arXiv subjects

Fuminobu Takahashi

Publications and source records attributed to Fuminobu Takahashi.

At least 163 records · Page 9Linked to original sources

arXiv2012

A Gravitino-rich Universe

The gravitino may well play an important role in cosmology, not only because its interactions are Planck-suppressed and therefore long-lived, but also because it is copiously produced via various processes such as particle scatterings in thermal plasma, and (pseudo) modulus and inflaton decays. We study a possibility that the early Universe was gravitino-rich from various aspects. In particular, a viable cosmology is possible, if high-scale supersymmetry is realized in nature as suggested by the recent discovery of the standard-model like Higgs boson of mass about 125-126 GeV. We find that the Universe can be even gravitino-dominated, in which case there will be an entropy dilution by the gravitino decay. If the gravitino abundance is proportional to the reheating temperature, both the maximal baryon asymmetry in leptogenesis and the dark matter from the gravitino decay become independent of the reheating temperature. The dark matter candidate is the Wino-like neutralino, whose mass is suppressed compared to the anomaly-mediation relation.

hep-ph↗

arXiv2012

Higgs, Moduli Problem, Baryogenesis and Large Volume Compactifications

We consider the cosmological moduli problem in the context of high-scale supersymmetry breaking suggested by the recent discovery of the standard-model like Higgs boson. In order to solve the notorious moduli-induced gravitino problem, we focus on the LARGE volume scenario, in which the modulus decay into gravitinos can be kinematically forbidden. We then consider the Affleck-Dine mechanism with or without an enhanced coupling with the inflaton, taking account of possible Q-ball formation. We show that the baryon asymmetry of the present Universe can be generated by the Affleck-Dine mechanism in LARGE volume scenario, solving the moduli and gravitino problems. We also find that the overall volume modulus decays into a pair of the axionic superpartners, which contribute to the extra relativistic degrees of freedom.

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arXiv2012

Cosmologically viable gauge mediation

Gauge mediation provides us with a complete picture of supersymmetry breaking and its mediation within the effective field theories, and thus allows us to discuss consistencies with low-energy particle physics as well as cosmological observations. We study in detail the cosmological evolution of the pseudo-modulus field in the supersymmetry breaking sector and also the production of the gravitinos in the early Universe in a simple (but a complete) model of gauge mediation. Under fairly reasonable assumptions, it is found that there exists a parameter region where dark matter of the Universe is explained by both thermally and non-thermally produced gravitinos, while the baryon asymmetry of the Universe is generated through the thermal leptogenesis.

hep-ph↗

arXiv2012

Light Higgsino from Axion Dark Radiation

The recent observations imply that there is an extra relativistic degree of freedom coined dark radiation. We argue that the QCD axion is a plausible candidate for the dark radiation, not only because of its extremely small mass, but also because in the supersymmetric extension of the Peccei-Quinn mechanism the saxion tends to dominate the Universe and decays into axions with a sizable branching fraction. We show that the Higgsino mixing parameter mu is bounded from above when the axions produced at the saxion decays constitute the dark radiation: mu \lesssim 300 GeV for a saxion lighter than 2m_W, and mu less than the saxion mass otherwise. Interestingly, the Higgsino can be light enough to be within the reach of LHC and/or ILC even when the other superparticles are heavy with mass about 1 TeV or higher. We also estimate the abundance of axino produced by the decays of Higgsino and saxion.

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arXiv2012

Anarchy and Leptogenesis

We study if leptogenesis works successfully together with the neutrino mass anarchy hypothesis. We find that the predicted neutrino mass spectrum is sensitive to the reheating temperature or the inflaton mass, while the distributions of the neutrino mixing angles and CP violation phases remain intact as determined by the invariant Haar measure of U(3). In the case of thermal leptogenesis, the light neutrino mass distribution agrees well with the observations if the reheating temperature is O(10^{9-11}) GeV. The mass spectrum of the right-handed neutrinos and the neutrino Yukawa matrix exhibit a certain pattern, as a result of the competition between random matrices with elements of order unity and the wash-out effect. Non-thermal leptogenesis is consistent with observation only if the inflaton mass is larger than or comparable to the typical right-handed neutrino mass scale. Cosmological implications are discussed in connection with the 125GeV Higgs boson mass.

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arXiv2012

Cosmological Moduli Problem in Low Cutoff Theory

We show that the cosmological moduli problem is solved, without relying on huge late-time entropy production, if the universal cutoff scale of the theory is a few orders of magnitude smaller than the Planck scale. We obtain a general estimate of the modulus abundance in terms of the inflationary scale and the reheating temperature, and find in particular that the reheating temperature can be high enough for the non-thermal leptogenesis to work.

