Keiko I. Nagao

Simplified models for dark matter interacting with quarks

A bstractWe investigate simplified models in which dark matter particles, taken to be either Dirac or Majorana fermions, couple to quarks via colored mediators. We determine bounds from colliders and direct detection experiments, and show how the interplay of the two leads to a complementary view of this class of dark matter models. Forecasts for future searches in light of the current constraints are presented.

LHC signature of supersymmetric models with non-universal sfermion masses

We study the LHC signature of the minimal supersymmetric standard model with non-universal sfermion masses. In the model, soft masses of gauginos and the 3rd generation of 10 of SU(5) are around the weak scale, while other sfermion soft mass is universal and around a few TeV. Such sfermion mass spectrum is motivated not only from flavor, CP and naturalness constraints but also from E6 grand unified model with non-Abelian horizontal (flavor) symmetry. The characteristic signature of the model at the LHC is the dominance of the events with 4 b partons in the final state together with high rate of mildly boosted top quark arising from gluino decay. The prominent high pT jet also arises from squark decay. We show it is possible to find the characteristic signature in the early stage of the LHC. The discrimination of our scenario from some CMSSM model points with similar signature may be possible with large integrated luminosity. The result of sparticle mass measurement using exclusive channel with the help of hemisphere analysis, and inclusive measurement of gluino and squark masses using MT2 and MT2min in some representative model points are presented.

Reconstruction of Higgs bosons in the di-tau channel via 3-prong decay

A bstractWe propose a method for reconstructing the mass of a particle, such as the Higgs boson, decaying into a pair of τ leptons, of which one subsequently undergoes a 3-prong decay. The kinematics is solved using information from the visible decay products, the missing transverse momentum, and the 3-prong τ decay vertex, with the detector resolution taken into account using a likelihood method. The method is shown to give good discrimination between a 125 GeV Higgs boson signal and the dominant backgrounds, such as Z0 decays to ττ and W± plus jets production. As a result, we find an improvement, compared to existing methods for this channel, in the discovery potential, as well as in measurements of the Higgs boson mass and production cross section times branching ratio.

Isospin-violating dark matter search by nuclear emulsion detector

Dark matter signal and its annual modulation of event number are observed by some direct searches in small mass region. However, the regions have been excluded by others. The isospin-violating dark matter is a hopeful candidate to explain the discrepancy. We study the possibility that a future project of dark matter search using nuclear emulsion can reach favored region by the isospin-violating dark matter. Since the detector has the directional sensitivity, it is expected to examine the region including the modulation property.

Neutralino dark matter in the minimal supersymmetric standard model with natural light Higgs sector

We study the neutralino relic density in the minimal supersymmetric standard model with natural light Higgs sector in which all Higgs masses, the supersymmetry (SUSY) breaking parameters, and the Higgsino mass parameter {mu} are of order the weak scale. To realize this situation we adopt nonuniversal Higgs masses at the grand unified scale. We show that in some parameter space in which the SUSY breaking parameters are comparatively small, not only the constraint from the observed relic density of dark matter but also the LEP Higgs bound and the constraint from the b{yields}s{gamma} process are satisfied. In most of the parameter space, the neutralino relic density becomes smaller than the observed relic density in contrast with the results in the constrained minimal SUSY standard model (CMSSM). The reason is that the neutralino coannihilation processes to Higgs bosons open even if the gaugino mass is small and the cross sections become large due to the small dimensionful parameters. Especially the small {mu} parameter and the light CP-odd Higgs, which are difficult to be realized in the CMSSM, are essential for the result.

Directional search for isospin-violating dark matter with nuclear emulsion

Some of direct dark matter searches reported not only positive signals but also annual modulation of the signal event. However, the parameter spaces have been excluded by other experiments. Isospin violating dark matter solves the contradiction by supposing different coupling to proton and neutron. We study the possibility to test the favored parameter region by isospin violating dark matter model with the future detector of dark matter using the nuclear emulsion. Since the nuclear emulsion detector has directional sensitivity, the detector is expected to examine whether the annual modulations observed other experiments is caused by dark matter or background signals.

Directional Search for Isospin-Violating Dark Matter with Nuclear Emulsion

Some of direct dark matter searches reported not only positive signals but also annual modulation of the signal event. However, the parameter spaces have been excluded by other experiments. Isospin violating dark matter solves the contradiction by supposing different coupling to proton and neutron. We study the possibility to test the favored parameter region by isospin violating dark matter model with the future detector of dark matter using the nuclear emulsion. Since the nuclear emulsion detector has directional sensitivity, the detector is expected to examine whether the annual modulations observed other experiments is caused by dark matter or background signals.