arXiv subjects

Hiromasa Nakatsuka

Publications and source records attributed to Hiromasa Nakatsuka.

24 records · Page 2Linked to original sources

arXiv2020

Probing Oscillons of Ultra-Light Axion-like Particle by 21cm Forest

Ultra-Light Axion-like Particle (ULAP) is motivated as one of the solutions to the small scale problems in astrophysics. When such a scalar particle oscillates with an $\mathcal{O}(1)$ amplitude in a potential shallower than quadratic, it can form a localized dense object, oscillon. Because of its longevity due to the approximate conservation of the adiabatic invariant, it can survive up to the recent universe as redshift $z \sim \mathcal{O}(10)$. The scale affected by these oscillons is determined by the ULAP mass $m$ and detectable by observations of 21cm line. In this paper, we examine the possibility to detect ULAP by 21cm line and find that the oscillon can enhance the signals of 21cm line observations when $m \lesssim 10^{-19}\ {\rm eV}$ and the fraction of ULAP to dark matter is much larger than $10^{-2}$ depending on the form of the potential.

astro-ph.CO↗

arXiv2020

Ultralight vector dark matter search with auxiliary length channels of gravitational wave detectors

Recently, a considerable amount of attention has been given to the search for ultralight dark matter by measuring the oscillating length changes in the arm cavities of gravitational wave detectors. Although gravitational wave detectors are extremely sensitive for measuring the differential arm length changes, the sensitivity to dark matter is largely attenuated, as the effect of dark matter is mostly common to arm cavity test masses. Here, we propose to use auxiliary length channels, which measure the changes in the power and signal recycling cavity lengths and the differential Michelson interferometer length. The sensitivity to dark matter can be enhanced by exploiting the fact that auxiliary interferometers are more asymmetric than two arm cavities. We show that the sensitivity to $U(1)_{B-L}$ gauge boson dark matter with masses below $7\times 10^{-14}$ eV can be greatly enhanced when our method is applied to a cryogenic gravitational wave detector KAGRA, which employs sapphire test masses and fused silica auxiliary mirrors. We show that KAGRA can probe more than an order of magnitude of unexplored parameter space at masses around $1.5 \times 10^{-14}$ eV, without any modifications to the existing interferometer.

hep-ph↗

arXiv2020

Q-ball decay through A-term in the gauge-mediated SUSY breaking scenario

Q-balls are non-topological solitons whose classical stability is ensured by a global $U(1)$ charge. In particular, Q-balls are produced in the framework of the Affleck-Dine baryogenesis where $U(1)$ charge is the baryon number. Since some type of Q-balls is stable against quantum decay into nucleons, it can work as the dark matter. When the dark matter Q-balls are captured into the neutron star, they absorb the surrounding neutrons and grow to consume all the neutrons, which leads to a stringent constraint on the Q-ball dark matter. However, the Q-ball growth stops due to the $U(1)$ breaking A-term. We revisit the constraint by the neutron star for the gauge-mediation type and the new type Q-ball, and find the allowed parameter space, in which Q-balls work as the dark matter.

hep-ph↗

arXiv2019

Generation of Primordial Black Holes and Gravitational Waves from Dilaton-Gauge Field Dynamics

We study the observational signatures from particle production of a $U(1)$ gauge field kinetically coupled to an inflaton. Regarding the form of gauge kinetic function, we consider the possibility that it becomes stabilized at a certain time, which makes the growing power of the gauge field evolve non-monotonically with a sharp transition. Remarkably, the copious production of the gauge field occurs on super-horizon scales at the late stage of inflation and perturbations are enhanced on the intermediate scales during inflation. We find that it can predict a bumpy shape of the curvature power spectrum which leads to the generation of primordial black holes as a dark matter after inflation. We also estimate two types of tensor modes sourced by the gauge field: the primordial gravitational waves generated during inflation and the induced gravitational waves provided by the enhanced curvature perturbation after inflation. We show that both of them are potentially testable with the future space-based gravitational wave interferometers.

astro-ph.CO↗

arXiv2019

Effect of nonlinearity between density and curvature perturbations on the primordial black hole formation

We study the effect of the nonlinear relation between density and curvature perturbations on the formation of PBHs. By calculating the variance and skewness of the density perturbation we derive the non-Gaussian property. As a criterion for PBH formation, the compaction function is used and it is found that larger curvature perturbations are required due to the nonlinear effect. We estimate the PBH abundance based on the Press-Schechter formalism with non-Gaussian probability density function during Radiation dominated era. It is found that the nonlinear effect slightly suppresses the PBH formation and the suppression is comparable to that expected if the primordial curvature perturbation would have the local form of non-Gaussianity with nonlinear parameter $f_\text{NL} \sim -1$.

astro-ph.CO↗

arXiv2018

Formation of primordial black holes in an axionlike curvaton model

We performed the detailed analysis of the primordial black hole (PBH) formation mechanism in an axionlike curvaton model with a coupling to inflaton. The phase direction of the complex scalar works as a curvaton and produces enough PBHs to explain the black hole binaries ($\sim 30 M_{\odot}$) observed in the LIGO-Virgo Collaboration or PBHs as dark matter (DM) ($\sim 10^{-12} M_{\odot}$). We examine whether our model satisfies the current constraints on the PBH mass spectrum, the curvature perturbation and the secondarily produced gravitational waves. We also take into account ambiguity about the choice of the window functions and effect of the non-Gaussianity.

astro-ph.CO↗