arXiv subjects

Kohei Kamada

Publications and source records attributed to Kohei Kamada.

At least 55 records · Page 3Linked to original sources

arXiv2020

On the violent preheating in the mixed Higgs-$R^2$ inflationary model

It has been argued that the mixed Higgs-$R^2$ model acts as the UV extension of the Higgs inflation, pushing up its cut-off scale in the vacuum close up to the Planck scale. In this letter, we study the inflaton oscillation stage after inflation, focusing on the effective mass of the phase direction of the Higgs field, which can cause a violent preheating process. We find that the "spikes" in the effective mass of the phase direction observed in the Higgs inflation still appear in the mixed Higgs-$R^2$ model. While the spikes appear above the cut-off scale in the Higgs-only case, they appear below the cut-off scale when the model is extended with $R^2$ term though reheating cannot be completed in the violent particle production regime since the spikes get milder.

hep-ph↗

arXiv2020

Gravitational leptogenesis with kination and gravitational reheating

We revisit the gravitational leptogenesis scenario in which the inflaton is coupled to gravity by the Chern-Simons term and the lepton asymmetry is generated through the gravitational anomaly in the lepton number current during inflation. We constrain the possible model parameter space by requiring the absence of ghost modes below the Planck scale, which would suggest the breakdown of the effective theory, and evaluate the net baryon asymmetry for various reheating processes. We find that the mechanism with these requirements is insufficient to explain the observed baryon asymmetry of the Universe for standard reheating scenarios. We show that, however, with the kination scenario realized in e.g. the k- and G-inflation models, a sufficient baryon asymmetry can be generated within a feasible range of the model parameters.

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arXiv2018

Phase Transitions in Twin Higgs Models

We study twin Higgs models at non-zero temperature and discuss cosmological phase transitions as well as their implications on electroweak baryogenesis and gravitational waves. It is shown that the expectation value of the Higgs field at the critical temperature of the electroweak phase transition is much smaller than the critical temperature, which indicates two important facts: (i) the electroweak phase transition cannot be analyzed perturbatively (ii) the electroweak baryogenesis is hardly realized in the typical realizations of twin Higgs models. We also analyze the phase transition associated with the global symmetry breaking, through which the Standard Model Higgs is identified with one of the pseudo-Nambu-Goldstone bosons in terms of its linear realization, with and without supersymmetry. For this phase transition, we show that, only in the supersymmetric case, there are still some parameter spaces, in which the perturbative approach is validated and the phase transition is the first order. We find that the stochastic gravitational wave background is generated through this first order phase transition, but it is impossible to be detected by DECIGO or BBO in the linear realization and the decoupling limit. The detection of stochastic gravitational wave background with the feature of first order phase transition, therefore, will give strong constraints on twin Higgs models.

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arXiv2018

Cosmological Helical Hypermagnetic Fields and Baryogenesis

Recent gamma-ray observations of TeV blazars exhibits the deficits of the secondary GeV cascade photons. This suggests the existence of the intergalactic magnetic fields, which may have a primordial origin. One of the mechanisms that can produce primordial magnetic fields is so-called the chiral plasma instability, where the (hyper) magnetic fields are destabilized when a large chiral asymmetry exists in the high-temperature plasma in the early Universe. We argue that such a large chiral asymmetry can be produced through the GUT baryogenesis. Note that the chiral asymmetry is a good conserved quantity at high temperature when the Yukawa interaction is weak enough. We also point out that the generated hypermagnetic fields are maximally helical, and hence baryon and lepton asymmetry is inevitably produced through the chiral anomaly in the Standard Model through $U(1)_Y$ gauge interaction at the electroweak symmetry breaking. Consequently, the magnetic fields suggested by the blazar observations over-produce baryon asymmetry. Thus the chiral plasma instability alone cannot be responsible for the intergalactic magnetic fields but can be responsible for the baryon asymmetry of the Universe. In other words, GUT baryogenesis without $B$-$L$ asymmetry generation is revived as a viable baryogenesis scenario, which otherwise has been thought to suffer from $B$+$L$ washout by sphalerons. This presentation is based on the work [1].

