Hitoshi Murayama

Publications (10)7.37 Total impact

• Source

  Available from: arxiv.org

  Article: Natural soft leptogenesis

  Yuval Grossman, Ryuichiro Kitano, Hitoshi Murayama
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  ABSTRACT: Successful soft leptogenesis requires small B-terms for the right-handed sneutrinos and a large CP violating phase between the A- and B-terms. We show that this situation is realized naturally within the framework of gauge mediated supersymmetry breaking. The A-term is dominated by contribution from gauge mediation, while supergravity effects are more important for the B-term. The different origins naturally explain simultaneously the smallness of the B-term and the large CP violating phase. The most stringent bounds on the model come from the cosmological gravitino problem. We find a viable parameter region with very light gravitino $m_{3/2} \lesssim 16 eV$, providing a consistent framework for supersymmetry phenomenology, soft leptogenesis and cosmology. Comment: 11 pages, 1 figure
  04/2005;
• Source

  Available from: arxiv.org

  Article: Models of Neutrino Mass with a Low Cutoff Scale

  Hooman Davoudiasl, Ryuichiro Kitano, Graham D. Kribs, Hitoshi Murayama
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  ABSTRACT: In theories with a low quantum gravity scale, global symmetries are expected to be violated, inducing excessive proton decay or large Majorana neutrino masses. The simplest cure is to impose discrete gauge symmetries, which in turn make neutrinos massless. We construct models that employ these gauge symmetries while naturally generating small neutrino masses. Majorana (Dirac) neutrino masses are generated through the breaking of a discrete (continuous) gauge symmetry at low energies, e.g., 2 keV to 1 GeV. The Majorana case predicts \Delta N_\nu \simeq 1 at BBN, neutrinoless double beta decay with scalar emission, and modifications to the CMB anisotropies from domain walls in the universe as well as providing a possible Dark Energy candidate. For the Dirac case, despite the presence of a new light gauge boson, all laboratory, astrophysical, and cosmological constraints can be avoided. Comment: 11 pages, 4 figures
  02/2005;
• Source

  Available from: ArXiv

  Article: A viable supersymmetric model with UV insensitive anomaly mediation

  Masahiro Ibe, Ryuichiro Kitano, Hitoshi Murayama
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  ABSTRACT: We propose an electroweak model which is compatible with the UV insensitive anomaly mediated supersymmetry breaking. The model is an extension of the NMSSM by adding vector-like matter fields which can drive the soft scalar masses of the singlet Higgs field negative and the successful electroweak symmetry breaking is achieved. Viable parameter regions are found to preserve perturbativity of all the coupling constants up to the Planck scale. With this success, the model becomes a perfect candidate of physics beyond the standard model without the FCNC and CP problem. The cosmology is also quite interesting. The lightest neutralino is the wino which is a perfect cold dark matter candidate assuming the non-thermal production from the gravitino decay. There is no gravitino problem because it decays before the BBN era, and thus the thermal leptogenesis works. The cosmological domain wall problem inherent in the NMSSM is absent since the Z_3 symmetry is broken by the QCD instanton effect in the presence of the vector-like quarks. We also briefly comment on a possible solution to the strong CP problem a la the Nelson-Barr mechanism. Comment: 22 pages, 4 figures, 1 table
  12/2004;
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  Available from: arxiv.org

  Article: Gravitational baryogenesis.

  Hooman Davoudiasl, Ryuichiro Kitano, Graham D Kribs, Hitoshi Murayama, Paul J Steinhardt
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  ABSTRACT: We show that a gravitational interaction between the derivative of the Ricci scalar curvature and the baryon-number current dynamically breaks CPT in an expanding Universe and, combined with baryon-number-violating interactions, can drive the Universe towards an equilibrium baryon asymmetry that is observationally acceptable.
  Physical Review Letters 12/2004; 93(20):201301. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Viable Supersymmetry and Leptogenesis with Anomaly Mediation

  Masahiro Ibe, Ryuichiro Kitano, Hitoshi Murayama, Tsutomu Yanagida
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  ABSTRACT: The seesaw mechanism that explains the small neutrino masses comes naturally with supersymmetric (SUSY) grand unification and leptogenesis. However, the framework suffers from the SUSY flavor and CP problems, and has a severe cosmological gravitino problem. We propose anomaly mediation as a simple solution to all these problems, which is viable once supplemented by the D-terms for U(1)_Y and U(1)_{B-L}. Even though the right-handed neutrino mass explicitly breaks U(1)_{B-L} and hence reintroduces the flavor problem, we show that it lacks the logarithmic enhancement and poses no threat to the framework. The thermal leptogenesis is then made easily consistent with the gravitino constraint. Comment: 5 pages, one figure, uses Revtex4; Discussion on the upper bound on the LSP mass added. The version published in PRD
  03/2004;
• Source

