arXiv subjects

Fumihiro Takayama

Publications and source records attributed to Fumihiro Takayama.

24 records · Page 2Linked to original sources

arXiv2003

Graviton Cosmology in Universal Extra Dimensions

In models of universal extra dimensions, gravity and all standard model fields propagate in the extra dimensions. Previous studies of such models have concentrated on the Kaluza-Klein (KK) partners of standard model particles. Here we determine the properties of the KK gravitons and explore their cosmological implications. We find the lifetimes of decays to KK gravitons, of relevance for the viability of KK gravitons as dark matter. We then discuss the primordial production of KK gravitons after reheating. The existence of a tower of KK graviton states makes such production extremely efficient: for reheat temperature T_RH and d extra dimensions, the energy density stored in gravitons scales as T_RH^{2+3d/2}. Overclosure and Big Bang nucleosynthesis therefore stringently constrain T_RH in all universal extra dimension scenarios. At the same time, there is a window of reheat temperatures low enough to avoid these constraints and high enough to generate the desired thermal relic density for KK WIMP and superWIMP dark matter.

hep-ph↗

arXiv2003

Superweakly Interacting Massive Particles

We investigate a new class of dark matter: superweakly-interacting massive particles (superWIMPs). As with conventional WIMPs, superWIMPs appear in well-motivated particle theories with naturally the correct relic density. In contrast to WIMPs, however, superWIMPs are impossible to detect in all conventional dark matter searches. We consider the concrete examples of gravitino and graviton cold dark matter in models with supersymmetry and universal extra dimensions, respectively, and show that superWIMP dark matter satisfies stringent constraints from Big Bang nucleosynthesis and the cosmic microwave background.

hep-ph↗

arXiv2002

3- and 2-body supersymmetric processes: predictions, challenges and prospectives

The processes subject of this talk are: 1) the 2 --> 3 processes contributing, together with the 2 --> 2 one to the hadron-collider production of a charged Higgs boson in association with a t-quark; 2) the 2 --> 2 and 2 --> 3 processes giving rise to the hadron-collider strahlung of a charged slepton from a t-quark in R_p-violating models in which lepton-number-violating trilinear couplings largely dominate over bilinear ones; 3) 3-body neutralino and chargino decays in similar R_p-violating scenarios. The significance of the R_p-violating processes is critically assessed against implications from neutrino physics. Contributions to neutrino masses arising at the tree level, at the one- and two-loop levels are reviewed. Comments are also made on the production of sneutrinos via gluon fusion and on the decay of sneutrinos into photons pairs, and on the influence that constraints from neutrino physics have on these processes.

hep-ph↗

arXiv2000

Neutrino Masses and Mixing from Bilinear R-Parity Violation

Neutrino masses and mixing are generated in a supersymmetric standard model when R-parity is violated in bilinear mass terms. The mixing matrix among the neutrinos takes a restrictive form if the lepton flavor universality holds in the R-parity violating soft masses. It turns out that only the small angle MSW solution to the solar neutrino problem is consistent with the result of the CHOOZ experiment and the atmospheric neutrino data.

hep-ph↗

arXiv2000

Gravitino Dark Matter without R-parity

Cosmological issues are examined when gravitino is the lightest superparticle (LSP) and R-parity is broken. Decays of the next lightest superparticles occur rapidly via R-parity violating interaction, and thus they do not upset the big-bang nucleosynthesis, unlike the R-parity conserving case. The gravitino LSP becomes unstable, but its lifetime is typically much longer than the age of the Universe. It turns out that observations of diffuse photon background coming from radiative decays of the gravitino do not severely constrain the gravitino abundance, and thus the gravitino weighing less than around 1 GeV can be dark matter of the Universe when bilinear R-parity violation generates a neutrino mass which accounts for the atmospheric neutrino anomaly.

hep-ph↗

arXiv1999

Pattern of Neutrino Oscillations in Supersymmetry with Bilinear R-parity Violation

Neutrino masses and their mixing are studied in detail in the framework of a supersymmetric standard model with bilinear R-parity violation. In this scenario, the mixing matrix among the neutrinos is in a restrictive form. We find that only the small angle MSW solution is allowed for the solar neutrino problem when the results of the CHOOZ experiment are combined with the mass squared difference and mixing angle suggested by the atmospheric neutrino data. Collider signals of this scenario are also discussed.

hep-ph↗