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arXiv2012

Alchemical Inflation: inflaton turns into Higgs

We propose a new inflation model in which a gauge singlet inflaton turns into the Higgs condensate after inflation. The inflationary path is characterized by a moduli space of supersymmetric vacua spanned by the inflaton and Higgs field. The inflation energy scale is related to the soft supersymmetry breaking, and the Hubble parameter during inflation is smaller than the gravitino mass. The initial condition for the successful inflation is naturally realized by the pre-inflation in which the Higgs plays a role of the waterfall field.

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arXiv2012

PeV-scale Supersymmetry from New Inflation

We show that heavy supersymmetric particles around O(100) TeV to O(1) PeV naturally appear in new inflation in which the Higgs boson responsible for the breaking of U(1)B-L plays the role of inflaton. Most important, the supersymmetric breaking scale is bounded above by the inflationary dynamics, in order to suppress the Coleman-Weinberg potential which would otherwise spoil the slow-roll inflation. Our scenario has rich phenomenological and cosmological implications: the Higgs boson mass at around 125 GeV can be easily explained, non-thermal leptogenesis works automatically, the gravitino production from inflaton decay is suppressed, the dark matter is either the lightest neutralino or the QCD axion, and the upper bound on the inflation scale for the modulus stabilization can be marginally satisfied.

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arXiv2012

Dark Radiation from Modulated Reheating

We show that the modulated reheating mechanism can naturally account for dark radiation, whose existence is hinted by recent observations of the cosmic microwave background radiation and the primordial Helium abundance. In this mechanism, the inflaton decay rate depends on a light modulus which acquires almost scale-invariant quantum fluctuations during inflation. We find that the light modulus is generically produced by the inflaton decay and therefore a prime candidate for the dark radiation. Interestingly, an almost scale-invariant power spectrum predicted in the modulated reheating mechanism gives a better fit to the observation in the presence of the extra radiation. We discuss the production mechanism of the light modulus in detail taking account of its associated isocurvature fluctuations. We also consider a case where the modulus becomes the dominant component of dark matter.

astro-ph.CO↗

arXiv2012

Gravity mediation without a Polonyi problem

Recent indications of the 125GeV Higgs at the LHC can be explained in a relatively high-scale SUSY scenario where the sparticle masses are multi-TeV as is realized in the focus-point region. However, it suffers from the notorious cosmological Polonyi problem. We argue that the Polonyi problem is solved and thermal or non-thermal leptogenesis scenario works successfully, if a certain Polonyi coupling to the inflaton is enhanced by a factor of 10-100.

hep-ph↗

arXiv2011

Higgs mass and inflation

We show that the standard-model Higgs boson mass m_h is correlated with the spectral index of density perturbation n_s in the inflation scenario with the inflaton being identified with the B-L Higgs boson. The Higgs boson mass ranges from m_h ~ 120GeV to 140GeV for n_s ~ 0.95 - 0.96. In particular, as n_s approaches to 0.96, the Higgs mass is predicted to be in the range of 125GeV to 140GeV in the case of relatively light gauginos, and 120GeV to 135GeV in the case where all SUSY particle masses are of the same order. This will be tested soon by the LHC experiment and the Planck satellite. The relation is due to the PeV-scale supersymmetry required by the inflationary dynamics. We also comment on the cosmological implications of our scenario such as non-thermal leptogenesis and dark matter.

hep-ph↗

arXiv2011

Non-Gaussianity from Curvatons Revisited

We investigate density perturbations sourced by a curvaton with a generic energy potential. The key feature of a curvaton potential which deviates from a quadratic is that the curvaton experiences a non-uniform onset of its oscillation. This sources additional contributions to the resulting density perturbations, and we especially find that the non-Gaussianity parameter f_NL can become large with either sign no matter whether the curvaton dominates or subdominates the universe when it decays. Such non-quadratic curvaton potentials are required in order to produce a red-tilted density perturbation spectrum (without invoking large-field inflation), and are also motivated by explicit curvaton models based on microscopic physics. We further apply our generic results to the case where the curvaton is a pseudo-Nambu-Goldstone (NG) boson of a broken U(1) symmetry, and show that the resulting density perturbations are strongly enhanced towards the hilltop region of the energy potential, accompanied by a mild increase of the non-Gaussianity. Such hilltop NG curvatons can produce observationally suggested density perturbations under wide ranges of inflation/reheating scales, and further predict the non-Gaussianity of the density perturbations to lie within the range 10 <~ f_NL <~ 30.

