arXiv subjects

Kristjan Müürsepp

Publications and source records attributed to Kristjan Müürsepp.

13 recordsLinked to original sources

arXiv2026

Gravitational Waves from Dimension-6 Assisted Peccei - Quinn Phase Transitions

We consider a simple model-independent extension of the minimal KSVZ axion model, augmenting the Peccei - Quinn effective potential with a dimension-6 operator invariant under the Peccei - Quinn symmetry. We show that this operator can trigger a first-order phase transition and study its cosmological consequences. We map out the parameter space, consistent with present observational constraints, in which the phase transition produces a gravitational wave signal observable at current and future interferometers, and we investigate the resulting dark matter phenomenology. Finally, we present specific ultraviolet completions that generate the dimension-6 operator, each leading to distinct phenomenological signatures, which motivates our model-independent approach.

hep-ph↗

arXiv2025

An EFT approach to the study of multi-phase criticality scenarios

Multi-phase critical scenarios explain the observed Higgs boson mass scale by the almost simultaneous occurrence of two smoothly connected phases of the theory, which differ by the selected vacuum configuration. A generic prediction of the framework is the presence of a further light scalar state, the dilaton, which naturally couples weakly to the Higgs boson. The implementation of the framework usually requires the presence of a third, heavier state, which plays the role of dark matter and ensures the couplings run so that the multi-phase criticality condition is met. In this paper we consider the multi-phase criticality limit of an extension of the Standard Model including two extra scalar singlets, addressing the scenario with effective field theory methods that are particularly suited for treating the hierarchical mass spectrum that this construction yields. The analysis improves on the approximated results available in the Literature and explores the phenomenology of the model at collider and dark matter experiments. We find that the running of scalar couplings in the EFT between the two scales cannot be ignored, but the quantum corrections from the dark matter candidate are not noticeably modified.

hep-ph↗

arXiv2025

CP-violation in production of heavy neutrinos from bubble collisions

First order phase transitions (FOPT) in the early Universe can be powerful emitters of both relativistic and heavy particles, upon the collision of ultra-relativistic bubble shells. If the particles coupling to the bubble wall have CP-violating interactions, the same collision process can also create a local lepton or baryon charge. This CP-violation can originate from different channels, which have only been partially addressed in the literature. We present a systematic analysis of the different channels inducing CP-violation during bubble collisions: 1) the decay of heavy particles 2) the production of heavy particles and 3) the production of light and relativistic Standard Model (SM) particles. As an illustration of the impact that such mechanisms can have on baryon number and dark matter (DM) abundance, we then introduce a simple model of cogenesis, separating a positive and a negative lepton number in the SM and a dark sector (DS). The lepton number asymmetry in the SM can be used to explain the baryon asymmetry of the Universe (BAU), while the opposite asymmetry in the DS is responsible for determining the abundance of DM. Moreover, the masses of light neutrinos can be understood via the inverse seesaw mechanism, with the lepton-violating Majorana mass originating from the FOPT. A typical signal produced by a FOPT is the irreducible gravitational wave (GW) background. We find that a substantial portion of the parameter space can be probed at future observatories like the Einstein Telescope (ET).

hep-ph↗

arXiv2025

Accelerated cosmic expansion, mass creation, and the QCD axion

We propose a mechanism in which the current acceleration of cosmic expansion is driven by continuous creation of energy density $ρ_b$ for a certain field $φ_b$. We accordingly modify Einstein equation, derive modified Friedmann equations and analyze the regimes in which cosmic acceleration occurs. The creation process requires $ρ_b\neq 0$ as initial condition, which we enforce by identifying $φ_b$ with the axion of a hidden gauge group that confined in recent cosmological times, leading to a level crossing between $φ_b$ and the QCD axion, which is assumed to comprise dark matter. The conversion of a small fraction of QCD axions into $φ_b$ shortly before matter-dark energy equality generates the initial $ρ_b$ needed to trigger the creation process and offers a solution to the coincidence puzzle.

