arXiv subjects

Masahide Yamaguchi

Publications and source records attributed to Masahide Yamaguchi.

At least 55 records · Page 3Linked to original sources

arXiv2022

A review of neutrino decoupling from the early universe to the current universe

We review the distortions of spectra of relic neutrinos due to the interactions with electrons, positrons, and neutrinos in the early universe. We solve integro-differential kinetic equations for the neutrino density matrix, including vacuum three-flavor neutrino oscillations, oscillations in electron and positron background, a collision term and finite temperature corrections to electron mass and electromagnetic plasma up to the next-to-leading order $\mathcal{O}(e^3)$. After that, we estimate the effects of the spectral distortions in neutrino decoupling on the number density and energy density of the Cosmic Neutrino Background (C$ν$B) in the current universe, and discuss the implications of these effects on the capture rates in direct detection of the C$ν$B on tritium, with emphasis on the PTOLEMY-type experiment. In addition, we find a precise value of the effective number of neutrinos, $N_{\rm eff}=3.044$. However, QED corrections to weak interaction rates at order $\mathcal{O}(e^2 G_F^2)$ and forward scattering of neutrinos via their self-interactions have not been precisely taken into account in the whole literature so far. Recent studies suggest that these neglections might induce uncertainties of $\pm(10^{-3} - 10^{-4})$ in $N_{\rm eff}$.

hep-ph↗

arXiv2022

Neutrino lines from MeV dark matter annihilation and decay in JUNO

We discuss the discovery potential of JUNO experiment for neutrino lines from MeV dark matter (DM) annihilation and decay in a model independent way. We find that JUNO will be able to give severe constraints on the cross section of DM annihilating into neutrinos and on the lifetime of DM decaying into neutrinos. More concretely, with $20$ years of data-taking in the fiducial volume $17$ kton, the cross section will be constrained smaller than $4\times 10^{-26}\,{\rm cm^{3}\,sec^{-1}}$ for the mass of a DM particle $15\,{\rm MeV} \lesssim m_χ \lesssim 50\,{\rm MeV}$ at $90\,\%$ C.L., which might be strong enough to test thermal production mechanism of DM particles for such range of DM mass. The lifetime will be constrained as strong as $1\times 10^{24}\,{\rm sec}$ for the mass of a DM particle $m_χ \simeq 100\,{\rm MeV}$ at $90\,\%$ C.L..

hep-ph↗

arXiv2022

Microlensing constraints on axion stars including finite lens and source size effects

A fraction of light scalar dark matter, especially axions, may organize into Bose-Einstein condensates, gravitationally bound clumps, "boson stars", and be present in large number in galactic halos today. We compute the expected number of gravitational microlensing events of clumps composed of the ordinary QCD axion and axion-like-particles and derive microlensing constraints from the EROS-2 survey and the Subaru Hyper Suprime-Cam observation. We perform a detailed lensing calculation, including the finite lens and source size effects in our analysis. We constrain the axion mass in terms of the fraction of dark matter collapsed into clumps, the individual clump densities, and the axion self-coupling. We also consider and constrain clumps composed of a generic scalar dark matter candidate with repulsive self-interactions. Our analysis opens up a new window for the potential discovery of dark matter.

hep-ph↗

arXiv2022

Unstable Cosmic Neutrino Capture

Future direct observations of the Cosmic Neutrino Background (C$ν$B) have the potential to explore a neutrino lifetime, especially in the region of the age of the universe, $t_0=4.35\times 10^{17}\ {\rm s}$. We forecast constraints on neutrino decay via capture of the C$ν$B on tritium, with emphasis on the PTOLEMY-type experiment. In addition, in some cases of invisible neutrino decay into lighter neutrinos in the Standard Model and invisible particles, we can constrain not only the neutrino lifetime but also the masses of the invisible particles. For this purpose, we also formulate the energy spectra of the lighter neutrinos produced by 2-body and 3-body decays, and those of the electrons emitted in the process of the detection of the lighter neutrinos.

