arXiv subjects

Motohiko Kusakabe

Publications and source records attributed to Motohiko Kusakabe.

At least 55 records · Page 3Linked to original sources

arXiv2015

Constraints on modified Gauss-Bonnet gravity during big bang nucleosynthesis

The modified gravity is considered to be one of possible explanations of the accelerated expansions of the present and the early universe. We study effects of the modified gravity on big bang nucleosynthesis (BBN). If effects of the modified gravity are significant during the BBN epoch, they should be observed as changes of primordial light element abundances. We assume a $f(G)$ term with the Gauss-Bonnet term $G$, during the BBN epoch. A power-law relation of $df/dG \propto t^p$ where $t$ is the cosmic time was assumed for the function $f(G)$ as an example case. We solve time evolutions of physical variables during BBN in the $f(G)$ gravity model numerically, and analyzed calculated results. It is found that a proper solution for the cosmic expansion rate can be lost in some parameter region. In addition, we show that calculated results of primordial light element abundances can be significantly different from observational data. Especially, observational limits on primordial D abundance leads to the strongest constraint on the $f(G)$ gravity. We then derive constraints on parameters of the $f(G)$ gravity taking into account the existence of the solution of expansion rate and final light element abundances.

astro-ph.CO↗

arXiv2015

Corrected constraints on big bang nucleosynthesis in a modified gravity model of $f(R) \propto R^n$

Big bang nucleosynthesis in a modified gravity model of $f(R)\propto R^n$ is investigated. The only free parameter of the model is a power-law index $n$. We find cosmological solutions in a parameter region of $1< n \leq (4+\sqrt{6})/5$. We calculate abundances of $^4$He, D, $^3$He, $^7$Li, and $^6$Li during big bang nucleosynthesis. We compare the results with the latest observational data. It is then found that the power-law index is constrained to be $(n-1)=(-0.86\pm 1.19)\times 10^{-4}$ (95 % C.L.) mainly from observations of deuterium abundance as well as $^4$He abundance.

astro-ph.CO↗

arXiv2014

Chemical separation of primordial Li$^+$ during structure formation caused by nanogauss magnetic field

During the structure formation, charged and neutral chemical species may have separated from each other at the gravitational contraction in primordial magnetic field (PMF). A gradient in the PMF in a direction perpendicular to the field direction leads to the Lorentz force on the charged species. Resultantly, an ambipolar diffusion occurs, and charged species can move differently from neutral species, which collapses gravitationally during the structure formation. We assume a gravitational contraction of neutral matter in a spherically symmetric structure, and calculate fluid motions of charged and neutral species. It is shown that the charged fluid, i.e., proton, electron and $^7$Li$^+$, can significantly decouple from the neutral fluid depending on the field amplitude. The charged species can, therefore, escape from the gravitational collapse. We take the structure mass, the epoch of the gravitational collapse, and the comoving Lorenz force as parameters. We then identify a parameter region for an effective chemical separation. This type of chemical separation can reduce the abundance ratio of Li/H in early structures because of inefficient contraction of $^7$Li$^+$ ion. Therefore, it may explain Li abundances of Galactic metal-poor stars which are smaller than the prediction in standard big bang nucleosynthesis model. Amplitudes of the PMFs are controlled by a magneto-hydrodynamic turbulence. The upper limit on the field amplitude derived from the turbulence effect is close to the value required for the chemical separation.

astro-ph.CO↗

arXiv2014

Effects of Long-lived 10 MeV Scale Sterile Neutrino on Primordial Elemental Abundances and Effective Neutrino Number