hep-ph↗

arXiv2025

Fuzzy dark matter fails to explain the dark matter cores

Ultrafaint dwarf galaxies (UFDs) are ideal for studying dark matter (DM) due to minimal baryonic effects. UFD observations suggest cored DM profiles. We find that the core radius -- stellar mass scaling predicted by fuzzy dark matter (FDM) is at $6.1σ$ tension with UFD observations. Combining observations from 27 UFDs, the required FDM mass $m_a = 3.2_{-0.6}^{+0.8}\times 10^{-21}\,{\rm eV}$ is also in conflict with existing Lyman-$α$ bounds. Our results suggest that FDM cannot provide a consistent explanation for DM cores and imply $m_a > 2.2\times 10^{-21}\,{\rm eV}$ at to $2σ$ CL.

astro-ph.CO↗

arXiv2024

On the viability of the QCD axion - dark energy transition

This conference thesis summarizes my presentation at Tartu Tuorla Cosmology meeting based on a collaborative work with E.Nardi and C.Smarra \cite{Muursepp:2024mbb}. It is well known that a pseudo-Nambu Goldstone boson (pNGB) coupled to a confining gauge group obtains a non-zero contribution to its mass through instanton effects. At non-zero temperature, this manifests in temperature-dependent mass $m^{2}(T) \propto T^{-n}$. If these particles come to dominate the energy density of the Universe in the non-relativistic regime they would accelerate the expansion of the Universe for $n>2$, thus providing a dark energy (DE) component. In this work, we outline a scenario in which a pNGB $ϕ_{b}$ presently undergoing confinement could realize such a scenario. Using energetic considerations we find that $ϕ_{b}$ alone is not enough to produce the experimentally observed amount of DE. However, coupling $ϕ_{b}$ to the QCD axion $ϕ_a$ allows the transfer of energy from the QCD axion to the pNGB via non-adiabatic level-crossing thus reproducing the observed amount of dark matter (DM) and DE.

hep-ph↗

arXiv2023

Dynamics of false vacuum bubbles with trapped particles

We study the impact of the ambient fluid on the evolution of collapsing false vacuum bubbles by simulating the dynamics of a coupled bubble-particle system. A significant increase in the mass of the particles across the bubble wall leads to a buildup of those particles inside the false vacuum bubble. We show that the backreaction of the particles on the bubble slows or even reverses the collapse. Consequently, if the particles in the true vacuum become heavier than in the false vacuum, the particle-wall interactions always decrease the compactness that the false vacuum bubbles can reach making their collapse to black holes less likely.

hep-ph↗

arXiv2022

Vacuum stability with radiative Yukawa couplings

We explore the electroweak vacuum stability in the framework of a recently proposed paradigm for the origin of Yukawa couplings. These arise as low energy effective couplings radiatively generated by portal interactions with a hidden, or dark, sector at the one-loop level. Possible tree-level Yukawa couplings are forbidden by a new underlying symmetry, assumed to be spontaneously broken by the vacuum expectation value of a new scalar field above the electroweak scale. As a consequence, the top Yukawa interaction ceases to behave as a local operator at energies above the new sector scale and, therefore, cannot contribute to the running of the quartic Higgs coupling at higher energies. By studying two complementary scenarios, we explicitly show that the framework can achieve the stability of the electroweak vacuum without particular tuning of parameters. The proposed mechanism requires the existence of a dark sector and new portal messenger scalar interactions that, connecting the Standard Model to the dark sector fields, could be tested at the LHC and future collider experiments.