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arXiv2022

Invertibility conditions for field transformations with derivatives: toward extensions of disformal transformation with higher derivatives

We discuss a field transformation from fields $ψ_a$ to other fields $ϕ_i$ that involves derivatives, $ϕ_i = \bar ϕ_i(ψ_a, \partial_αψ_a, \ldots ;x^μ)$, and derive conditions for this transformation to be invertible, primarily focusing on the simplest case that the transformation maps between a pair of two fields and involves up to their first derivatives. General field transformation of this type changes number of degrees of freedom, hence for the transformation to be invertible, it must satisfy certain degeneracy conditions so that additional degrees of freedom do not appear. Our derivation of necessary and sufficient conditions for invertible transformation is based on the method of characteristics, which is used to count the number of independent solutions of a given differential equation. As applications of the invertibility conditions, we show some non-trivial examples of the invertible field transformations with derivatives, and also give a rigorous proof that a simple extension of the disformal transformation involving a second derivative of the scalar field is not invertible.

hep-th↗

arXiv2021

Revisiting non-Gaussianity in non-attractor inflation models in the light of the cosmological soft theorem

We revisit the squeezed-limit non-Gaussianity in the single-field non-attractor inflation models from the viewpoint of the cosmological soft theorem. In the single-field attractor models, inflaton's trajectories with different initial conditions effectively converge into a single trajectory in the phase space, and hence there is only one \emph{clock} degree of freedom (DoF) in the scalar part. Its long-wavelength perturbations can be absorbed into the local coordinate renormalization and lead to the so-called \emph{consistency relation} between $n$- and $(n+1)$-point functions. On the other hand, if the inflaton dynamics deviates from the attractor behavior, its long-wavelength perturbations cannot necessarily be absorbed and the consistency relation is expected not to hold any longer. In this work, we derive a formula for the squeezed bispectrum including the explicit correction to the consistency relation, as a proof of its violation in the non-attractor cases. First one must recall that non-attractor inflation needs to be followed by attractor inflation in a realistic case. Then, even if a specific non-attractor phase is effectively governed by a single DoF of phase space (represented by the exact ultra-slow-roll limit) and followed by a single-DoF attractor phase, its transition phase necessarily involves two DoF in dynamics and hence its long-wavelength perturbations cannot be absorbed into the local coordinate renormalization. Thus, it can affect local physics, even taking account of the so-called \emph{local observer effect}, as shown by the fact that the bispectrum in the squeezed limit can go beyond the consistency relation. More concretely, the observed squeezed bispectrum does not vanish in general for long-wavelength perturbations exiting the horizon during a non-attractor phase.

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arXiv2021

Precise Capture Rates of Cosmic Neutrinos and Their Implications on Cosmology

We explore the potential of measurements of cosmological effects, such as neutrino spectral distortions from the neutrino decoupling and neutrino clustering in our Galaxy, via cosmic neutrino capture on tritium. We compute the precise capture rates of each neutrino species including such cosmological effects to probe them. These precise estimates of capture rates are also important in that the would-be deviation of the estimated capture rate could suggest new neutrino physics and/or a non-standard evolution of the universe. In addition, we discuss the precise differences between the capture rates of Dirac and Majorana neutrinos for each species, the required energy resolutions to detect each neutrino species and the method of reconstruction of the spectrum of cosmic neutrinos via the spectrum of emitted electrons, with emphasis on the PTOLEMY experiment.

hep-ph↗

arXiv2021

Losing the trace to find dynamical Newton or Planck constants

We show that promoting the trace part of the Einstein equations to a trivial identity results in the Newton constant being an integration constant. Thus, in this formulation the Newton constant is a global dynamical degree of freedom which is also a subject to quantization and quantum fluctuations. This is similar to what happens to the cosmological constant in the unimodular gravity where the trace part of the Einstein equations is lost in a different way. We introduce a constrained variational formulation of these modified Einstein equations. Then, drawing on analogies with the Henneaux-Teitelboim action for unimodular gravity, we construct different general-covariant actions resulting in these dynamics. The inverse of dynamical Newton constant is canonically conjugated to the Ricci scalar integrated over spacetime. Surprisingly, instead of the dynamical Newton constant one can formulate an equivalent theory with a dynamical Planck constant. Finally, we show that an axion-like field can play a role of the gravitational Newton constant or even of the quantum Planck constant.

