Searcharxiv⌕ Search

arXiv subjects

Koichi Funakubo

Publications and source records attributed to Koichi Funakubo.

14 recordsLinked to original sources
arXiv2025

One-loop analysis of dark matter constraints in a complex scalar extension of the Standard Model

We investigate the complex singlet extension of the Standard Model, which provides a scalar dark matter candidate. We impose the constraints from the observed relic abundance together with the most stringent limits from direct detection experiments on the model. The counterterms are determined so that the vacuum and mass conditions are consistently satisfied at one-loop order, and the one-loop corrections to scalar self-interactions are fully included in the amplitudes involving the dark matter particle. As a result, the allowed parameter region shows clear deviations from the tree-level analysis, demonstrating the impact of quantum corrections on the phenomenology of dark matter.

hep-ph↗
arXiv2024

A renormalization group improvement for thermally resummed effective potential

We propose a novel method for renormalization group improvement of thermally resummed effective potential. In our method, $β$-functions are temperature dependent as a consequence of the divergence structure in resummed perturbation theory. In contrast to the ordinary $\overline{\text{MS}}$ scheme, the renormalization group invariance of the resummed finite-temperature effective potential holds order by order, which significantly mitigates a notorious renormalization scale dependence of phase transition quantities such as a critical temperature even at the one-loop order. We also devise a tractable method that enables one to incorporate temperature-dependent higher-order corrections by fully exploiting the renormalization group invariance.

hep-ph↗
arXiv2024

Refined renormalization group improvement for thermally resummed effective potential

We newly develop a renormalization group (RG) improvement for thermally resummed effective potentials. In this method, $β$-functions are consistently defined in resummed perturbation theories, so that order-by-order RG invariance is not spoiled after thermal resummation. With this improvement, scale dependences of phase transition quantities such as a critical temperature, which are known to be notoriously large at the one-loop order, are greatly reduced compared to calculations with the conventional $\overline{\text{MS}}$ scheme. By taking advantage of the RG invariance, we also devise a resummation method that can incorporate potentially harmful large logarithmic terms and temperature-dependent power corrections in a generic form. We point out that a resummed one-loop effective potential refined by the method can give results that agree with those obtained by resummed two-loop effective potentials within errors.

hep-ph↗
arXiv2020

Magnetic mass effect on the sphaleron energy

We elucidate a magnetic mass effect on a sphaleron energy that is crucial for baryon number preservation needed for successful electroweak baryogenesis. It is found that the sphaleron energy increases in response to the magnetic mass. As an application, we study the sphaleron energy and electroweak phase transition with the magnetic mass in a two-Higgs-doublet model. Although the magnetic mass can screen the gauge boson loops, it relaxes a baryon number preservation criterion more effectively, broadening the baryogenesis-possible region. Our findings would be universal in any new physics models as long as the gauge sector is common to the standard model.

hep-ph↗
arXiv2016

Does a band structure affect sphaleron processes?

Inspired by a recent work of Tye and Wong, we examine an effect of a band structure on baryon number preservation criteria requisite for successful electroweak baryogenesis. Action of a reduced model is fully constructed including a time component of the gauge field that is missing in the original work. The band structure is estimated more precisely in wider energy range based on WKB framework with three connection formulas to find that the band structure has little effect on the criteria at around 100 GeV temperature. We also address an issue of suppression factors peculiar to the $(B+L)$-changing process in high-energy collisions at zero temperature.

hep-ph↗
arXiv2013

Two-loop effective potential, thermal resummation and first-order phase transitions: Beyond the high-temperature expansion

We study a finite temperature two-loop resummed effective potential in the Abelian gauge theory. A tractable calculation scheme without using a high-temperature expansion is devised. We apply it to the Abelian-Higgs model and its extension to a minimal supersymmetric standard model-like model and study the thermal phase transition. It is shown that our scheme improves the previous results about 10% in the quantities relevant to the phase transition, and its impacts on bubble dynamics could be even more sizable. It still holds that the stop-stop-gluon sunset diagram enhances the strength of the first-order phase transition even without the high-temperature expansion.

hep-ph↗
arXiv2009

Electroweak phase transition, critical bubbles and sphaleron decoupling condition in the MSSM