hep-ph↗

arXiv2021

Phenomenology of a Fake Inert Doublet Model

We introduce a new way of modeling the physics beyond the Standard Model by considering fake, strictly off-shell degrees of freedom: the fakeons. To demonstrate the approach and exemplify its reach, we re-analyze the phenomenology of the Inert Doublet Model under the assumption that the second doublet is a fakeon. Remarkably, the fake doublet avoids the most stringent $Z$-pole constraints regardless of the chosen mass scale, thereby allowing for the presence of new effects well below the electroweak scale. Furthermore, the absence of on-shell propagation prevents fakeons from inducing missing energy signatures in collider experiments. The distinguishing features of the model appear at the loop level, where fakeons modify the Higgs boson $h\toγγ$ decay width and the Higgs trilinear coupling. The running of Standard Model parameters proceeds as in the usual Inert Doublet Model case. Therefore, the fake doublet can also ensure the stability of the Standard Model vacuum. Our work shows that fakeons are a valid alternative to the usual tools of particle physics model building, with the potential to shape a new paradigm, where the significance of the existing experimental constraints towards new physics must necessarily be reconsidered.

hep-ph↗

arXiv2021

A fake doublet solution to the muon anomalous magnetic moment

Extensions to the Standard Model that use strictly off-shell degrees of freedom - the fakeons - allow for new measurable interactions at energy scales usually precluded by the constraints that target the on-shell propagation of new particles. Here we employ the interactions between a new fake scalar doublet and the muon to explain the recent Fermilab measurement of its anomalous magnetic moment. Remarkably, unlike in the case of usual particles, the experimental result can be matched for fakeon masses below the electroweak scale without contradicting the stringent precision data and collider bounds on new light degrees of freedom. Our analysis, therefore, demonstrates that the fakeon approach offers unexpected viable possibilities to model new physics naturally at low scales.

hep-ph↗

arXiv2021

An effective field theory of the Delta-resonance

We present an effective field theory of the $Δ$-resonance as an interacting Weinberg's $(3/2,0)\oplus (0,3/2)$ field in the multi-spinor formalism. We derive its interactions with nucleons $N$, pions $π$ and photons $γ$, and compute the $Δ$-resonance cross-sections in pion-nucleon scattering and pion photo-production. The theory contains only the physical spin-3/2 degrees of freedom. Thus, it is intrinsically consistent at the Hamiltonian level and, unlike the commonly used Rarita-Schwinger framework, does not require any additional ad hoc manipulation of couplings or propagators. The symmetries of hadronic physics select a unique operator for each coupling $NπΔ$ and $γπΔ$. The proposed framework can be extended to also describe other higher-spin hadronic resonances.

hep-ph↗

arXiv2021

Confronting spin-3/2 and other new fermions with the muon g-2 measurement

The new measurement of the muon's anomalous magnetic moment released by the Muon g-2 experiment at Fermilab sets strong constraints on the properties of many new particles. Using an effective field theory approach to the interactions of higher-spin fields, we evaluate the contribution of an electrically neutral and colour singlet spin-3/2 fermion to $(g-2)_μ$ and derive the corresponding constraints on its mass and couplings. These constraints are then compared with the ones on spin-1/2 fermions, such as the vector-like leptons that are predicted by various extensions of the Standard Model, the excited leptons which appear in composite models, as well as the charginos and neutralinos of supersymmetric theories. Unlike these new spin-1/2 fermions, the spin-3/2 particles generate only small contributions to the muon anomalous magnetic moment.

hep-ph↗

arXiv2019

Dark origin of quark flavor hierarchy and mixing

We provide a dynamical mechanism for the generation of the Cabibbo-Kobayashi-Maskawa matrix in the context of a recently proposed model of flavor. The framework, based on the paradigm that Yukawa couplings are effective low energy couplings generated radiatively by dark sector interactions, is here extended to include a new scalar field which plays the role of a {\it dark flavon}. Being singlet under the Standard Model gauge group but charged under the $U(1)$ symmetry of the dark sector, this particle sources new two- and three-loop diagrams that result in off-diagonal Yukawa interactions, thereby providing a simple explanation for the observed structure of the CKM matrix. By using an effective parametrization of these new loop contributions, we show that the CKM matrix elements can be correctly reproduced for perturbative values of the involved couplings. We outline the rich phenomenology predicted by the scenario and detail its implications for the LHC and future collider experiments.

hep-ph↗