gr-qc↗

arXiv2021

Disentangling mass spectra of multiple fields in cosmological collider

We study effects of multiple scalar fields (scalar isocurvatons) with the Hubble scale masses on the inflationary bispectrum in the squeezed limit, particular paying attention to the question how to disentangle mass spectra of such fields. We consider two isocurvatons with almost degenerate masses and the coupling of an inflaton to both isocurvatons as an example. We find that the characteristic feature associated with nearly degenerate masses appears in the oscillating part of the bispectrum, which is dominated by a waveform with a specific wavelength roughly given by an inverse of the mass difference. Such a waveform with a relatively longer wavelength can be easily identified and useful for disentangling almost degenerate mass spectra. This situation is in sharp contrast with the case of collider experiments on earth, where the very precise energy resolution corresponding to the mass difference is required to disentangle almost degenerate mass spectra. Therefore, if future observations could detect this kind of a characteristic feature in bispectrum of the primordial curvature perturbations, it can prove the existence of degenerate multiple particles around the Hubble scale and resolve their mass degeneracies.

hep-th↗

arXiv2021

Analytic formula for the dynamics around inflation end and implications on primordial gravitational waves

We argue that primordial gravitational waves have a spectral break and its information is quite useful for exploring the early universe. Indeed, such a spectral break can be a fingerprint of the end of inflation, and the amplitude and the frequency at the break can tell us the energy scale of inflation and the reheating temperature simultaneously. In order to investigate the spectral break, we give an analytic formula for evolution of the Hubble parameter around the end of inflation where the slow roll approximation breaks down. We also evaluate the spectrum of primordial gravitational waves around the break point semi-analytically using the analytic formula for the inflation dynamics.

astro-ph.CO↗

arXiv2021

Reconstruction of Primordial Power Spectrum of curvature perturbation from the merger rate of Primordial Black Hole Binaries

The properties of primordial curvature perturbations on small scales are still unknown while those on large scales have been well probed by the observations of the cosmic microwave background anisotropies and the large scale structure. In this paper, we propose the reconstruction method of primordial curvature perturbations on small scales through the merger rate of binary primordial black holes, which could form from large primordial curvature perturbation on small scales.

astro-ph.CO↗

arXiv2021

Local observer effect on the cosmological soft theorem

Non-Gaussianities of primordial perturbations in the soft limit provide the important information about the light degrees of freedom during inflation. The soft modes of the curvature perturbations, unobservable for a local observer, act as rescaling the spatial coordinates. We determine how the trispectrum in the collapsed limit is shifted by the rescaling due to the soft modes. We find the form of the inequality between $f_{\rm NL}$ and $τ_{\rm NL}$ parameters is not affected by the rescaling, demonstrating that the role of the inequality as an indicator of the light degrees of freedom remains intact. We also comment on the local observer effect on the consistency relation for ultra slow-roll inflation.

astro-ph.CO↗

arXiv2020

Gravitational wave spectra from oscillon formation after inflation

We systematically investigate the preheating behavior of single field inflation with an oscillon-supporting potential. We compute the properties of the emitted gravitational waves (GWs) and the number density and characteristics of the produced oscillons. By performing numerical simulations for a variety of potential types, we divide the analyzed potentials in two families, each of them containing potentials with varying large- or small-field dependence. We find that the shape and amplitude of the emitted GW spectrum have a universal feature, with the peak around the physical wavenumber $k/a \sim m$ at the inflaton oscillation period, irrespective of the exact potential shape. This can be used as a smoking-gun for deducing the existence of a violent preheating phase and possible oscillon formation after inflation. Despite this apparent universality, we find differences in the shape of the emitted GW spectra between the two potential families, leading to discriminating features between them. In particular, all potentials show the emergence of a two-peak structure in the GW spectrum, arising at the time of oscillon formation. However, potentials exhibiting efficient parametric resonance tend to smear out this structure and by the end of the simulation the GW spectrum exhibits a single broad peak. We further compute the properties of the produced oscillons for each potential, finding differences in the number density and size distribution of stable oscillons and transient overdensities. We perform a linear fluctuation analysis and use Floquet charts to relate the results of our simulations to the structure of parametric resonance. We find that the growth rate of scalar perturbations and the associated oscillon formation time are sensitive to the small-field potential shape while the macroscopic physical properties of oscillons (e.g. total number) depend on the large-field potential shape.

