arXiv subjects

Masaki Yamada

Publications and source records attributed to Masaki Yamada.

At least 55 records · Page 3Linked to original sources

arXiv2022

Baryon Asymmetric Universe from Spontaneous CP Violation

Spontaneous CP violation, such as the Nelson-Barr (NB) mechanism, is an attractive scenario for addressing the strong CP problem while realizing the observed phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. However, not only the CKM phase but also the baryon asymmetric Universe requires sources of CP violation. In this study, we show that a supersymmetric NB mechanism can naturally accommodate the Affleck-Dine (AD) baryogenesis within a CP-invariant Lagrangian. The model provides flat directions associated with new heavy quarks. Focusing on one of the directions, we find that the correct baryon asymmetry is obtained with a sufficiently low reheating temperature which does not cause the gravitino problem. Some parameter space is consistent with the gravitino dark matter. We assess radiative corrections to the strong CP phase induced by gauge-mediated supersymmetry breaking and CP-violating heavy fields and show that the strong CP problem is solved in a viable parameter space where the visible sector supersymmetric particles must be lighter than O(100) TeV. Even in the case that they are heavier than the TeV scale, our model predicts the neutron electric dipole moment within the reach of the near future experiments. Our model addresses the electroweak naturalness problem, strong CP problem, baryon asymmetric Universe, and dark matter. Then, the model may give a new compelling paradigm of physics beyond the Standard Model.

hep-ph↗

arXiv2022

Gravitational production of dark photon dark matter with mass generated by the Higgs mechanism

We study the gravitational production of dark photon dark matter during inflation, when dark photons acquire mass by the Higgs mechanism. In the previous study, it was assumed that the dark photon has a Stückelberg mass, or a mass generated by the Higgs mechanism with a sufficiently heavy Higgs boson. In this paper we consider a case in which the Higgs boson is not fully decoupled; the Higgs field changes its vacuum expectation value after inflation. Then, the dark photon mass also changes with time after inflation, and the time evolution of the longitudinal mode is different from the case with a Stückelberg mass. Consequently, the spectrum of the dark photon energy density can have two peaks at an intermediate scale and a small scale. We show that the dark photon can explain the dark matter if its current mass is larger than $6 \, μ{\rm eV} \times (H_I / 10^{14} \, {\rm GeV})^{-4}$ and smaller than $0.8 \, {\rm GeV} \times (H_I / 10^{14} \, {\rm GeV})^{-3/2}$, with $H_I$ being the Hubble parameter during inflation. A higher mass is required if one considers a larger gauge coupling constant. The result for the Stückelberg mass can be reproduced in the limit of a small gauge coupling constant. We also comment on the constraints set by various conjectures in quantum gravity theory.

hep-ph↗

arXiv2021

Wash-In Leptogenesis

We present a leptogenesis mechanism based on the standard type-I seesaw model that successfully operates at right-handed-neutrino masses as low as a few 100 TeV. This mechanism, which we dub "wash-in leptogenesis", does not require any CP violation in the neutrino sector and can be implemented even in the regime of strong wash-out. The key idea behind wash-in leptogenesis is to generalize standard freeze-out leptogenesis to a nonminimal cosmological background in which the chemical potentials of all particles not in chemical equilibrium at the temperature of leptogenesis are allowed to take arbitrary values. This sets the stage for building a plethora of new baryogenesis models where chemical potentials generated at high temperatures are reprocessed to generate a nonvanishing B-L asymmetry at low temperatures. As concrete examples, we discuss wash-in leptogenesis after axion inflation and in the context of grand unification.

hep-ph↗

arXiv2021

Learning Mobile CNN Feature Extraction Toward Fast Computation of Visual Object Tracking

In this paper, we construct a lightweight, high-precision and high-speed object tracking using a trained CNN. Conventional methods with trained CNNs use VGG16 network which requires powerful computational resources. Therefore, there is a problem that it is difficult to apply in low computation resources environments. To solve this problem, we use MobileNetV3, which is a CNN for mobile terminals.Based on Feature Map Selection Tracking, we propose a new architecture that extracts effective features of MobileNet for object tracking. The architecture requires no online learning but only offline learning. In addition, by using features of objects other than tracking target, the features of tracking target are extracted more efficiently. We measure the tracking accuracy with Visual Tracker Benchmark and confirm that the proposed method can perform high-precision and high-speed calculation even in low computation resource environments.

