arXiv subjects

Kimmo Kainulainen

Publications and source records attributed to Kimmo Kainulainen.

At least 19 recordsLinked to original sources

arXiv2026

Consistent Thermal Resummation and Phase Transitions with 2PI Methods

We apply the two-particle irreducible (2PI) formalism as a framework for a consistent thermal resummation in studies of cosmological phase transitions. Considering a model with two mixing real scalar fields, we work within the Hartree approximation and renormalize the 2PI effective action, while introducing a connection to physical parameters. This yields the Hartree-resummed finite-temperature effective potential, which is valid for all temperatures and avoids the limitations of conventional methods based on the high-temperature approximation. With this potential, we study one- and two-step transitions within the model, and compare our results with those obtained using resummation schemes widely employed in the literature. Finally, we evaluate the gravitational wave spectrum generated from first-order phase transitions, demonstrating the impact of the choice of resummation scheme on the predicted spectrum.

hep-ph↗

arXiv2026

Tachyonic particle production: quantum 2PI formalism with momentum exchanging collisions

Oscillating spacetime curvature can drive particle production during reheating, whose accurate modeling requires the use of non-perturbative out-of-equilibrium methods. Tachyonic instabilities have previously been studied using 2-Particle Irreducible (2PI) formalism in the Hartree approximation, which however misses important momentum exchanging interactions. We present a self-consistent approximation scheme for reducing the non-local next-to-leading order 2PI equations of motion to local quantum kinetic equations, which can be solved with standard methods. We pay special attention to interactions involving unstable modes during tachyonic instabilities.

hep-ph↗

arXiv2024

Coherent collision integrals for neutrino transport equations

We present quantum kinetic equations for neutrinos and derive Feynman rules for computing scattering rates involving coherent states. Our rules encompass both flavour- and particle-antiparticle coherence and allow writing down the scattering matrix elements and collision integrals with the same intuitive ease as with the usual non-coherent Feynman rules. Our results are useful for computing collision rates that arise routinely in the context of coherently mixing neutrinos with arbitrary masses. We give several explicit examples, including some collision integrals for coherently mixing neutrinos in supernovae.

hep-ph↗

arXiv2024

Systematic moment expansion for electroweak baryogenesis

We present a systematic moment expansion for solving the semiclassical Boltzmann equations for electroweak baryogenesis. The expansion is developed in powers of adiabatic coordinate velocity, and it is used for computing the CP-violating seed asymmetry at the front of the phase transition wall, that sources the eventual baryon asymmetry of the universe (BAU). We implement the method in a benchmark model, with a CP-violating mass arising from a dimension-5 operator coupling the fermion to a singlet scalar field. We find that the higher moment calculations yield a BAU that can significantly differ from the commonly used two-moment approximation. We discuss in detail the underlying approximations in the moment method and propose a new truncation scheme for the expansion, that appears to give more numerically robust results than the previous schemes.

hep-ph↗

arXiv2024

Tachyonic production of dark relics: classical lattice vs. quantum 2PI in Hartree truncation

We study the out-of-equilibrium production of non-minimally coupled self-interacting scalar dark matter during reheating using classical lattice simulations. The outcomes of the classical simulations are in qualitative agreement with the previous results obtained using the quantum 2PI approach in the Hartree truncation. In particular, the novel non-linear resonance found in the 2PI Hartee study is present also in the classical lattice simulations and can dominate the final dark matter yield. For the parameters considered, the difference in final value of the scalar two-point function between the two approaches is a factor of O(1).

hep-ph↗

arXiv2023

Anatomy of real intermediate state-subtraction scheme

We study the origin of the real intermediate state subtraction problem and compare its different solutions. We show that the ambiguity in subtraction schemes arises from the on-shell approximation for the 2-point functions that reduces the Schwinger-Dyson equations to the Boltzmann limit. We also suggest a new subtraction scheme which, unlike the earlier definitions, never leads to negative scattering rates. This scheme also quantifies the validity of the on-shell limit in terms of an effective one-particle weight function $R(Δ)$, where $Δ$ measures the region around the resonance associated with the real state.

hep-ph↗

arXiv2023

Quantum kinetic equations with flavor and particle-antiparticle coherences for neutrinos

We develop a formalism to model neutrino evolution encompassing both flavor and particle-antiparticle mixings and decohering collisions. Our results include a quantum kinetic equation (a set of coupled scalar equations) for the generalized neutrino density matrix, valid for arbitrary neutrino masses and kinematics, and a comprehensive set of Feynman rules to compute collision integrals for coherently evolving states. We expose a novel shell structure describing the phase space of mixing neutrinos and show how the prior information on the system can enter into the theory and modify the neutrino flavor evolution. Potential applications of our results include modelling neutrino distributions in hot and dense environments and studies of neutrino mixing effects in colliders and in the early Universe.

hep-ph↗

arXiv2023

Quantum transport theory for neutrinos with flavor and particle-antiparticle mixing