astro-ph.CO↗

arXiv2011

On the Adiabatic Solution to the Polonyi/Moduli Problem

One of the solutions to the cosmological Polonyi problem is to introduce a large coupling between the Polonyi field and the inflaton so that the Polonyi field adiabatically tracks the temporal minimum of the potential. We study general conditions for the adiabatic suppression mechanism to work, and find that a non-negligible amount of the Polonyi field is induced in the form of coherent oscillations at the end of inflation. In the case of low reheating temperature, this contribution is so small that it does not cause cosmological problems. On the other hand, this contribution may be significant for a relatively high reheating temperature and we still need some amount of tuning in order to avoid the Polonyi problem. We also point out that Polonyi particles produced from thermal plasma pose a severe constraint on the reheating temperature. Furthermore, we extend the original framework to include enhanced couplings of the Polonyi field with the visible particles as well as with itself, and derive upper bounds on the reheating temperature after inflation. We also investigate the adiabatic solution to the cosmological moduli problem in gauge and anomaly mediation.

hep-ph↗

arXiv2011

Low-scale Supersymmetry from Inflation

We investigate an inflation model with the inflaton being identified with a Higgs boson responsible for the breaking of U(1)B-L symmetry. We show that supersymmetry must remain a good symmetry at scales one order of magnitude below the inflation scale, in order for the inflation model to solve the horizon and flatness problems, as well as to account for the observed density perturbation. The upper bound on the soft supersymmetry breaking mass lies between 1TeV and 10^3TeV. Interestingly, our finding opens up a possibility that universes with the low-scale supersymmetry are realized by the inflationary selection. Our inflation model has rich implications; non-thermal leptogenesis naturally works, and the gravitino and moduli problems as well as the moduli destabilization problem can be solved or ameliorated; the standard-model higgs boson receives a sizable radiative correction if the supersymmertry breaking takes a value on the high side ~10^3TeV.

hep-ph↗

arXiv2011

Number-Theory Dark Matter

We propose that the stability of dark matter is ensured by a discrete subgroup of the U(1)B-L gauge symmetry, Z_2(B-L). We introduce a set of chiral fermions charged under the U(1)B-L in addition to the right-handed neutrinos, and require the anomaly-cancellation conditions associated with the U(1)B-L gauge symmetry. We find that the possible number of fermions and their charges are tightly constrained, and that non-trivial solutions appear when at least five additional chiral fermions are introduced. The Fermat theorem in the number theory plays an important role in this argument. Focusing on one of the solutions, we show that there is indeed a good candidate for dark matter, whose stability is guaranteed by Z_2(B-L).

hep-ph↗

arXiv2011

Constraint on the gravitino mass in hybrid inflation

We revisit the F-term hybrid inflation model in supergravity. In particular, we point out that a constant term in the superpotential has significant effects on the inflaton dynamics. It is shown that the hybrid inflation model suffers from several potential problems: tuning of the initial condition, gravitino overproduction and formation of cosmic strings, for both minimal and non-minimal Kahler potentials. These problems can only be avoided in gauge-mediated SUSY breaking models where the gravitino is relatively light and the constant term in the superpotential is not important. Implications on the non-thermal leptogenesis scenario are also described.

hep-ph↗

arXiv2011

Why have supersymmetric particles not been observed?

If low-energy supersymmetry is the solution to the hierarchy problem, it is a puzzle why supersymmetric particles have not been observed experimentally to date. We show that supersymmetric particles in the TeV region can be explained if the fundamental cut-off scale of the theory is smaller than the 4-dimensional Planck scale and if thermal leptogenesis is the source of the observed baryon asymmetry. The supersymmetric particles such as sfermions and gauginos are predicted to be in the TeV region, while the gravitino is the LSP with mass of O(100)GeV and is a good candidate for dark matter. Interestingly, the cosmological moduli problem can be solved in the theory with the low cut-off scale.

hep-ph↗

arXiv2011

Strong dynamics at the Planck scale as a solution to the cosmological moduli problem

We argue that strong dynamics at the Planck scale can solve the cosmological moduli problem. We discuss its implications for inflation models, and find that a certain type of multi-field inflation model is required for this mechanism to work, since otherwise it would lead to the serious eta-problem. Combined with the inflaton-induced gravitino problem, we show that a chaotic inflation with a discrete symmetry naturally avoids both problems. Interestingly, the focus point supersymmetry is predicted when this mechanism is applied to the Polonyi model.

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