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Search Results: 1 - 10 of 1223 matches for " Kazunori Nakayama "
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Axions : Theory and Cosmological Role
Kawasaki, Masahiro;Nakayama, Kazunori
High Energy Physics - Phenomenology , 2013,
Abstract: We review recent developments on axion cosmology. Topics include : axion cold dark matter, axions from topological defects, axion isocurvature perturbation and its non-Gaussianity and axino/saxion cosmology in supersymmetric axion model.
Direct/indirect detection signatures of nonthermally produced dark matter
Nagai, Minoru;Nakayama, Kazunori
High Energy Physics - Phenomenology , 2008, DOI: 10.1103/PhysRevD.78.063540
Abstract: We study direct and indirect detection possibilities of neutralino dark matter produced non-thermally by e.g. the decay of long-lived particles, as is easily implemented in the case of anomaly or mirage mediation models. In this scenario, large self-annihilation cross sections are required to account for the present dark matter abundance, and it leads to significant enhancement of the gamma-ray signature from the Galactic Center and the positron flux from the dark matter annihilation. It is found that GLAST and PAMELA will find the signal or give tight constraints on such nonthermal production scenarios of neutralino dark matter.
Baryon Asymmetry in a Heavy Moduli Scenario
Kawasaki, Masahiro;Nakayama, Kazunori
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.76.043502
Abstract: In some models of supersymmetry breaking, modulus fields are heavy enough to decay before BBN. But the large entropy produced via moduli decay significantly dilutes the preexisting baryon asymmetry of the universe. We study whether Affleck-Dine mechanism can provide enough baryon asymmetry which survives the dilution, and find several situations in which desirable amount of baryon number remains after the dilution. The possibility of non-thermal dark matter is also discussed. This provides the realistic cosmological scenario with heavy moduli.
Cosmological Moduli Problem from Thermal Effects
Nakayama, Kazunori;Takahashi, Fuminobu
High Energy Physics - Phenomenology , 2008, DOI: 10.1016/j.physletb.2008.11.046
Abstract: We estimate the cosmological abundance of a modulus field that has dilatonic couplings to gauge fields, paying particular attention to thermal corrections on the modulus potential. We find that a certain amount of the modulus coherent oscillations is necessarily induced by a linear thermal effect. We argue that such an estimate provides the smallest possible modulus abundance for a given thermal history of the Universe. As an example we apply our results to a saxion, a bosonic supersymmetric partner of an axion, and derive a tight bound on the reheating temperature. We emphasize that the problem cannot be avoided by fine-tuning the initial deviation of the modulus field, since the minimal amount of the modulus is induced by the dynamics of the scalar potential.
Nonthermal dark matter in mirage mediation
Nagai, Minoru;Nakayama, Kazunori
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.76.123501
Abstract: In mirage-mediation models there exists a modulus field whose mass is O(1000) TeV and its late-decay may significantly change the standard thermal relic scenario of the dark matter. We study nonthermal production of the dark matter directly from the modulus decay, and find that for some parameter regions non-thermally produced neutralinos can become the dark matter.
The R-axion and non-Gaussianity
Kazunori Nakayama,Fuminobu Takahashi
Physics , 2009, DOI: 10.1016/j.physletb.2009.08.020
Abstract: We study cosmological implications of an R-axion, a pseudo Nambu-Goldstone boson associated with spontaneous breaking of an U(1)_R symmetry, focusing on its quantum fluctuations generated during inflation. We show that, in the anomaly mediation, the R-axion decays into a pair of gravitinos, which eventually decay into the visible particles producing the neutralino LSP. As a result, the quantum fluctuations of the R-axion are inherited by the cold dark matter isocurvature density perturbation with potentially large non-Gaussianity. The constraints on the inflation scale and the initial misalignment are derived.
Higgs Chaotic Inflation in Standard Model and NMSSM
Kazunori Nakayama,Fuminobu Takahashi
Physics , 2010, DOI: 10.1088/1475-7516/2011/02/010
Abstract: We construct a chaotic inflation model in which the Higgs fields play the role of the inflaton in the standard model as well as in the singlet extension of the supersymmetric standard model. The key idea is to allow a non-canonical kinetic term for the Higgs field. The model is a realization of the recently proposed running kinetic inflation, in which the coefficient of the kinetic term grows as the inflaton field. The inflaton potential depends on the structure of the Higgs kinetic term. For instance, the inflaton potential is proportional to phi^2 and phi^{2/3} in the standard model and NMSSM, respectively. It is also possible to have a flatter inflaton potential.
Running Kinetic Inflation
Kazunori Nakayama,Fuminobu Takahashi
Physics , 2010, DOI: 10.1088/1475-7516/2010/11/009
Abstract: We study a recently proposed running kinetic inflation model in which the inflaton potential becomes flat due to rapid growth of the kinetic term at large inflaton field values. As concrete examples, we build a variety of chaotic inflation models in supergravity with e.g. quadratic, linear, and fractional-power potentials. The power of the potential generically increases after inflation, and the inflaton is often massless at the potential minimum in the supersymmetric limit, which leads to many interesting phenomena. First, the light inflaton mass greatly relaxes severe thermal and non-thermal gravitino problems. Secondly, the kination epoch is naturally present after inflation, which may enhance the gravity waves. Thirdly, since the inflaton is light, it is likely coupled to the Higgs sector for successful reheating. The inflaton and its superpartner, inflatino, may be produced at the LHC. Interestingly, the inflatino can be dark matter, if it is the lightest supersymmetric particle.
Curvature perturbation from velocity modulation
Kazunori Nakayama,Teruaki Suyama
Physics , 2011, DOI: 10.1103/PhysRevD.84.063520
Abstract: We propose a new variant model of the modulated reheating. If particles have large scale fluctuations on their velocities, or equivalently their Lorentz factors, the decay rate also fluctuates and the curvature perturbation is induced via their decay processes in analogy with modulated reheating. For example, if they are produced nonthermally by the decay of another field with its mass fluctuating on large scales, such a situation is realized. We explicitly calculate the resulting curvature perturbation and non-linearity parameters and show that the effect of velocity-modulation is not negligible if the particles are semi-relativistic at the decay.
Low-scale Supersymmetry from Inflation
Kazunori Nakayama,Fuminobu Takahashi
Physics , 2011, DOI: 10.1088/1475-7516/2011/10/033
Abstract: 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.
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