arXiv subjects

Yutaro Shoji

Publications and source records attributed to Yutaro Shoji.

At least 19 recordsLinked to original sources

arXiv2026

Topological Blocking of the Schwinger Effect in the Salpeter Equation: A Lefschetz Thimble Analysis

We present a comprehensive Lefschetz thimble analysis of the one-dimensional Salpeter equation under a strong electric field. By treating the non-local square-root operator within the framework of algebraic analysis, we construct the full solution space, which includes relativistic generalizations of the Airy Ai and Bi functions and their negative-energy counterparts. Through a direct comparison with the Dirac and Klein-Gordon equations, we provide a geometric explanation for the absence of Klein paradox and the Schwinger effect in the Salpeter equation. Furthermore, our findings establish a unified geometric interpretation of the Schwinger effect across different relativistic wave equations.

hep-th↗

arXiv2026

Symmetry of Bounce Solutions at Finite Temperature

The seminal work of Coleman, Glaser, and Martin established that, at zero temperature, any non-trivial solution to the equations of motion with the least Euclidean action is $O(D)$-symmetric. This paper extends their foundational analysis to finite temperature. We rigorously prove that for a broad class of scalar potentials, any saddle-point configuration with the least action is necessarily $O(D\!-\!1)$-symmetric and monotonic in the spatial directions. This result provides a firm mathematical justification for the symmetry properties widely assumed in studies of thermal vacuum decay and cosmological phase transitions.

hep-th↗

arXiv2026

Stable Evaluation of Lefschetz Thimble Intersection Numbers: Towards Real-Time Path Integrals

We introduce a robust numerical method for determining intersection numbers of Lefschetz thimbles in multivariable settings. Our approach employs the multiple shooting method to solve the upward flow equations from the saddle points to the original integration cycle, which also enables us to determine the signs of the intersection numbers. The method demonstrates stable and reliable performance, and has been tested for systems with up to $20$ variables, which can be further extended by adopting quadruple-precision arithmetic. We determine intersection numbers for several complex saddle points in a discretized path integral, providing new insights into the structure of real-time path integrals. The proposed method is broadly applicable to a wide range of problems involving oscillatory integrals in physics and mathematics.

hep-th↗

arXiv2025

A novel realization of linear seesaw model in a non-invertible selection rule with the assistance of $\mathbb Z_3$ symmetry

We propose a novel realization of linear seesaw model in a non-invertible selection rule with the assistance of $\mathbb Z_3$ symmetry. In our framework, Dirac mass matrices are generated at one-loop level, dynamically breaking the non-invertible symmetry while the symmetry is invariant under the tree-level. In addition to the active neutrino masses, the model exhibits rich and testable phenomenology such as non-unitarity bound, lepton flavor violations, lepton anomalous magnetic moment, and dark matter candidate. After describing our model, we carry out numerical analysis and show some results for our physical parameters.

hep-ph↗

arXiv2025

Regularization of Functional Determinants of Radial Operators via Heat Kernel Coefficients

We propose an efficient regularization method for functional determinants of radial operators using heat kernel coefficients. Our key finding is a systematic way to identify heat kernel coefficients in the angular momentum space. We explicitly obtain the formulas up to sixth order in the heat kernel expansion, which suffice to regularize up to 13-dimensional functional determinants. We find that the heat kernel coefficients accurately approximate the large angular momentum dependence of functional determinants, and make numerical computations more efficient. In the limit of a large angular momentum, our formulas reduce to the Wentzel-Kramers-Brillouin formulas in previous studies, but are extended to higher orders. All the results are available in both the zeta function regularization and the dimensional regularization.

hep-th↗

arXiv2025

False Vacuum Decay Rate From Thin To Thick Walls

We consider a single real scalar field in flat spacetime with a polynomial potential up to $ϕ^4$, that has a local minimum, the false vacuum, and a deeper global minimum, the true vacuum. When the vacua are almost degenerate we are in the thin wall regime, while as their difference in potential energy increases, we approach the thick wall regime. We give explicit simple formulae for the decay rate of the false vacuum in 3 and 4 spacetime dimensions. Our results include a careful treatment both of the bounce action, which enters at the exponent of the decay rate, and of the functional determinant at one loop, which determines the prefactor. The bounce action is computed analytically as an expansion in the thin wall parameter in generic $D$ dimensions. We find that truncating such an expansion at second order we obtain a remarkably accurate bounce action also deep into thick wall regimes. We calculate the functional determinant numerically in 3 and 4 dimensions and fit the results with simple polynomials of the same thin wall parameter. This allows us to write the complete one-loop decay rate as a compact expression, which works accurately from thin to thick wall regimes.