cs.CV↗

arXiv2021

A more attractive scheme for radion stabilization and supercooled phase transition

We propose a new radion stabilization mechanism in the Randall-Sundrum spacetime, introducing a bulk ${SU(N_H)}$ gauge field which confines at a TeV scale. It turns out that the radion is stabilized by the balance between a brane tension and a pressure due to the Casimir energy of the strong ${SU(N_H)}$ gauge field. We investigate the phase transition between the Randall-Sundrum (compactified) spacetime and a de-compactified spacetime and determine the parameter regime in which eternal (old) inflation is avoided and the phase transition can be completed. In comparison to the Goldberger-Wise mechanism, the 5D Planck mass can be larger than the AdS curvature and a classical description of the gravity is reliable in our stabilization mechanism. We also discuss the effect of the phase transition in cosmology such as an entropy dilution and a production of gravitational waves.

hep-ph↗

arXiv2021

Cosmic Birefringence Triggered by Dark Matter Domination

Cosmic birefringence is predicted if an axion-like particle (ALP) moves after the recombination. We show that this naturally happens if the ALP is coupled to the dark matter density because it then acquires a large effective mass after the matter-radiation equality. Our scenario applies to a broad range of the ALP mass $m_ϕ\lesssim 10^{-28}$ eV, even smaller than the present Hubble constant. We give a simple model to realize this scenario, where dark matter is made of hidden monopoles, which give the ALP such a large effective mass through the Witten effect. The mechanism works if the ALP decay constant is of order the GUT scale without a fine-tuning of the initial misalignment angle. For smaller decay constant, the hidden monopole can be a fraction of dark matter. We also study the implications for the QCD axion, and show that the domain wall problem can be solved by the effective mass.

hep-ph↗

arXiv2021

A natural and simple UV completion of the QCD axion model

The novel PQ mechanism replaces the strong CP problem with some challenges in a model building. In particular, the challenges arise regarding i) the origin of an anomalous global symmetry called a PQ symmetry, ii) the scale of the PQ symmetry breaking, and iii) the quality of the PQ symmetry. In this letter, we provide a natural and simple UV completed model that addresses these challenges. Extra quarks and anti-quarks are separated by two branes in the Randall-Sundrum ${\bf R}^4 \times S^1 / {\bf Z}_2$ spacetime while a hidden SU($N_H$) gauge field condensates in the bulk. The brane separation is the origin of the PQ symmetry and its breaking scale is given by the dynamical scale of the SU($N_H$) gauge interaction. The (generalized) Casimir force of SU($N_H$) condensation stabilizes the 5th dimension, which guarantees the quality of the PQ symmetry.

hep-ph↗

arXiv2021

Dark Matter Production during the Thermalization Era

We revisit the non-thermal dark matter (DM) production during the thermalization and reheating era after inflation. The decay of inflaton produces high-energy particles that are thermalized to complete the reheating of the Universe. Before the thermalization is completed, DM can be produced from a collision between the high-energy particles and/or the ambient plasma. We calculate the DM abundance produced from these processes for the case where the cross section of the DM production is proportional to the $n$-th power of the center of mass energy. We find that the collision between the high-energy particles is almost always dominant for $n \gtrsim 4$ while it is subdominant for $n \lesssim 2$. The production from the ambient plasma is dominant when $n \lesssim 3$ and the reheating temperature is of the order of or larger than the DM mass. The production from a collision between the high-energy particle and the ambient plasma is important for $n \lesssim 2$ and the reheating temperature is much lower than the DM mass.