We derive quantum kinetic equations for mixing neutrinos including consistent forward scattering terms and collision integrals for coherent neutrino states. In practice, we reduce the general Kadanoff--Baym equations in a few clearly justified steps to a generalized density matrix equation that describes both the flavour- and particle-antiparticle coherences and is valid for arbitrary neutrino masses and kinematics. We then reduce this equation to a simpler particle-antiparticle diagonal limit and eventually to the ultra-relativistic limit. Our derivation includes simple Feynman rules for computing collision integrals with the coherence information. We also expose a novel spectral shell structure underlying the mixing phenomenon and quantify how the prior information on the system impacts on the QKE's, leading to a direct effect on its evolution. Our results can be used for example to accurately model neutrino distributions in hot and dense environments and to study the production and decay of mixing heavy neutrinos in colliders.

hep-ph↗

arXiv2022

Tachyonic production of dark relics: a non-perturbative quantum study

We study production of dark relics during reheating after the end of inflation in a system consisting of a non-minimally coupled spectator scalar field and the inflaton. We derive a set of renormalized quantum transport equations for the one-point function and the two-point function of the spectator field and solve them numerically. We find that our system can embody both tachyonic and parametric instabilities. The former is an expected result due to the non-minimal coupling, but the latter displays new features driven by a novel interplay of the two-point function with the Ricci scalar. We find that when the parametric instability driven by the two-point function takes place, it dominates the total particle production. The quantitative results are also found to be highly sensitive to the model parameters.

hep-ph↗

arXiv2022

Momentum distributions of cosmic relics: Improved analysis

We solve coupled momentum-dependent Boltzmann equations for the phase space distribution of cosmic relic particles, without resorting to approximations of assuming kinetic equilibrium or neglecting backscattering or elastic interactions. Our method is amendable to precision numerical computations. To test it, we consider two benchmark models where the momentum dependence of dark matter distribution function is potentially important: a real singlet scalar extension near the Higgs resonance and a sterile neutrino dark matter model with a singlet scalar mediator. The singlet scalar example shows that the kinetic equilibrium may hold surprisingly well even near sharp resonances. However, the integrated method may underestimate the relic density by up to 40% in extreme cases. In the sterile neutrino dark matter model, we studied how the inclusion of previously ignored elastic interactions and processes with initial state sterile neutrinos could affect the nonthermal nature of their resulting distributions. Here the effects turned out to be negligible, proving the robustness of the earlier predictions.

hep-ph↗

arXiv2022

Can Primordial Black Holes as all Dark Matter explain Fast Radio Bursts?

Primordial black holes (PBHs) are one of the most interesting nonparticle dark matter (DM) candidates. They may explain all the DM content in the Universe in the mass regime from about $10^{-14}M_{\odot}$ to $10^{-11}M_{\odot}$. We study PBHs as the source of fast radio bursts (FRBs) via magnetic reconnection in the event of collisions between them and neutron stars (NSs) in galaxies. We investigate the energy loss of PBHs during PBH-NS encounters to model their capture by NSs. To an order-of-magnitude estimation, we conclude that the parameter space of PBHs being all DM is accidentally consistent with that to produce FRBs with a rate which is the order of the observed FRB rate.

astro-ph.HE↗

arXiv2022

Non-equilibrium dynamics of a scalar field with quantum backreaction

We study the dynamical evolution of coupled one- and two-point functions of a scalar field in the 2PI framework at the Hartree approximation, including backreaction from out-of-equilibrium modes. We renormalize the 2PI equations of motion in an on-shell scheme in terms of physical parameters. We present the Hartree-resummed renormalized effective potential at finite temperature and critically discuss the role of the effective potential in a non-equilibrium system. We follow the decay and thermalization of a scalar field from an initial cold state with all energy stored in the potential, into a fully thermalized system with a finite temperature. We identify the non-perturbative processes of parametric resonance and spinodal instability taking place during the reheating stage. In particular we study the unstable modes in the region where the vacuum 1PI effective action becomes complex and show that such spinodal modes can have a dramatic effect on the evolution of the one-point function. Our methods can be easily adapted to simulate reheating at the end of inflation.

hep-ph↗

arXiv2021

Flavour mixing transport theory and resonant leptogenesis

We derive non-equilibrium quantum transport equations for flavour-mixing fermions. We develop the formalism mostly in the context of resonant leptogenesis with two mixing Majorana fermions and one lepton flavour, but our master equations are valid more generally in homogeneous and isotropic systems. We give a hierarchy of quantum kinetic equations, valid at different approximations, that can accommodate helicity and arbitrary mass differences. In the mass-degenerate limit the equations take the familiar form of density matrix equations. We also derive the semiclassical Boltzmann limit of our equations, including the CP-violating source, whose regulator corresponds to the flavour coherence damping rate. Boltzmann equations are accurate and insensitive to the particular form of the regulator in the weakly resonant case $Δm \gg Γ$, but for $Δm \lesssim Γ$ they are qualitatively correct at best, and their accuracy crucially depends on the form of the CP-violating source.