  Available from: arxiv.org

  Article: Electroweak Symmetry Breaking via UV Insensitive Anomaly Mediation

  Ryuichiro Kitano, Graham D. Kribs, Hitoshi Murayama
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  ABSTRACT: Anomaly mediation solves the supersymmetric flavor and CP problems. This is because the superconformal anomaly dictates that supersymmetry breaking is transmitted through nearly flavor-blind infrared physics that is highly predictive and UV insensitive. Slepton mass squareds, however, are predicted to be negative. This can be solved by adding D-terms for U(1)_Y and U(1)_{B-L} while retaining the UV insensitivity. In this paper we consider electroweak symmetry breaking via UV insensitive anomaly mediation in several models. For the MSSM we find a stable vacuum when tanbeta < 1, but in this region the top Yukawa coupling blows up only slightly above the supersymmetry breaking scale. For the NMSSM, we find a stable electroweak breaking vacuum but with a chargino that is too light. Replacing the cubic singlet term in the NMSSM superpotential with a term linear in the singlet we find a stable vacuum and viable spectrum. Most of the parameter region with correct vacua requires a large superpotential coupling, precisely what is expected in the ``Fat Higgs'' model in which the superpotential is generated dynamically. We have therefore found the first viable UV complete, UV insensitive supersymmetry breaking model that solves the flavor and CP problems automatically: the Fat Higgs model with UV insensitive anomaly mediation. Moreover, the cosmological gravitino problem is naturally solved, opening up the possibility of realistic thermal leptogenesis. Comment: 27 pages, 3 figures, 1 table
  02/2004;
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  Available from: arxiv.org

  Article: Lepton-flavor mixing and $K \to \pi \nu\bar\nu$ decays

  Yuval Grossman, Gino Isidori, Hitoshi Murayama
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  ABSTRACT: The impact of possible sources of lepton-flavor mixing on $K \to \pi \nu\bar\nu$ decays is analysed. At the one-loop level lepton-flavor mixing originated from non-diagonal lepton mass matrices cannot generate a CP-conserving $K_L \to \pi^0\nu\bar\nu$ amplitude. The rates of these modes are sensitive to leptonic flavor violation when there are at least two different leptonic mixing matrices. New interactions that violate both quark and lepton universalities could enhance the CP-conserving component of $\Gamma(K_L \to \pi^0\nu\bar\nu)$ and have a substantial impact. Explicit examples of these effects in the context of supersymmetric models, with and without R-parity conservation, are discussed. Comment: 10 pages, 3 figures
  11/2003;
• Source

  Available from: ArXiv

  Article: Unified origin of baryons and dark matter

  Ryuichiro Kitano, Hitoshi Murayama, Michael Ratz
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  ABSTRACT: We investigate the possibility that both the baryon asymmetry of the universe and the observed cold dark matter density are generated by decays of a heavy scalar field which dominates the universe before nucleosynthesis. Since baryons and cold dark matter have common origin, this mechanism yields a natural explanation of the similarity of the corresponding energy densities. The cosmological moduli and gravitino problems are avoided.
  Physics Letters B.
• Source

  Available from: arxiv.org

  Article: The new Minimal Standard Model

  Hooman Davoudiasl, Ryuichiro Kitano, Tianjun Li, Hitoshi Murayama
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  ABSTRACT: We construct the new Minimal Standard Model that incorporates the new discoveries of physics beyond the Minimal Standard Model (MSM): dark energy, non-baryonic dark matter, neutrino masses, as well as baryon asymmetry and cosmic inflation, adopting the principle of minimal particle content and the most general renormalizable Lagrangian. We base the model purely on empirical facts rather than aesthetics. We need only six new degrees of freedom beyond the MSM. It is free from excessive flavor-changing effects, CP violation, too-rapid proton decay, problems with electroweak precision data, and unwanted cosmological relics. Any model of physics beyond the MSM should be measured against the phenomenological success of this model.
  Physics Letters B.
• Article: Lepton–flavor mixing and K→πνν̄ decays

  Yuval Grossman, Gino Isidori, Hitoshi Murayama
  [show abstract] [hide abstract]
  ABSTRACT: The impact of possible sources of lepton–flavor mixing on decays is analysed. At the one-loop level lepton–flavor mixing originated from non-diagonal lepton mass matrices cannot generate a CP-conserving amplitude. The rates of these modes are sensitive to leptonic flavor violation when there are at least two different leptonic mixing matrices. New interactions that violate both quark and lepton universalities could enhance the CP-conserving component of and have a substantial impact. Explicit examples of these effects in the context of supersymmetric models, with and without R-parity conservation, are discussed.
  Physics Letters B. 588:74-80.