hep-ph↗

arXiv2020

Cosmological Perturbations in Palatini Formalism

We investigate cosmological perturbations of scalar-tensor theories in Palatini formalism. First we introduce an action where the Ricci scalar is conformally coupled to a function of a scalar field and its kinetic term and there is also a k-essence term consisting of the scalar and its kinetic term. This action has three frames that are equivalent to one another: the original Jordan frame, the Einstein frame where the metric is redefined, and the Riemann frame where the connection is redefined. For the first time in the literature, we calculate the quadratic action and the sound speed of scalar and tensor perturbations in three different frames and show explicitly that they coincide. Furthermore, we show that for such action the sound speed of gravitational waves is unity. Thus, this model serves as dark energy as well as an inflaton even though the presence of the dependence of the kinetic term of a scalar field in the non-minimal coupling, different from the case in metric formalism. We then proceed to construct the L3 action called Galileon terms in Palatini formalism and compute its perturbations. We found that there are essentially 10 different(inequivalent) definitions in Palatini formalism for a given Galileon term in metric formalism. We also see that,in general, the L3 terms have a ghost due to Ostrogradsky instability and the sound speed of gravitational waves could potentially deviate from unity, in sharp contrast with the case of metric formalism. Interestingly, once we eliminate such a ghost, the sound speed of gravitational waves also becomes unity. Thus, the ghost-free L3 terms in Palatini formalism can still serve as dark energy as well as an inflaton, like the case in metric formalism.

hep-th↗

arXiv2020

Effective Field Theory of Anisotropic Inflation and Beyond

We develop an effective-field-theory (EFT) framework for inflation with various symmetry breaking pattern. As a prototype, we formulate anisotropic inflation from the perspective of EFT and construct an effective action of the Nambu-Goldstone bosons for the broken time translation and rotation symmetries. We also calculate the statistical anisotropy in the scalar two-point correlation function for concise examples of the effective action.

hep-th↗

arXiv2020

A precision calculation of relic neutrino decoupling

We study the distortions of equilibrium spectra of relic neutrinos due to the interactions with electrons, positrons, and neutrinos in the early Universe. We solve the integro-differential kinetic equations for the neutrino density matrix, including three-flavor oscillations and finite temperature corrections from QED up to the next-to-leading order $\mathcal{O}(e^3)$ for the first time. In addition, the equivalent kinetic equations in the mass basis of neutrinos are directly solved, and we numerically evaluate the distortions of the neutrino spectra in the mass basis as well, which can be easily extrapolated into those for non-relativistic neutrinos in the current Universe. In both bases, we find the same value of the effective number of neutrinos, $N_{\rm eff} = 3.044$, which parameterizes the total neutrino energy density. The estimated error for the value of $N_{\rm eff}$ due to the numerical calculations and the choice of neutrino mixing parameters would be at most 0.0005.

hep-ph↗

arXiv2020

Disformal invariance of cosmological observables

We study the frame dependence/independence of cosmological observables under disformal transformations, extending the previous results regarding conformal transformations, and provide the correspondence between Jordan-frame and Einstein-frame variables. We consider quantities such as the gravitational constant in the Newtonian limit, redshift, luminosity and angular diameter distances, as well as the distance-duality relation. Also, the Boltzmann equation, the observed specific intensity, and the adiabaticity condition are discussed. Since the electromagnetic action changes under disformal transformations, photons in the Einstein frame no longer propagate along null geodesics. As a result, several quantities of cosmological interest are modified. Nevertheless, we show that the redshift is invariant and the distance-duality relation (the relation between the luminosity distance and the angular diameter distance) still holds in general spacetimes even though the reciprocity relation (the relation between two geometrical distances) is modified.

gr-qc↗

arXiv2020

Generalized ghost-free propagators in nonlocal field theories

In this paper we present an iterative method to generate an infinite class of new nonlocal field theories whose propagators are ghost-free. We first examine the scalar field case and show that the pole structure of such generalized propagators possesses the standard two derivative pole and in addition can contain complex conjugate poles which, however, do not spoil at least tree level unitarity as the optical theorem is still satisfied. Subsequently, we define analogous propagators for the fermionic sector which is also devoid of unhealthy degrees of freedom. As a third case, we apply the same construction to gravity and define a new set of theories whose graviton propagators around the Minkowski background are ghost-free. Such a wider class also includes nonlocal theories previously studied, and Einstein's general relativity as a peculiar limit. Moreover, we compute the linearized gravitational potential generated by a static point-like source for several gravitational theories belonging to this new class and show that the nonlocal nature of gravity regularizes the singularity at the origin.

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