arXiv subjects

Natsumi Nagata

Publications and source records attributed to Natsumi Nagata.

104 records · Page 6Linked to original sources

arXiv2014

QCD Corrections to Quark (Chromo)Electric Dipole Moments in High-scale Supersymmetry

Recent results from the LHC experiments, both for the Higgs mass measurement and the direct search for supersymmetric (SUSY) particles, might indicate that the SUSY breaking scale is much higher than the electroweak scale. Although it is difficult to investigate such a scenario at collider experiments, the measurement of the hadronic electric dipole moments is one of promising ways to detect the effects of the SUSY particles. These effects are expressed in terms of the CP-violating effective operators defined at the SUSY breaking scale, which involve quarks, gluons, photons, and gluinos. In this paper, we discuss the QCD corrections to the effective operators in the high-scale SUSY scenario. To appropriately evaluate the radiative corrections in the presence of large mass hierarchy among the SUSY particles, we exploit an effective theoretical approach based on the renormalization-group equations. As a result, it is found that the low-energy quark electric and chromoelectric dipole moments may differ from those evaluated in previous works by O(100) % and O(10) %, respectively.

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arXiv2013

Grand Unification in High-scale Supersymmetry

A constraint on masses of superheavy gauge and Higgs multiplets at the grand unification (GUT) scale is obtained from the gauge coupling unification in the case of high-scale supersymmetry. We found that all of the particles may lie around a scale of 10^16 GeV so that the threshold corrections to the gauge coupling constants at the GUT scale are smaller than those in the case of the low-energy supersymmetry. In addition, the GUT scale tends to be slightly lower when the gauginos are heavier and, thus, the proton decay rate via the X-boson exchange process is expected to be enhanced.

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arXiv2013

Decoupling Can Revive Minimal Supersymmetric SU(5)

We revisit proton decay via the color-triplet Higgs multiplets in the minimal supersymmetric grand unified model with heavy sfermions. Although the model has been believed to be excluded due to the too short lifetime of proton, we have found that it is possible to evade the experimental constraints on the proton decay rate if the supersymmetric particles have masses much heavier than the electroweak scale. With such heavy sfermions, the 126GeV Higgs boson is naturally explained, while they do not spoil the gauge coupling unification and the existence of dark matter candidates. Since the resultant proton lifetime lies in the regions which may be reached in the future experiments, proton decay searches may give us a chance to verify the scenario as well as the supersymmetric grand unified models.

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arXiv2013

Two-loop Renormalization Factors of Dimension-six Proton Decay Operators in the Supersymmetric Standard Models

The renormalization factors of the dimension-six effective operators for proton decay are evaluated at two-loop level in the supersymmetric grand unified theories. For this purpose, we use the previous results in which the quantum corrections to the effective Kahler potential are evaluated at two-loop level. Numerical values for the factors are presented in the case of the minimal supersymmetric SU(5) grand unified model. We also derive a simple formula for the one-loop renormalization factors for any higher-dimensional operators in the Kahler potential, assuming that they are induced by the gauge interactions.

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arXiv2013

Neutron Electric Dipole Moment Induced by the Strangeness Revisited

We have revisited the calculation of the neutron electric dipole moment in the presence of the CP-violating operators up to dimension five based on the chiral perturbation theory. Especially, we focus on the contribution of strangeness content. In the calculation, we extract the nucleon matrix elements of scalar-type quark operators from the results of the lattice QCD simulations, while those of the dipole-type quark-gluon operators are evaluated by using the method of the QCD sum rules. As a result, it is found that although the strangeness quantity in nucleon is small, the contribution of the chromoelectric dipole moment of strange quark may be still sizable, and thus may offer a sensitive probe for the CP-violating interactions in physics beyond the Standard Model.

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arXiv2013

Direct Search of Dark Matter in High-Scale Supersymmetry

We study direct detection of dark matter in supersymmetric (SUSY) model where most of SUSY particles have very high-scale masses beyond the weak scale. In the scenario, Wino-like or Higgsino-like neutralino is a good candidate for the dark matter in the universe. The neutralino scatters off nuclei by a Higgs-boson exchange diagram and also electroweak loop diagrams. It is found that the elastic-scattering cross section with nuclei is enhanced or suppressed due to constructive or deconstructive interference among the diagrams. Such a cross section is within the reach of future experiment in some parameter region.

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arXiv2012

Enhancement of Proton Decay Rates in Supersymmetric SU(5) Grand Unified Models

In the supersymmetric grand unified theories (SUSY GUTs), gauge bosons associated with the unified gauge group induce proton decay. We investigate the proton decay rate via the gauge bosons in the SUSY GUTs under the two situations; one is with extra vector-like multiplets, and the other is with heavy sfermions. It is found that the proton lifetime is significantly reduced in the former case, while in the latter case it is slightly prolonged. Determination of the particle contents and their mass spectrum below the GUT scale is important to predict the proton lifetime. The proton decay searches have started to access to the 10^16 GeV scale.