The electroweak phase transition and the sphaleron decoupling condition in the MSSM are revisited taking the latest experimental data into account. The light Higgs boson scenario and the ordinary decoupling limit which are classified by the relative size between the CP-odd Higgs boson mass and Z boson mass are considered within the context of electroweak baryogenesis. We investigate v/T at not only the critical temperature at which the effective potential has two degenerate minima but also the nucleation temperature of the critical bubbles, where v is a vacuum expectation value of the Higgs boson and T denotes a temperature. It is found that v/T at the nucleation temperature can be enhanced by about 10% compared to that at the critical temperature. We also evaluate the sphaleron decoupling condition including the zero mode factors of the fluctuations around sphaleron. It is observed that the sphaleron decoupling condition at the nucleation temperature is given by v/T>1.38 for the typical parameter sets. In any phenomenologically allowed region, v/T at both the critical and nucleation temperatures cannot be large enough to satisfy such a sphaleron decoupling condition.

hep-ph↗
arXiv2005

Sphalerons in the NMSSM

We study sphaleron solutions in the next-to-minimal supersymmetric standard model. We find that the boundary condition on the singlet field at the origin of the radial coordinate is of Neumann type, while the other boundary conditions are of Dirichlet type. The sphaleron energy takes almost the same value as in the MSSM for wide range of parameters, in spite of the negative contribution from the cubic term in the Higgs potential.

hep-ph↗
arXiv2005

The Higgs sector in the Next-to-MSSM

We study the Higgs sector in the Next-to-Minimal Supersymmetric Standard Model with and without explicit CP violation, focusing on the case of weak scale expectation value of the singlet field. We scan a wide range of the parameter space to find out allowed regions by requiring that the electroweak vacuum be the global minimum of the effective potential and that the neutral Higgs bosons with moderate gauge coupling be heavier than the lower bound on the Higgs boson in the standard model. Among the allowed parameters, some sets admit the situation in which the light Higgs bosons couple with the $Z$ boson too weak to be found in present collider experiments. For such parameter sets, we find an upper bound on the charged Higgs mass which is reachable in LHC.

hep-ph↗
arXiv2005

Phase Transitions in the NMSSM

We study phase transitions in the Next-to-Minimal Supersymmetric Standard Model (NMSSM) with the weak scale vacuum expectation values of the singlet scalar, constrained by Higgs spectrum and vacuum stability. We find four different types of phase transitions, three of which have two-stage nature. In particular, one of the two-stage transitions admits strongly first order electroweak phase transition, even with heavy squarks. We introduce a tree-level explicit CP violation in the Higgs sector, which does not affect the neutron electric dipole moment. In contrast to the MSSM with the CP violation in the squark sector, a strongly first order phase transition is not so weakened by this CP violation.

hep-ph↗
arXiv1999

Transitional CP Violation in the MSSM and Electroweak Baryogenesis

Electroweak baryogenesis depends on the profile of the bubble wall created in the first-order phase transition. It is pointed out that CP violation in the Higgs sector of the MSSM could become large enough to explain the baryon asymmetry. We confirm this by solving the equations of motion for the Higgs fields with the effective potential at the transition temperature. That is, we present an example such that the transitional CP violation is realized and show the possibility that the baryon asymmetry of the universe may be produced, if marginally, by the $τ$ lepton interacting with the wall, when an explicit CP breaking in the Higgs sector, which is consistent with experimental bounds, is induced at the phase transition.

hep-ph↗
arXiv1996

Explicit CP Breaking and Electroweak Baryogenesis

We investigate spatial behaviors of the CP-violating angle $θ$ by solving the equation of motion of the two-Higgs-doublet model in the presence of a small explicit CP breaking $δ$. The moduli of the two Higgs scalars are fixed to be the kink shape with a common width. In addition to solutions $θ\sim O(δ)$ in all the region, we find several sets of two solutions of opposite signs, whose magnitudes become as large as $O(1)$ around the surface of the bubble wall while the CP violation in the broken phase limit is of $O(δ)$. Such set of solutions not only yield sufficient amount of the chiral charge flux, but also avoid the cancellation in the net baryon number because of a large discrepancy in their energy densities driven nonperturbatively by the small $δ$.

hep-ph↗
arXiv1992

Chiral Invariance and Species Doublers in Generic Fermion Models on the Lattice

Discussions are made on the structures of chirally invariant lattice actions without any restriction of hermiticity. With the help of the Ward-Takahashi identity a general conclusion can be derived that there must be species doublers in any chirally invariant model provided that the model is chosen as well-regularized, that is, there is no singularity in the propagator after introducing fermion mass on the lattice. Various examples are discussed to pick up better models defined in the sense that the number of species doubler is smaller than that of the naive Dirac action.

hep-lat↗