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arXiv2018

Return of the grand unified theory baryogenesis: Source of helical hypermagnetic fields for the baryon asymmetry of the universe

It has been considered that baryogenesis models without a generation of $B$-$L$ asymmetry such as the GUT baryogenesis do not work since the asymmetry is washed out by the electroweak sphalerons. Here, we point out that helical hypermagnetic fields can be generated through the chiral magnetic effect with a chiral asymmetry generated in such baryogenesis models. The helical hypermagnetic fields then produce baryon asymmetry mainly at the electroweak symmetry breaking, which remains until today. Therefore, the baryogenesis models without $B$-$L$ asymmetry can still be the origin of the present baryon asymmetry. In particular, if it can produce chiral asymmetry mainly carried by right-handed electrons of order of $10^{-3}$ in terms of the chemical potential to temperature ratio, the resultant present-day baryon asymmetry can be consistent with our Universe, although simple realizations of the GUT baryogenesis are hard to satisfy the condition. We also argue the way to overcome the difficulty in the GUT baryogenesis. The intergalactic magnetic fields with $B_0\sim 10^{-16 \sim 17}$ G and $λ_0 \sim 10^{-2\sim3}$ pc are the smoking gun of the baryogenesis scenario as discussed before.

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arXiv2018

Magnetic Field Transfer From A Hidden Sector

Primordial magnetic fields in the dark sector can be transferred to magnetic fields in the visible sector due to a gauge kinetic mixing term. We show that the transfer occurs when the evolution of magnetic fields is dominated by dissipation due to finite electric conductivity, and does not occur at later times if the magnetic fields evolve according to magnetohydrodynamics scaling laws. The efficiency of the transfer is suppressed by not only the gauge kinetic mixing coupling but also the ratio between the large electric conductivity and the typical momentum of the magnetic fields. We find that the transfer gives nonzero visible magnetic fields today. However, without possible dynamo amplifications, the field transfer is not efficient enough to obtain the intergalactic magnetic fields suggested by the gamma-ray observations, although there are plenty of possibilities for efficient dark magnetogenesis, which are experimentally unconstrained.

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arXiv2017

Axion production from primordial magnetic fields

Production of axionlike particles (ALPs) by primordial magnetic fields may have significant impacts on cosmology. We discuss the production of ALPs in the presence of the primordial magnetic fields. We find a region of the ALP mass and photon coupling which realizes the observed properties of the dark matter with appropriate initial conditions for the magnetic fields. This region may be interesting in light of recent indications for the 3.5 keV lines from galaxy clusters. For a small axion mass, a region of previously allowed parameter spaces is excluded by overproduction of ALPs as a hot/warm dark matter component. Since the abundance of ALPs strongly depends on the initial conditions of primordial magnetic fields, our results provide implications for scenarios of magnetogenesis.

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arXiv2017

Baryon Asymmetry and Gravitational Waves from Pseudoscalar Inflation

In models of inflation driven by an axion-like pseudoscalar field, the inflaton, a, may couple to the standard model hypercharge via a Chern-Simons-type interaction, $L \subset a/(4Λ) F\tilde{F}$. This coupling results in explosive gauge field production during inflation, especially at its last stage, which has interesting phenomenological consequences: For one thing, the primordial hypermagnetic field is maximally helical. It is thus capable of sourcing the generation of nonzero baryon number, via the standard model chiral anomaly, around the time of electroweak symmetry breaking. For another thing, the gauge field production during inflation feeds back into the primordial tensor power spectrum, leaving an imprint in the stochastic background of gravitational waves (GWs). In this paper, we focus on the correlation between these two phenomena. Working in the approximation of instant reheating, we (1) update the investigation of baryogenesis via hypermagnetic fields from pseudoscalar inflation and (2) examine the corresponding implications for the GW spectrum. We find that successful baryogenesis requires a suppression scale Lambda of around Lambda ~ 3 x 10^17 GeV, which corresponds to a relatively weakly coupled axion. The gauge field production at the end of inflation is then typically accompanied by a peak in the GW spectrum at frequencies in the MHz range or above. The detection of such a peak is out of reach of present-day technology; but in the future, it may serve as a smoking-gun signal for baryogenesis from pseudoscalar inflation. Conversely, models that do yield an observable GW signal suffer from the overproduction of baryon number, unless the reheating temperature is lower than the electroweak scale.

hep-ph↗

arXiv2016

Evolution of the Baryon Asymmetry through the Electroweak Crossover in the Presence of a Helical Magnetic Field

We elaborate upon the model of baryogenesis from decaying magnetic helicity by focusing on the evolution of the baryon number and magnetic field through the Standard Model electroweak crossover. The baryon asymmetry is determined by a competition between the helical hypermagnetic field, which sources baryon number, and the electroweak sphaleron, which tends to wash out baryon number. At the electroweak crossover both of these processes become inactive; the hypermagnetic field is converted into an electromagnetic field, which does not source baryon number, and the weak gauge boson masses grow, suppressing the electroweak sphaleron reaction. An accurate prediction of the relic baryon asymmetry requires a careful treatment of the crossover. We extend our previous study [K. Kamada and A. J. Long, Phys. Rev. D94, 065301 (2016), arXiv:1606.08891[astro-ph.CO]], taking into account the gradual conversion of the hypermagnetic into the electromagnetic field. If the conversion is not completed by the time of sphaleron freeze-out, as both analytic and numerical studies suggest, the relic baryon asymmetry is enhanced compared to previous calculations. The observed baryon asymmetry of the Universe can be obtained for a primordial magnetic field that has a present-day field strength and coherence length of $B_0 \sim 10^{-17} \, {\rm G}$ and $λ_0 \sim 10^{-3} \, {\rm pc}$ and a positive helicity. For larger $B_0$ the baryon asymmetry is overproduced, which may be in conflict with blazar observations that provide evidence for an intergalactic magnetic field of strength $B_0 \gtrsim 10^{-14\sim16} \, {\rm G}$.

