark matter particle masses: from a few keV up to $\sim 10^8~$GeV. We discuss potential observational consequences of keV-scale dark matter produced this way, in particular for right-handed neutrinos. We conclude that a determination of the primordial dark matter momentum distribution might be able to shed light on the gravity-induced fermionic interactions.

hep-ph↗