The primordial lithium abundance inferred from observations of metal-poor stars is ~3 times smaller than the theoretical value in standard big bang nucleosynthesis (BBN) model. We assume a simple model including a sterile neutrino nu_H with mass of O(10) MeV which decays long after BBN. We then investigate cosmological effects of a sterile neutrino decay. We formulate the injection spectrum of nonthermal photons induced by electrons and positrons generated at the nu_H decay, as a function of the nu_H mass and the photon temperature. We then consistently solve (1) the cosmic thermal history, (2) nonthermal nucleosynthesis induced by the nonthermal photons, (3) the baryon-to-photon ratio eta, and (4) the effective neutrino number N_eff. Amounts of energy injection at the nu_H decay are constrained from limits on primordial D and 7Li abundances, the N_eff value, and the cosmic microwave background energy spectrum. We find that 7Be is photodisintegrated and the Li problem is partially solved for the lifetime 10^4-10^5 s and the mass >~ 14 MeV. 7Be destruction by more than a factor of 3 is not possible because of an associated D over-destruction. In the parameter region, the eta value is decreased slightly, while the N_eff value is increased by a factor of <~ 1. In this study, errors in photodisintegration cross sections of 7Be(g, a)3He and 7Li(g, a)3H that have propagated through literatures are corrected. It is then found that the new photodisintegration rates are 2.3 to 2.5 times smaller than the old rates, so that efficiencies of 7Be and 7Li photodisintegration are significantly smaller.

astro-ph.CO↗

arXiv2014

Cosmological solutions to the Lithium problem: Big-bang nucleosynthesis with photon cooling, $X$-particle decay and a primordial magnetic field

The $^7$Li abundance calculated in BBN with the baryon-to-photon ratio fixed from fits to the CMB power spectrum is inconsistent with the observed lithium abundances on the surface of metal-poor halo stars. Previous cosmological solutions proposed to resolve this $^7$Li problem include photon cooling (possibly via the Bose-Einstein condensation of a scalar particle) or the decay of a long-lived $X-$particle (possibly the next-to-lightest supersymmetric particle). In this paper we reanalyze these solutions, both separately and in concert. We also introduce the possibility of a primordial magnetic field (PMF) into these models. We constrain the $X-$particles and the PMF parameters by the observed light element abundances using a likelihood analysis to show that the inclusion of all three possibilities leads to an optimum solution to the lithium problem. We deduce allowed ranges for the $X-$particle parameters and energy density in the PMF that can solve $^7$Li problem.

astro-ph.CO↗

arXiv2014

Revised Big Bang Nucleosynthesis with long-lived negatively charged massive particles: updated recombination rates, primordial 9Be nucleosynthesis, and impact of new 6Li limits

We extensively reanalyze effects of a long-lived negatively charged massive particle, X-, on big bang nucleosynthesis (BBN). The BBN model with an X- particle was originally motivated by the discrepancy between 6,7Li abundances predicted in standard BBN model and those inferred from observations of metal-poor stars. In this model 7Be is destroyed via the recombination with an X- particle followed by radiative proton capture. We calculate precise rates for the radiative recombinations of 7Be, 7Li, 9Be, and 4He with X-. In nonresonant rates we take into account respective partial waves of scattering states and respective bound states. The finite sizes of nuclear charge distributions cause deviations in wave functions from those of point-charge nuclei. For a heavy X- mass, m_X \gtrsim 100 GeV, the d-wave --> 2P transition is most important for 7Li and 7,9Be, unlike recombination with electrons. Our new nonresonant rate of the 7Be recombination for m_X=1000 GeV is more than 6 times larger than the existing rate. Moreover, we suggest a new important reaction for 9Be production: the recombination of 7Li and X- followed by deuteron capture. We derive binding energies of X-nuclei along with reaction rates and Q-values. We then calculate BBN and find that the amount of 7Be destruction depends significantly on the charge distribution of 7Be. Finally, updated constraints on the initial abundance and the lifetime of the X- are derived in the context of revised upper limits to the primordial 6Li abundance. Parameter regions for the solution to the 7Li problem are revised, and the primordial 9Be abundances is revised.

astro-ph.CO↗

arXiv2014

General limit on the relation between abundances of D and $^7$Li in big bang nucleosynthesis with nucleon injections