hep-ph↗

arXiv2016

Baryogenesis from Decaying Magnetic Helicity

As a result of the Standard Model chiral anomalies, baryon number is violated in the early universe in the presence of a hypermagnetic field with varying helicity. We investigate whether the matter / anti-matter asymmetry of the universe can be created from the decaying helicity of a primordial (hyper)magnetic field before and after the electroweak phase transition. In this model, baryogenesis occurs without $(B-L)$-violation, since the $(B+L)$ asymmetry generated by the hypermagnetic field counteracts the washout by electroweak sphalerons. At the electroweak crossover, the hypermagnetic field becomes an electromagnetic field, which does not source $(B+L)$. Although the sphalerons remain in equilibrium for a time, washout is avoided since the decaying magnetic helicity sources chirality. The relic baryon asymmetry is fixed when the electroweak sphaleron freezes out. Under reasonable assumptions, a baryon asymmetry of $n_B / s \simeq 4 \times 10^{-12}$ can be generated from a maximally helical, right-handed (hyper)magnetic field that has a field strength of $B_0 \simeq 10^{-14} \, {\rm Gauss}$ and coherence length of $λ_{0} \simeq 1 \, {\rm pc}$ today. Relaxing an assumption that relates $λ_0$ to $B_0$, the model predicts $n_B / s \gtrsim 10^{-10}$, which could potentially explain the observed baryon asymmetry of the universe.

astro-ph.CO↗

arXiv2016

Large-scale magnetic fields can explain the baryon asymmetry of the Universe

Helical hypermagnetic fields in the primordial Universe can produce the observed amount of baryon asymmetry through the chiral anomaly without any ingredients beyond the standard model of particle physics. While they generate no $B-L$ asymmetry, the generated baryon asymmetry survives the spharelon washout effect, because the generating process remains active until the electroweak phase transition. Solving the Boltzmann equation numerically and finding an attractor solution, we show that the baryon asymmetry of our Universe can be explained, if the present large-scale magnetic fields indicated by the blazar observations have a negative helicity and existed in the early Universe before the electroweak phase transition. We also derive the upper bound on the strength of the helical magnetic field, which is tighter than the cosmic microwave background constraint, to avoid the overproduction of baryon asymmetry.

hep-ph↗

arXiv2015

On the strong coupling scale in Higgs G-inflation

Higgs G-inflation is an inflation model that takes advantage of a Galileon-like derivative coupling. It is a non-renormalizable operator and is strongly coupled at high energy scales. Perturbative analysis does not have a predictive power any longer there. In general, when the Lagrangian is expanded around the vacuum, the strong coupling scale is identified as the mass scale that appears in non-renormalizable operators. In inflationary models, however, the identification of the strong coupling scale is subtle, since the structures of the kinetic term as well as the interaction itself can be modified by the background inflationary dynamics. Therefore, the strong coupling scale depends on the background. In this letter, we evaluate the strong coupling scale of the fluctuations around the background in the Higgs G-inflation including the Nambu-Goldstone modes associated with the symmetry breaking. We find that the system is sufficiently weakly coupled when the scales which we now observe exit the horizon during inflation and the observational predictions with the semiclassical treatment are valid. However, we also find that the inflaton field value at which the strong coupling scale and the Hubble scale meet is less than the Planck scale. Therefore, we cannot describe the model from the Planck scale, or the chaotic initial condition.

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arXiv2015

A Minimal Supersymmetric Model of Particle Physics and the Early Universe

We consider a minimal supersymmetric extension of the Standard Model, with right-handed neutrinos and local B-L, the difference between baryon and lepton number, a symmetry which is spontaneously broken at the scale of grand unification. To a large extent, the parameters of the model are determined by gauge and Yukawa couplings of quarks and leptons. We show that this minimal model can successfully account for the earliest phases of the cosmological evolution: Inflation is driven by the energy density of a false vacuum of unbroken B-L symmetry, which ends in tachyonic preheating, i.e. the decay of the false vacuum, followed by a matter dominated phase with heavy B-L Higgs bosons. Nonthermal and thermal processes produce an abundance of heavy neutrinos whose decays generate primordial entropy, baryon asymmetry via leptogenesis and dark matter consisting of gravitinos or nonthermal WIMPs. The model predicts relations between neutrino and superparticle masses and a characteristic spectrum of gravitational waves.