hep-th↗

arXiv2025

Functional Determinants for False Vacuum Decay

We derive simple expressions to regularise functional determinants from fluctuations of fields with spin 0, 1/2, and 1. These are important for the precise dimensionful determination of false vacuum decay rates. We work in $D = 4$ Euclidean dimensions and use familiar Feynman diagrammatic techniques with a double expansion in interactions and masses, together with dimensional regularisation in momentum space. We Fourier transform to coordinate space and end up with a simple regularisation prescription in terms of single integrals over the Euclidean radius of field-dependent masses and their derivatives. Our results apply to models with an arbitrary scalar potential and with any number of scalars, fermions, gauge bosons and associated ghosts. We exemplify this approach on the Standard Model with a streamlined calculation of the renormalisation and isolation of divergences in fluctuation determinants.

hep-ph↗

arXiv2024

Revising the full one-loop gauge prefactor in electroweak vacuum stability

We revisit the decay rate of the electroweak vacuum in the Standard Model with the full one-loop prefactor. We focus on the gauge degrees of freedom and derive the degeneracy factors appearing in the functional determinant using group theoretical arguments. Our treatment shows that the transverse modes were previously overcounted, so we revise the calculation of that part of the prefactor. The new result modifies the gauge fields' contribution by $6\%$ and slightly decreases the previously predicted lifetime of the electroweak vacuum, which remains much longer than the age of the universe. Our discussion of the transverse mode degeneracy applies to any calculation of functional determinants involving gauge fields in four dimensions.

hep-ph↗

arXiv2024

Heating Galaxy Clusters with Interacting Dark Matter

The overcooling of cool core clusters is a persistent puzzle in the astrophysics of galaxy clusters. We propose that it may naturally be resolved via interactions between the baryons of the intracluster medium (ICM) and its dark matter (DM). DM-baryon interactions can inject heat into the ICM to offset bremmstrahlung cooling, but these interactions are also strongly constrained by existing experiments and astrophysical observations. We survey existing constraints and combine these with the energetic needs of an observed sample of cool core clusters. We find that a robust parameter space exists for baryon-DM scattering solutions to the cooling flow problem, provided that only a sub-component of DM interacts strongly with the baryons. Interestingly, baryon-DM scattering is a thermally stable heating source so long as the baryon temperature is greater than $1/3-1/2$ the DM temperature, a condition that seems to be satisfied observationally.

astro-ph.CO↗

arXiv2023

Migdal Effect in Dark Matter Direct Detection Experiments

The elastic scattering of an atomic nucleus plays a central role in dark matter direct detection experiments. In those experiments, it is usually assumed that the atomic electrons around the nucleus of the target material immediately follow the motion of the recoil nucleus. In reality, however, it takes some time for the electrons to catch up, which results in ionization and excitation of the atoms. In previous studies, those effects are taken into account by using the so-called Migdal's approach, in which the final state ionization/excitation are treated separately from the nuclear recoil. In this paper, we reformulate the Migdal's approach so that the "atomic recoil" cross section is obtained coherently, where we make transparent the energy-momentum conservation and the probability conservation. We show that the final state ionization/excitation can enhance the detectability of rather light dark matter in the GeV mass range via the {\it nuclear} scattering. We also discuss the coherent neutrino-nucleus scattering, where the same effects are expected.

hep-ph↗

arXiv2023

State-of-the-Art Calculation of the Decay Rate of Electroweak Vacuum in Standard Model

The decay rate of the electroweak (EW) vacuum is calculated in the framework of the standard model (SM) of particle physics, using the recent progresses in the understanding of the decay rate of metastable vacuum in gauge theories. We give a manifestly gauge-invariant expression of the decay rate. We also perform a detailed numerical calculation of the decay rate. With the best-fit values of the SM parameters, we find that the decay rate of the EW vacuum per unit volume is about $10^{-577}\ {\rm Gyr^{-1}Gpc^{-3}}$; with the uncertainty in the top mass, the decay rate is estimated as $10^{-295}-10^{-1465}\ {\rm Gyr^{-1}Gpc^{-3}}$.

hep-ph↗

arXiv2023

Low-Energy Compton Scattering in Materials

Low-energy Compton scattering is an important background for sub-GeV dark matter direct-detection and other experiments. Current Compton scattering calculations typically rely on assumptions that are not valid in the low-energy region of interest, beneath ~ 50 eV. Here we relate the low-energy Compton scattering differential cross section to the dielectric response of the material. Our new approach can be used for a wide range of materials and includes all-electron, band structure, and collective effects, which can be particularly relevant at low energies. We demonstrate the strength of our approach in several solid-state systems, in particular Si, Ge, GaAs, and SiC, which are relevant for current and proposed experiments searching for dark matter, neutrinos, and millicharged particles.

