Speaker1: Leo Tsukada

Title: Modeling and Searching for Stochastic Gravitational-Wave Backgrounds from Ultra-light Boson Particles

Ultralight bosons, which are predicted in a variety of beyond-Standard-Model scenarios as dark- matter candidates, can trigger the superradiant instability around spinning black holes. This in- stability gives rise to oscillating boson condensates which then dissipate through the emission of nearly monochromatic gravitational waves. Such systems are promising sources for current and future gravitational-wave detectors. In this work, we consider minimally-coupled, massive scalar and vector bosons, each of which could emit a population of signals that incoherently superpose with one another, called stochastic gravitational-wave background. We adopt recently obtained numerical results for the gravitational-wave flux, and astrophysical models of black hole populations that include both isolated black holes and binary merger remnants, to compute and study in detail the background spectrum produced by these sources. Using a Bayesian framework, we search for such a background signal using data from the first and second observing runs of Advanced LIGO. We find no evidence for such a signal, which allows us to place constraints on those boson masses. We find that our results disfavor the scalar (vector) boson mass in the range [2.0 - 6.0] * 10^-13 eV ([0.8 - 7.0] * 10^-13 eV ) at 95% credibility, assuming the optimistic spin distribution for the isolated black hole population.

Speaker2: Fumio Uchida

Title: Generation of baryon isocurvature perturbations from primordial magnetic fields and its cosmological consequence. (初期磁場によるバリオン等曲率揺らぎの生成とその宇宙論的帰結)

As a possible origin of the baryon asymmetry of the universe, helical magnetic fields before the electroweak symmetry breaking are studied. In the scenario, the chiral anomaly in the Standard Model converts the hypermagnetic helicity of the magnetic field into baryon (and lepton) number.
Conditions on the strength and the coherence length of the magnetic fields are imposed to reproduce the observed baryon-to-entropy ratio, and the generation of baryon isocurvature perturbations gives another constraint because it affects the big-bang nucleosynthesis. I will show the constraints in terms of those on the present intergalactic magnetic fields, and discuss their cosmological implications.

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Last-modified: 2020-12-03 (木) 17:08:29 (140d)