hep-ph↗

arXiv2021

CP-violating transport theory for Electroweak Baryogenesis with thermal corrections

We derive CP-violating transport equations for fermions for electroweak baryogenesis from the CTP-formalism including thermal corrections at the one-loop level. We consider both the VEV-insertion approximation (VIA) and the semiclassical (SC) formalism. We show that the VIA-method is based on an {\em assumption} that leads to an ill-defined source term containing a pinch singularity, whose regularisation by thermal effects leads to ambiguities including spurious ultraviolet and infrared divergences. We then carefully review the derivation of the semiclassical formalism and extend it to include thermal corrections. We present the semiclassical Boltzmann equations for thermal WKB-quasiparticles with source terms up to the second order in gradients that contain both dispersive and finite width corrections. We also show that the SC-method reproduces the current divergence equations and that a correct implementation of the Fick's law captures the semiclassical source term even with conserved total current $\partial_μj^μ= 0$. Our results show that the VIA-source term is not just ambiguous, but that it does not exist. Finally, we show that the collisional source terms reported earlier in the semiclassical literature are also spurious, and vanishes in a consistent calculation.

hep-ph↗

arXiv2021

Baryogenesis and gravity waves from a UV-completed electroweak phase transition

We study gravity wave production and baryogenesis at the electroweak phase transition, in a real singlet scalar extension of the Standard Model, including vector-like top partners to generate the CP violation needed for electroweak baryogenesis (EWBG). The singlet makes the phase transition strongly first-order through its coupling to the Higgs boson, and it spontaneously breaks CP invariance through a dimension-5 contribution to the top quark mass term, generated by integrating out the heavy top quark partners. We improve on previous studies by incorporating updated transport equations, compatible with large bubble wall velocities. The wall speed and thickness are computed directly from the microphysical parameters rather than treating them as free parameters, allowing for a first-principles computation of the baryon asymmetry. The size of the CP-violating dimension-5 operator needed for EWBG is constrained by collider, electroweak precision, and renormalization group running constraints. We identify regions of parameter space that can produce the observed baryon asymmetry or observable gravitational (GW) wave signals. Contrary to standard lore, we find that for strong deflagrations, the efficiencies of large baryon asymmetry production and strong GW signals can be positively correlated. However we find the overall likelihood of observably large GW signals to be smaller than estimated in previous studies. In particular, only detonation-type transitions are predicted to produce observably large gravitational waves.

hep-ph↗

arXiv2020

Precision calculations of dark matter relic abundance

The dark matter annihilation channels sometimes involve sharp resonances. In such cases the usual momentum averaged approximations for computing the DM abundance may not be accurate. We develop an easily accessible momentum dependent framework for computing the DM abundance accurately and efficiently near such features. We apply the method to the case of a singlet scalar dark matter $s$ interacting with SM through higgs portal $λ_{\rm hs}s^2 h^2$ and compare the results with different momentum averaged methods. The accuracy of the latter depend strongly on the strength of the elastic interactions and corrections are large if WIMP has negligible interactions beyond the main annihilation channel. In the singlet scalar model however, the standard model scatterings induce an efficient kinetic equilibrium that validates the momentum averaged computation to 20 per cent accuracy. We update the current extent of the allowed region in the light singlet scalar dark matter to $m_{\rm S} \in [56,62.5]$ GeV.

hep-ph↗

arXiv2020

Quantum transport and the phase space structure of the Wightman functions

We study the phase space structure of exact quantum Wightman functions in spatially homogeneous, temporally varying systems. In addition to the usual mass shells, the Wightman functions display additional coherence shells around zero frequency $k_0=0$, which carry the information of the local quantum coherence of particle-antiparticle pairs. We find also other structures, which encode non-local correlations in time, and discuss their role and decoherence. We give a simple derivation of the cQPA formalism, a set of quantum transport equations, that can be used to study interacting systems including the local quantum coherence. We compute quantum currents created by a temporal change in a particle's mass, comparing the exact Wightman function approach, the cQPA and the semiclassical methods. We find that the semiclassical approximation, which is fully encompassed by the cQPA, works surprisingly well even for very sharp temporal features. This is encouraging for the application of semiclassical methods in electroweak baryogenesis with strong phase transitions.

hep-th↗

arXiv2020

Electroweak baryogenesis at high wall velocities

It is widely believed that electroweak baryogenesis should be suppressed in strong phase transitions with fast-moving bubble walls, but this effect has never been quantitatively studied. We rederive fluid equations describing transport of particle asymmetries near the bubble wall without making the small-wall-velocity approximation. We show that the suppression of the baryon asymmetry is a smooth function of the wall speed and that there is no special behavior when crossing the sound speed barrier. Electroweak baryogenesis can thus be efficient also with strong detonations, generically associated with models with observably large gravitational waves. We also make a systematic and critical comparison of our improved transport equations to another one commonly used in the literature, based on the VEV-insertion formalism.

hep-ph↗