hep-ph↗

arXiv2020

Trapping Effect for QCD Axion Dark Matter

In the early universe, the potential of a scalar field can be significantly modified, and the scalar field may be trapped for a long time in a different location than the current vacuum. The trapping effect can increase or decrease the scalar abundance. For instance, in thermal inflation, a scalar field is trapped at the top of the potential by a thermal effect and dominates the universe to drive inflation for a short period of time. On the other hand, a scalar abundance can be exponentially suppressed in the adiabatic suppression mechanism, where a scalar field moves adiabatically by a time-dependent trapping potential. In this study, we investigate such a trapping effect on the abundance of scalar fields. Specifically, we investigate how the abundance of a scalar field depends on its initial position in the case of a double well potential and identify the physical quantity that controls the abundance. Then, we study the QCD axion abundance for various values of the misalignment angle, where the axon potential receives a large temporal mass due to the Witten effect. We find that the axion abundance is suppressed due to the adiabatic suppression mechanism even when it is trapped near the maximum of the potential, if the trapping effect is sufficiently large.

hep-ph↗

arXiv2020

XENON1T excess from anomaly-free ALP dark matter and its implications for stellar cooling anomaly

Recently, an anomalous excess was found in the electronic recoil data collected at the XENON1T experiment. The excess may be explained by an axion-like particle (ALP) with mass of a few keV and a coupling to electron of $g_{ae} \sim 10^{-13}$, if the ALP constitutes all or some fraction of local dark matter (DM). In order to satisfy the X-ray constraint, the ALP coupling to photons must be significantly suppressed compared to that to electrons. This strongly suggests that the ALP has no anomalous couplings to photons, i.e., there is no U(1)$_{\rm PQ}$-U(1)$_{\rm em}$-U(1)$_{\rm em}$ anomaly. We show that such anomaly-free ALP DM predicts an X-ray line signal with a definite strength through the operator arising from threshold corrections, and compare it with the projected sensitivity of the ATHENA X-ray observatory. The abundance of ALP DM can be explained by the misalignment mechanism, or by thermal production if it constitutes a part of DM. In particular, we find that the anomalous excess reported by the XENON1T experiment as well as the stellar cooling anomalies from white dwarfs and red giants can be explained simultaneously better when the ALP constitutes about 10\% of DM. As concrete models, we revisit the leptophilic anomaly-free ALP DM considered in [Phys.Lett.B 734 (2014) 178-182] as well as an ALP model based on a two Higgs doublet model in the supplemental material.

hep-ph↗

arXiv2020

Gravitational Waves and Dark Radiation from Dark Phase Transition: Connecting NANOGrav Pulsar Timing Data and Hubble Tension

Recent pulsar timing data reported by the NANOGrav collaboration may indicate the existence of a stochastic gravitational wave background around $f \sim 10^{-8}$ Hz. We explore a possibility to generate such low-frequency gravitational waves from a dark sector phase transition. Assuming that the dark sector is completely decoupled from the visible sector except via the gravitational interaction, we find that some amount of dark radiation should remain until present. The NANOGrav data implies that the amount of dark radiation is close to the current upper bound, which may help mitigate the so-called Hubble tension. If the existence of dark radiation is not confirmed in the future CMB-S4 experiment, it would imply the existence of new particles feebly interacting with the standard model sector at an energy scale of O(1-100) MeV.

astro-ph.CO↗

arXiv2020

Spherically Symmetric Scalar Hair for Charged Black Holes

The no-hair theorem by Mayo and Bekenstein states that there exists no non-extremal static and spherical charged black hole endowed with hair in the form of a charged scalar field with a self-interaction potential. In our recent work, we showed that the effect of a scalar mass term is important at an asymptotic infinity, which was omitted to prove the no-hair theorem. In this paper, we demonstrate that there actually exists static and spherical charged scalar hair, dubbed as Q-hair, around charged black holes, by taking into account the backreaction to the metric and gauge field. We also discuss that Q-cloud, which is constructed without the backreaction around a Reissner-Nordström black hole, is a good approximation to Q-hair under a certain limit.