The injections of energetic hadrons could have occurred in the early universe by decays of hypothetical long-lived exotic particles. The injections induce the showers of nonthermal hadrons via nuclear scattering. Neutrons generated at these events can react with $^7$Be nuclei and reduce $^7$Be abundance solving a problem of the primordial $^7$Li abundance. We suggest that thermal neutron injection is a way to derive a model independent conservative limit on the relation between abundances of D and $^7$Li in a hadronic energy injection model. We emphasize that an uncertainty in cross sections of inelastic $n+p$ scattering affects the total number of induced neutrons, which determines final abundances of D and $^7$Li. In addition, the annihilations of antinucleons with $^4$He result in higher D abundance and trigger nonthermal $^6$Li production. It is concluded that a reduction of $^7$Li abundance from a value in the standard big bang nucleosynthesis (BBN) model down to an observational two $σ$ upper limit is necessarily accompanied by an undesirable increase of D abundance up to at least an observational 12 $σ$ upper limit from observations of quasi-stellar object absorption line systems. The effects of antinucleons and secondary particles produced in the hadronic showers always lead to a severer constraint. The BBN models involving any injections of extra neutrons are thus unlikely to reproduce a small $^7$Li abundance consistent with observations.

astro-ph.CO↗

arXiv2013

7Be charge exchange between 7Be3+ ion and exotic long-lived negatively charged massive particle in big bang nucleosynthesis

The existence of an exotic long-lived negatively charged massive particle, i.e., X-, during big bang nucleosynthesis can affect primordial light element abundances. Especially, the final abundance of 7Li, mainly originating from the electron capture of 7Be, has been suggested to reduce by the 7Be destruction via the radiative X- capture of 7Be followed by the radiative proton capture of the bound state of 7Be and X- (7BeX). We suggest a new route of 7BeX formation, that is the 7Be charge exchange at the reaction of 7Be3+ ion and X-. The formation rate depends on the number fraction of 7Be3+ ion, the charge exchange cross section of 7Be3+, and the probability that produced excited states 7BeX* are converted to the ground state. We estimate respective quantities affecting the 7BeX formation rate, and find that this reaction pathway can be more important than ordinary radiative recombination of 7Be and X-. The effect of the charge exchange reaction is then shown in a latest nuclear reaction network calculation. Quantum physical model calculations for related reactions are needed to precisely estimate the efficiency of this pathway in future.

astro-ph.CO↗

arXiv2013

A Big-Bang Nucleosynthesis Limit on the Neutral Fermion Decays into Neutrinos

Using the primordial helium abundance, an upper limit to the magnetic moments for Dirac neutrinos had been provided by imposing restrictions on the number of the additional helicity states. Considering non-thermal photons produced in the decay of the heavy sterile mass eigenstates due to the neutrino magnetic moment, we explore the constraints imposed by the observed abundances of all the light elements produced during the Big Bang nucleosynthesis.

astro-ph.CO↗

arXiv2013

Production of 9Be through alpha-fusion reaction of metal-poor cosmic ray and stellar flare

Spectroscopic observations of metal-poor stars have indicated possible 6Li abundances that are much larger than the primordial abundance predicted in the standard big bang nucleosynthesis model. Possible mechanisms of 6Li production in metal-poor stars include pregalactic and cosmological cosmic ray (CR) nucleosynthesis and nucleosynthesis by flare-accelerated nuclides. We study 9Be production via two-step alpha-fusion reactions of CR or flare-accelerated 3,4He through 6He and 6,7Li, in pregalactic structure, intergalactic medium, and stellar surfaces. We solve transfer equations of CR or flare particles and calculate nuclear yields of 6He, 6,7Li, and 9Be taking account of probabilities of processing 6He and 6,7Li into 9Be via fusions with alpha particles. Yield ratios, i.e., 9Be/6Li, are then calculated for the CR and flare nucleosynthesis models. We suggest that the future observations of 9Be in metal-poor stars may find enhanced abundances originating from metal-poor CR or flare activities.