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arXiv2012

A calculation for vector dark matter direct detection

We evaluate the elastic scattering cross section of vector dark matter with nucleon based on the method of effective field theory. The dark matter is assumed to behave as a vector particle under the Lorentz transformation and to interact with colored particles including quarks in the Standard Model. After formulating general formulae for the scattering cross sections, we apply them to the case of the first Kaluza-Klein photon dark matter in the minimal universal extra dimension model. The resultant cross sections are found to be larger than those calculated in previous literature.

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arXiv2012

Reevaluation of Neutron Electric Dipole Moment with QCD Sum Rules

We study the neutron electric dipole moment in the presence of the CP-violating operators up to the dimension five in terms of the QCD sum rules. It is found that the OPE calculation is robust when exploiting a particular interpolating field for neutron, while there exist some uncertainties on the phenomenological side. By using input parameters obtained from the lattice calculation, we derive a conservative limit for the contributions of the CP violating operators. We also show the detail of the derivation of the sum rules.

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arXiv2012

Natural Supersymmetry and b -> s gamma constraints

We investigate constraints from the observed branching ratio for b -> s gamma and fine-tuning in the framework of natural supersymmetry. The natural supersymmetry requires the large trilinear coupling of the stop sector, light higgsinos (a small mu parameter) and light stops, in order to reduce the fine-tuning in the Higgs sector while avoiding the LEP constraint. It is found that in such a scenario 5% (10%) level of fine-tuning is inevitable due to the b -> s gamma constraint even if the messenger scale is as low as 10^5 GeV (10^4 GeV), provided that the gaugino masses satisfy the GUT relation.

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arXiv2011

Direct Detection of Dark Matter Degenerate with Colored Particles in Mass

In this Letter we explore the direct detection of the dark matter in the universe, assuming the dark matter particles are degenerate in mass with new colored particles below TeV scale. The scenario with such a mass spectrum is difficult to be confirmed or excluded by the present analysis at the LHC experiments because the QCD jets in the cascade decay of the new particles produced in the proton-proton collision are too soft to be triggered in the event selection. It is shown that both of the spin-independent and spin-dependent couplings of the dark matter with a nucleon are enhanced and the scattering cross section may reach even the current bound of the direct detection experiments. Then such a degenerate scenario may be tested in the direct detection experiments.

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arXiv2011

Direct Detection of Electroweak-Interacting Dark Matter

Assuming that the lightest neutral component in an SU(2)L gauge multiplet is the main ingredient of dark matter in the universe, we calculate the elastic scattering cross section of the dark matter with nucleon, which is an important quantity for the direct detection experiments. When the dark matter is a real scalar or a Majorana fermion which has only electroweak gauge interactions, the scattering with quarks and gluon are induced through one- and two-loop quantum processes, respectively, and both of them give rise to comparable contributions to the elastic scattering cross section. We evaluate all of the contributions at the leading order and find that there is an accidental cancellation among them. As a result, the spin-independent cross section is found to be O(10^-(46-48)) cm^2, which is far below the current experimental bounds.

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arXiv2011

Gluon contribution to the dark matter direct detection

In this article we have calculated the spin-independent cross section of nucleon-dark matter scattering process at loop level, which is relevant to dark matter direct detection. Paying particular attention to the scattering of gluon with dark matter, which contributes as leading order in the perturbation, we have systematically evaluated loop diagrams with tracking the characteristic loop momentum which dominates in the loops. Here loop diagrams whose typical loop momentum scales are the masses of quarks and other heavier particles are separately presented. Then, we have properly taken into account each contribution to give the cross section. We assume that the dark matter is pure bino or wino in the supersymmetric models. The application to other models is straightforward.

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arXiv2010

A complete calculation for direct detection of Wino dark matter

In the anomaly-mediated supersymmetry (SUSY) breaking scenario, neutral gaugino of SU(2)_L multiplet, Wino, can be the lightest SUSY particle and become a candidate for dark matter. We calculated scattering cross section of Wino dark matter with nucleon, which is responsible for direct detection of the dark matter, on the assumption that the SUSY particles and the heavier Higgs bosons have masses of the order of the gravitino mass in the SUSY standard model. In such a case, the Wino-nucleon coupling is generated by loop processes. We have included two-loop contribution to Wino-gluon interaction in the calculation, since it is one of the leading contributions to the Wino-nucleon coupling. It was found that the spin-independent scattering cross section with proton is 10^-(46-48) cm^2. While it is almost independent of the Wino mass, the result is quite sensitive to the Higgs boson mass due to the accidental cancellation.

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