gr-qc↗

arXiv2020

What if ALP dark matter for the XENON1T excess is the inflaton

The recent XENON1T excess in the electron recoil data can be explained by anomaly-free axion-like particle (ALP) dark matter with mass $m_ϕ= 2.3 \pm 0.2\,$keV and the decay constant $f_ϕ/q_e \simeq 2 \times 10^{10} \sqrt{Ω_ϕ/Ω_{\rm DM}}\,{\rm GeV}$. Intriguingly, the suggested mass and decay constant are consistent with the relation, $f_ϕ\sim 10^3 \sqrt{m_ϕM_p}$, predicted in a scenario where the ALP plays the role of the inflaton. This raises a possibility that the ALP dark matter responsible for the XENON1T excess also drove inflation in the very early universe. We study implications of the XENON1T excess for the ALP inflation and thermal history of the universe after inflation.

hep-ph↗

arXiv2020

Strongly-interacting massive particle and dark photon in the era of intensity frontier

A strongly interacting massive particle (SIMP) is an interesting candidate for dark matter (DM) because its self-interaction cross section can be naturally strong enough to address the astrophysical problem of small-scale structure formation. A simple model was proposed by assuming a monopole condensation, where composite SIMP comes from a "strongly interacting" U(1)$_{\rm d}$ gauge theory. In the original model, the DM relic abundance is determined by the $3\to2$ annihilation process via the Wess-Zumino-Witten term. In this letter, we discuss that the DM relic abundance is naturally determined also by a semi-annihilation process via a kinetic mixing between the hypercharge gauge boson and the dark U(1)$_{\rm d}$ gauge boson (dark photon). The dark photon can be discovered by LDMX-style missing momentum experiments in the near future.

hep-ph↗

arXiv2020

Fundamental Forces and Scalar Field Dynamics in the Early Universe

Scalar weak gravity conjectures (SWGCs) attempt to pinpoint the ranges of couplings consistent with a fundamental theory of all interactions. We identify a generic dynamical consequence of these conjectures for cosmology and show that SWGCs imply a particular behavior of the scalar fields in the early universe. A scalar field that develops a large expectation value during inflation must relax to the minimum of effective potential at a later time. SWGCs imply that a homogeneous distribution of the field is unstable with respect to fragmentation into localized lumps, which could potentially lead to significant consequences for cosmology.

hep-th↗

arXiv2020

Charged black holes in non-linear Q-clouds with O(3) symmetry

We construct charged soliton solutions around spherical charged black holes with no angular momentum in asymptotically flat spacetime. These solutions are non-linear generalizations of charged scalar clouds, dubbed Q-ball hair or Q-clouds, and they do not contradict the non-existence theorem for free (linear) scalar clouds around charged black holes. These solutions are the first examples of O(3) solutions for Q-clouds around a non-extremal and non-rotating BH in the Abelian gauge theory. We show that a solution exists with an infinitely short hair in the limit of extremal black holes. We discuss the evolution of scalar hair in a system with fixed total charge and describe how the existence of Q-clouds is related to the weak-gravity conjecture. The reason that the no-hair theorem by Mayo and Bekenstein cannot be applied to the massive scalar field is also discussed.

gr-qc↗

arXiv2019

4-volume cutoff measure of the multiverse

Predictions in an eternally inflating multiverse are meaningless unless we specify the probability measure. The scale-factor cutoff is perhaps the simplest and most successful measure which avoid catastrophic problems such as the youngness paradox, runaway problem, and Boltzmann brain problem, but it is not well defined in contracting regions with a negative cosmological constant. In this paper, we propose a new measure with properties similar to the scale-factor cutoff which is well defined everywhere. The measure is defined by a cutoff in the 4-volume spanned by infinitesimal comoving neighborhoods in a congruence of timelike geodesics. The probability distributions for the cosmological constant and for the curvature parameter in this measure are similar to those for the scale factor cutoff and are in a good agreement with observations.

hep-th↗

arXiv2019

Light majoron cold dark matter from topological defects and the formation of boson stars

We show that for a relatively light majoron ($\ll 100 $ eV) non-thermal production from topological defects is an efficient production mechanism. Taking the type I seesaw as benchmark scheme, we estimate the primordial majoron abundance and determine the required parameter choices where it can account for the observed cosmological dark matter. The latter is consistent with the scale of unification. Possible direct detection of light majorons with future experiments such as PTOLEMY and the formation of boson stars from the majoron dark matter are also discussed.

hep-ph↗