astro-ph.CO↗

arXiv2012

Effects of power law primordial magnetic field on big bang nucleosynthesis

Big bang nucleosynthesis (BBN) is affected by the energy density of a primordial magnetic field (PMF). For an easy derivation of constraints on models for PMF generations, we assume a PMF with a power law (PL) distribution in wave number defined with a field strength, a PL index, and maximum and minimum scales at a generation epoch. We then show a relation between PL-PMF parameters and the scale invariant (SI) strength of PMF for the first time. We perform a BBN calculation including PMF effects, and show abundances as a function of baryon to photon ratio $η$. The SI strength of the PMF is constrained from observational constraints on abundances of $^4$He and D. The minimum abundance of $^7$Li/H as a function of $η$ slightly moves to a higher $^7$Li/H value at a larger $η$ value when a PMF exists during BBN. We then discuss degeneracies between the PL-PMF parameters in the PMF effect. In addition, we assume a general case in which both the existence and the dissipation of PMF are possible. It is then found that an upper limit on the SI strength of the PMF can be derived from a constraint on $^4$He abundance, and that a lower limit on the allowed $^7$Li abundance is significantly higher than those observed in metal-poor stars.

astro-ph.CO↗

arXiv2012

Updated constraint on a primordial magnetic field during big bang nucleosynthesis and a formulation of field effects

A new upper limit on the amplitude of primordial magnetic field (PMF) is derived by a comparison between a calculation of elemental abundances in big bang nucleosynthesis (BBN) model and the latest observational constraints on the abundances. Updated nuclear reaction rates are adopted in the calculation. Effects of PMF on the abundances are consistently taken into account in the numerical calculation with the precise formulation of changes in physical variables. We find that abundances of 3He and 6Li increase while that of 7Li decreases when the PMF amplitude increases, in the case of the baryon-to-photon ratio determined from the measurement of cosmic microwave background radiation. We derive a constraint on the present amplitude of PMF, i.e., B(0)<1.5 micro G [corresponding to the amplitude less than 2.0x10^{11} G at BBN temperature of T=10^9 K] based on the rigorous calculation.

astro-ph.CO↗

arXiv2012

Solution to Big-Bang Nucleosynthesis in Hybrid Axion Dark Matter Model

Following a recent suggestion of axion cooling of photons between the nucleosynthesis and recombination epochs in the Early Universe, we investigate a hybrid model with both axions and relic supersymmetric particles. In this model we demonstrate that the 7Li abundance can be consistent with observations without destroying the important concordance of deuterium abundance.

astro-ph.CO↗

arXiv2011

Resonant annihilation of long-lived massive colored particles through hadronic collisions

Hypothetical long-lived massive colored particles (MCPs or Ys) would be confined in colorless exotic strongly interacting massive particles (SIMPs) at color confinement temperature of T_C /sim 200 MeV. Two long-lived MCPs form a bound state (Y /bar{Y}) at collisions of two SIMPs. We study sensitivities of MCP annihilation to decay properties of resonances (Y /bar{Y}), and binding energies or energy levels of exotic SIMPs. The (Y /bar{Y}) formation is assumed to dominantly proceeds through resonances of (Y/bar{Y}) in this paper. We make a toy model of the effective cross section for Y /bar{Y} annihilation. Abundances of SIMPs are then calculated for different sets of parameters specifying properties of (Y /bar{Y}) resonances, binding energies of SIMPs, the initial abundance and the mass of MCP. Calculated relic abundances for respective SIMP species are 2 x 10^{-8}--3 x 10^{-4} times that of baryon. They can be much higher but cannot be much smaller than the previous estimate. The abundances can be consistent depending on parameters with the possible scenario that SIMPs bind to nuclei and subsequent exotic nuclear reactions reduce the primordial abundance of 7Li or enhance those of 9Be and/or B in the early universe. A unique information on the quark-hadron phase transition in the early universe may become available in future by elaborated studies on the annihilation process with light element abundances as observables.

hep-ph↗

arXiv2011

Signals of the cosmological reionization in the radio sky through C and O fine structure lines

