arXiv subjects

Philipp Klose

Publications and source records attributed to Philipp Klose.

9 recordsLinked to original sources

arXiv2026

Thermal Resummation for Very Strong First-order Phase Transitions

Effective potentials are a key ingredient for predicting stochastic gravitational wave backgrounds from strong first-order phase transitions in the early universe. Established techniques for a robust computation, including dimensional reduction, rely on a high-temperature expansion that is expected to break down for very strong transitions capable of producing observable backgrounds at next-generation gravitational wave detectors such as LISA. We argue that existing 2PI effective action techniques enable consistent resummation for such transitions, and use them to compute the next-to-leading order effective potential of the Abelian Higgs model for a strong transition in a general covariant gauge. We find that our result can be recovered from a Daisy resummed potential by modifying the power counting, and show explicitly that it satisfies the leading-order Nielsen identity needed for gauge-independent predictions of the bubble nucleation rate and is consistent with prior results for small Higgs condensates.

hep-ph↗

arXiv2026

Higher-dimensional operators at finite-temperature affect gravitational-wave predictions

We investigate the effect of higher-dimensional marginal operators on the thermodynamics of cosmological phase transitions. Using the Abelian Higgs model as a representative for radiatively-generated one-step transitions, we systematically match these operators, which arise at higher orders in the underlying high-temperature expansion of thermal effective field theory, and use field redefinitions to construct a complete, minimal, and gauge-invariant operator basis. The Abelian Higgs model shares the essential infrared structure of more realistic gauge-Higgs theories at high temperatures, allowing us to test the validity of dimensional reduction in a simplified setting. We argue that for strong transitions, temporal gauge modes, which enhance the transition strength, should be treated on equal footing with spatial ones. Marginal operators are found to weaken the transition and introduce significant uncertainties for strong transitions. For transitions strong enough to produce gravitational waves detectable by LISA, our findings suggest that the high-temperature expansion may break down entirely. This would limit the applicability of effective theory techniques, including their use in non-perturbative lattice studies.

hep-ph↗

arXiv2026

On the importance of radiation-era initial conditions for tensor perturbations

Conservation of super-horizon tensor fluctuations is crucial for connecting inflation to observations. Starting from first principles, recent works have found violations of this conservation if free-streaming radiation is produced during reheating. We show that the non-conservation is sensitive to the radiation initial state, and argue that the physical state should be affected by tensor perturbations that are already present during reheating. The deviation from super-horizon conservation is then negligible, recovering the standard result from kinetic theory. In contrast, a globally homogeneous and isotropic plasma state leads to a large suppression of tensor amplitudes. This difference between the local (physical) and global thermal equilibrium settles the discrepancy between the older and recent literature.

hep-th↗

arXiv2026

QCD corrections to the electroweak sphaleron rate

The electroweak sphaleron rate in the high temperature phase of the Standard Model is inversely proportional to the weak-isospin conductivity. So far, only electroweak interactions were included in its computation. Here we take into account quark scattering through strong interactions at leading-log order. These reduce the quark contribution to the conductivity by up to 15 %, and the total conductivity by up to 6 %.

hep-ph↗

arXiv2024

Upper Bound on Thermal Gravitational Wave Backgrounds from Hidden Sectors

Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation. We study the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe. While this contribution can easily dominate over that from Standard Model particles, we find that both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future. We illustrate our results for axions and heavy neutral leptons. Finally, our results suggest that previous works overestimated the gravitational wave background from particle decays out of thermal equilibrium.

hep-ph↗

arXiv2022

Factorizing Hidden Particle Production Rates

A method is proposed to streamline the computation of hidden particle production rates by factorizing them into i) a model-independent SM contribution, and ii) a observable-independent hidden sector contribution. The Standard Model (SM) contribution can be computed once for each observable and re-used for a wide array of hidden sector models, while the hidden sector contribution can be computed once for each model, and re-used for a wide array of observables. The SM contribution also facilitates extracting model independent constraints on hidden particle production. The method is compatible with effective field theory (EFT) and simplified model approaches. It is illustrated by factorizing the rate of charged kaon decays into a charged lepton and a number of hidden particles, and a single form factor $F_{\ell}$ is found to parametrize the impact of general hidden sectors. We derive model-independent constraints on the form factor $F_e$ that governs decays into positrons and hidden particles.