hep-ph↗

arXiv2015

Inflationary cosmology and the standard model Higgs with a small Hubble induced mass

We study the dynamics of the standard model Higgs field in the inflationary cosmology. Since metastability of our vacuum is indicated by the current experimental data of the Higgs boson and top quark, inflation models with a large Hubble parameter may have a problem: In such models, the Higgs field rolls down towards the unwanted true vacuum due to the large fluctuation in the inflationary background. However, this problem can be relaxed by supposing an additional mass term for the Higgs field generated during and after inflation. We point out that it does not have to be larger than the Hubble parameter if the number of $e$-folds during inflation is not too large. We demonstrate that a high reheating temperature is favored in such a relatively small mass case and it can be checked by future gravitational wave observations. Such an induced mass can be generated by, {\it e.g.,} a direct coupling to the inflaton field or nonminimal coupling to gravity.

hep-ph↗

arXiv2015

Topological inflation from the Starobinsky model in supergravity

We consider the ghost-free higher order corrections to the Starobinsky model in the old minimal supergravity. In general, higher order corrections cannot be forbidden by symmetries, which likely violate the flatness of the scalaron potential and makes inflation difficult to explain the present Universe. We find a severe constraint on the dimensionless coupling of the $R^4$ correction as $-5.5 \times 10^{-8}<s<9.1 \times 10^{-8}$ from the recent results of the Planck observation. If we start from the chaotic initial condition, the constraint becomes much severer. However, in the case where the coupling of the $R^4$ correction is positive, the scalaron potential has a local maximum with two local minimum at the origin and infinity, which admits topological inflation. In this case, inflation can take place naturally if the coupling satisfies the observational constraints.

hep-th↗

arXiv2014

Effects of cosmic strings with delayed scaling on CMB anisotropy

The network of cosmic strings generated in a phase transition during inflation enters the scaling regime later than that of usual strings. If it occurs after the recombination, temperature anisotropies of the cosmic microwave background (CMB) at high multipole moments are significantly reduced. In this paper, we study such effects qualitatively and show that the constraint on the cosmic string tension from the CMB temperature anisotropies and B-mode polarizations can be relaxed. It is shown to be difficult to explain the recent BICEP2 and POLARBEAR results in terms of signals induced by cosmic strings alone even if we take into account the delayed scaling. However, the inflationary tensor-to-scalar ratio required to explain the observed B-mode signals can be slightly reduced to be consistent with the Planck constraint.

astro-ph.CO↗

arXiv2014

Hybrid Inflation in the Complex Plane

Supersymmetric hybrid inflation is an exquisite framework to connect inflationary cosmology to particle physics at the scale of grand unification. Ending in a phase transition associated with spontaneous symmetry breaking, it can naturally explain the generation of entropy, matter and dark matter. Coupling F-term hybrid inflation to soft supersymmetry breaking distorts the rotational invariance in the complex inflaton plane---an important fact, which has been neglected in all previous studies. Based on the delta-N formalism, we analyze the cosmological perturbations for the first time in the full two-field model, also taking into account the fast-roll dynamics at and after the end of inflation. As a consequence of the two-field nature of hybrid inflation, the predictions for the primordial fluctuations depend not only on the parameters of the Lagrangian, but are eventually fixed by the choice of the inflationary trajectory. Recognizing hybrid inflation as a two-field model resolves two shortcomings often times attributed to it: The fine-tuning problem of the initial conditions is greatly relaxed and a spectral index in accordance with the PLANCK data can be achieved in a large part of the parameter space without the aid of supergravity corrections. Our analysis can be easily generalized to other (including large-field) scenarios of inflation in which soft supersymmetry breaking transforms an initially single-field model into a multi-field model.

hep-ph↗

arXiv2014

Instability of colliding metastable strings

The breaking of $U(1)_R$ symmetry plays a crucial role in modeling the breaking of supersymmetry (SUSY). In the models that possess both SUSY preserving and SUSY breaking vacua, tube-like cosmic strings called R-tubes, whose surfaces are constituted by domain walls interpolating a false and a true vacuum with some winding numbers, can exist. Their (in)stability can strongly constrain SUSY breaking models theirselves. In the present study, we investigate the dynamical (in)stability of two colliding metastable tube-like strings by field-theoretic simulations. From them, we find that the strings become unstable, depending on the relative collision angle and speed of two strings, and the false vacuum is eventually filled out by the true vacuum owing to rapid expansion of the strings or unstable bubbles created as remnants of the collision.

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