We study the excitation of fine structure levels of C I, C II and O I by ultraviolet (UV) photons around strong UV sources which are also ionizing sources of the cosmological reionization at redshift of $\sim 10$. The evolutions of ionized regions around a point source are calculated by solving rate equations for non-equilibrium chemistry. Signals of UV photons through the fine structure lines are considered to be stronger at locations of more abundant chemical species of C I, C II and O I. Such environments would be realized where strong fluxes of non-ionizing UV line photons available for the pumping up of fine structure levels exist, and simultaneously ionizing UV photons are effectively shielded by dense H I regions. Signals from H I regions of moderately large densities induced by redshifted UV photons emitted at the point sources are found to be dominantly large over those of others. We discuss the detectability of the signals, and show that signals from idealized environments will be possibly detected by radio observations with next-generation arrays to come after the Atacama Large Millimeter/submillimeter Array (ALMA).

astro-ph.CO↗

arXiv2011

Time Dependent Quark Masses and Big Bang Nucleosynthesis Revisited

We reinvestigate the constraints from primordial nucleosynthesis on a possible time-dependent quark mass. The limits on such quark-mass variations are particularly sensitive to the adopted observational abundance constraints. Hence, in the present study we have considered updated light-element abundances and uncertainties deduced from observations. We also consider new nuclear reaction rates and an independent analysis of the influence of such quark-mass variations on the resonance properties of the important 3He(d,p)4He reaction. We find that the updated abundance and resonance constraints imply a narrower range on the possible quark-mass variations in the early universe. We also find that, contrary to previous investigations, the optimum concordance region reduces to a (95% C.L.) value of -0.005 < delta m_q/m_q < 0.007 consistent with no variation in the averaged quark mass.

astro-ph.CO↗

arXiv2011

Destruction of 7Be in big bang nucleosynthesis via long-lived sub-strongly interacting massive particles as a solution to the Li problem

We identify reactions which destroy 7Be and 7Li during big bang nucleosynthesis (BBN) in the scenario of BBN catalyzed by a long-lived sub-strongly interacting massive particle (sub-SIMP or X particle). The destruction associated with non radiative X captures of the nuclei can be realized only if the interaction strength between an X particle and a nucleon is properly weaker than that between two nucleons to a degree depending on the mass of X. Binding energies of nuclei to an X particle are estimated taking the mass and the interaction strength to nuclei of the X as input parameters. Nuclear reaction rates associated with the X are estimated naively, and adopted in calculating evolutions of nuclear abundances. We suggest that the 7Li problem, which might be associated with as-yet-unrecognized particle processes operating during BBN, can be solved if the X particle interacts with nuclei strongly enough to drive 7Be destruction but not strongly enough to form a bound state with 4He of relative angular momentum L=1. Justifications of this scenario by rigorous calculations of reaction rates using quantum mechanical many-body models are highly desirable since this result involves many significant uncertainties.

hep-ph↗

arXiv2010

Quantum Statistical Corrections to Astrophysical Photodisintegration Rates

Tabulated rates for astrophysical photodisintegration reactions make use of Boltzmann statistics for the photons involved as well as the interacting nuclei. Here we derive analytic corrections for the Planck-spectrum quantum statistics of the photon energy distribution. These corrections can be deduced directly from the detailed-balance condition without the assumption of equilibrium as long as the photons are represented by a Planck spectrum. Moreover we show that these corrections affect not only the photodisintegration rates but also modify the conditions of nuclear statistical equilibrium as represented in the Saha equation. We deduce new analytic corrections to the classical Maxwell-Boltzmann statistics which can easily be added to the reverse reaction rates of existing reaction network tabulations. We show that the effects of quantum statistics, though generally quite small, always tend to speed up photodisintegration rates and are largest for nuclei and environments for which Q/kT ~ 1. As an illustration, we examine possible effects of these corrections on the r-process, the rp-process, explosive silicon burning, the $γ$-process and big bang nucleosynthesis. We find that in most cases one is quite justified in neglecting these corrections. The correction is largest for reactions near the drip line for an r-process with very high neutron density, or an rp-process at high-temperature.

astro-ph.SR↗