hep-ph↗

arXiv2023

Is the Coleman de Luccia action minimum?: AdS/CFT approach

We use the anti-de Sitter/conformal field theory (AdS/CFT) correspondence to find the least bounce action in an AdS false vacuum state, i.e., the most probable decay process of the metastable AdS vacuum within the Euclidean formalism by Callan and Coleman. It was shown that the $O(4)$ symmetric bounce solution leads to the action minimum in the absence of gravity, but it is non-trivial in the presence of gravity. The AdS/CFT duality is used to evade the difficulties particular to a metastable gravitational system, such as the problems of negative modes and unbounded action. To this end, we show that the Fubini bounce solution in CFT, corresponding to the Coleman de Luccia bounce in AdS, gives the least action among all finite bounce solutions in a conformal scalar field theory. Thus, we prove that the Coleman de Luccia action is the least action when (i) the background is AdS, (ii) the AdS radii, $L_+$ and $L_-$, in the false and true vacua, respectively, satisfy $L_+ / L_- \simeq 1$, and (iii) a metastable potential gives a thin-wall bounce much larger than the AdS radii.

hep-th↗

arXiv2023

Stability of Electroweak Vacuum and Supersymmetric Contribution to Muon g-2

We study the stability of the electroweak vacuum in the supersymmetric (SUSY) standard model (SM), paying particular attention to its relation to the SUSY contribution to the muon anomalous magnetic moment $a_μ$. If the SUSY contribution to $a_μ$ is sizable, the electroweak vacuum may become unstable because of enhanced trilinear scalar interactions in particular when the sleptons are heavy. Consequently, assuming enhanced SUSY contribution to $a_μ$, an upper bound on the slepton masses is obtained. We give a detailed prescription to perform a full one-loop calculation of the decay rate of the electroweak vacuum for the case that the SUSY contribution to $a_μ$ is enhanced. We also give an upper bound on the slepton masses as a function of the SUSY contribution to $a_μ$.

hep-ph↗

arXiv2023

Path Integral for Mixed Tunneling, Polychronic Tunneling and Quantum Gravity

Quantum tunneling in a many-body system is much more non-trivial than that in a one-body system. The most characteristic phenomenon is the mixed tunneling, which has been studied in many fields for decades. For instance, let us consider a system where there are two coupled particles and only one of them feels a potential barrier. Quantum tunneling of such a system is not described by either Euclidean or Lorentzian time evolution and the exponent of the WKB wave function becomes complex. Recently, a similar phenomenon, polychronic tunneling, has been proposed in quantum gravity, which enhances the decay rate of a meta-stable vacuum by many orders of magnitude. In this paper, we present path integral formalism that is applicable to such systems. The formalism can be directly extended to quantum gravity and has some implications on the problem of time in quantum gravity. We also discuss a possible relation to the conventional path integral.

hep-th↗

arXiv2023

Polychronic Tunneling: New Tunneling Processes Experiencing Euclidean and Lorentzian Evolution Simultaneously

We discuss new possible tunneling processes in the presence of gravity. We formulate quantum tunneling using the Wheeler-deWitt canonical quantization and the WKB approximation. The distinctive feature of our formulation is that it accommodates the coexistence of Euclidean and Lorentzian evolution. It opens up a new possibility of quantum tunneling; e.g. a bubble wall itself tunnels the potential barrier pulling the field nearby, where the wall region experiences the Euclidean evolution while the other regions experience the Lorentzian evolution simultaneously. We execute numerical analysis and find that such a process can have a much higher tunneling rate than that of the Coleman-De Luccia bounce. We also find that the new tunneling processes exist even in the decoupling regime of gravity and affect low energy phenomenology.

hep-th↗

arXiv2023

$SU(4)_C \times SU(2)_L \times U(1)_R$ models with modular $A_4$ symmetry

We study $SU(4)_C \times SU(2)_L \times U(1)_R$ models with modular $A_4$ symmetry that provide unified description of the quark and lepton sector including the flavor structures. The models are distinguished by the assignments of the modular weight on matter superfields. We carry out numerical $χ^2$ analysis and search for parameter sets that accomodate the experimental results. We provide a benchmark point for each model to illustrate implications of our models.

hep-ph↗

arXiv2022

Upper bound on the smuon mass from vacuum stability in the light of muon $g-2$ anomaly

We derive an upper bound on the smuon mass assuming that the muon $g-2$ anomaly is explained by the supersymmetric (SUSY) contribution. In the minimal SUSY standard model, the SUSY contribution to the muon $g-2$ is enhanced when the Higgsino mass parameter is large. Then, the smuon-smuon-Higgs trilinear coupling is enhanced, which may destabilize the electroweak vacuum. We calculate precisely the decay rate of the electroweak vacuum in such a case. We include one-loop effects which are crucial to determine the overall normalization of the decay rate. Requiring that the theoretical prediction of the muon anomalous magnetic moment is consistent with the observed value at the $1$ and $2σ$ levels (equal to the central value of the observed value), we found that the lightest smuon mass should be smaller than $1.38$ and $1.68\ {\rm TeV}$ ($1.20\ {\rm TeV}$) for $\tanβ=10$ (with $\tanβ$ being the ratio of the vacuum expectation values of the two Higgs bosons), respectively, and the bound is insensitive to the value of $\tanβ$.

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