hep-ph↗

arXiv2021

Portal Effective Theories: A framework for the model independent description of light hidden sector interactions

We present a framework for the construction of portal effective theories (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encompass all portal operators up to dimension five, while the strong scale PETs additionally contain all portal operators of dimension six and seven that contribute at leading order to quark-flavour violating transitions. Using the strong scale PETs, we define a set of portal currents that couple hidden sectors to QCD, and construct portal chiral perturbation theories ($χ$PTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal $χ$PT Lagrangian that are not fixed by SM observations using non-perturbative matching techniques and give a complete list of the resulting one- and two-meson portal interactions. From those, we compute transition amplitudes for three golden channels that are used in hidden sector searches at fixed target experiments: i) charged kaon decay into a charged pion and a spin zero messenger, ii) charged kaon decay into a charged lepton and a spin one half messenger, and iii) neutral pion decay into a photon and a spin one messenger. Finally, we compare these amplitudes to specific expressions for models featuring light scalar particles, axion-like particles, heavy neutral leptons, and dark photons.

hep-ph↗

arXiv2020

Relativistic and spectator effects in leptogenesis with heavy sterile neutrinos

For leptogenesis with heavy sterile neutrinos above the electroweak scale, asymmetries produced at early times (in the relativistic regime) are relevant, if they are protected from washout. This can occur for weak washout or when the asymmetry is partly protected by being transferred to spectator fields. We thus study the relevance of relativistic effects for leptogenesis in a minimal seesaw model with two sterile neutrinos in the strongly hierarchical limit. Starting from first principles, we derive a set of momentum-averaged fluid equations to calculate the final $B-L$ asymmetry as a function of the washout strength and for different initial conditions at order one accuracy. For this, we take the leading fluid approximation for the relativistic $CP$-even and odd rates. Assuming that spectator fields remain in chemical equilibrium, we find that for weak washout, relativistic corrections lead to a sign flip and an enhancement of the asymmetry for a vanishing initial abundance of sterile neutrinos. As an example for the effect of partially equilibrated spectators, we consider bottom-Yukawa and weak-sphaleron interactions in leptogenesis driven by sterile neutrinos with masses $\gtrsim 5\times10^{12}$ GeV. For a vanishing initial abundance of sterile neutrinos, this can give rise to another flip and an absolute enhancement of the final asymmetry in the strong washout regime by up to two orders of magnitude relative to the cases either without spectators or with fully equilibrated ones. These effects are less pronounced for thermal initial conditions for the sterile neutrinos. The $CP$-violating source in the relativistic regime at early times is important as it is proportional to the product of lepton-number violating and lepton-number conserving rates, and therefore less suppressed than an extrapolation of the nonrelativistic approximations may suggest.

hep-ph↗

arXiv2019

On Lepton Number Violation in Heavy Neutrino Decays at Colliders

We study the perspective to observe lepton number violating signatures from heavy Majorana neutrino decays at colliders in view of the requirement to explain the light neutrino masses via the seesaw mechanism. In the minimal model with only two heavy neutrinos and in the $ν$MSM one can identify three distinct regions in the mass-mixing plane. For Majorana masses above the electroweak scale the branching ratio for lepton number violating processes at the LHC is generically suppressed. For masses well below the electroweak scale that are probed in displaced vertex searches or at fixed target experiments lepton number violation is the rule and can only be avoided at the cost of fine tuning. In between there is a mass regime where both possibilities coexist. In models with more than two heavy neutrinos the larger parameter space allows for more freedom, but our results remain qualitatively correct unless there is a mass degeneracy amongst more than two of the heavy neutrinos.

hep-ph↗