arXiv subjects

Mitsuru Kakizaki

Publications and source records attributed to Mitsuru Kakizaki.

36 records · Page 2Linked to original sources

arXiv2012

Vector WIMP Miracle

Weakly interacting massive particle (WIMP) is well known to be a good candidate for dark matter, and it is also predicted by many new physics models beyond the standard model at the TeV scale. We found that, if the WIMP is a vector particle (spin one particle) which is associated with some gauge symmetry broken at the TeV scale, the higgs mass is often predicted to be 120--125 GeV, which is very consistent with the result of higgs searches recently reported by ATLAS and CMS collaborations at the Large Hadron Collider experiment. In this letter, we consider the vector WIMP using a non-linear sigma model in order to confirm this result as general as possible in a bottom-up approach. Near-future prospects to detect the vector WIMP at both direct and indirect detection experiments of dark matter are also discussed.

hep-ph↗

arXiv2011

Thermal abundance of non-relativistic relics with Sommerfeld enhancement

We propose an analytic treatment for computing the relic abundances of non-relativistic particles whose annihilation rate at chemical decoupling is increased by Sommerfeld enhancement. We find approximate rational functions that closely fit the thermal average of Sommerfeld-enhanced cross sections in the massless limit of force carriers for s- and p-wave annihilations. We demonstrate that, with the approximate thermally-averaged cross sections implemented, the standard analytic method for the final relic abundances provides accuracy to within 1% even for the case of Sommerfeld enhancement.

astro-ph.CO↗

arXiv2009

The Thermal Abundance of Semi-Relativistic Relics

Approximate analytical solutions of the Boltzmann equation for particles that are either extremely relativistic or non-relativistic when they decouple from the thermal bath are well established. However, no analytical formula for the relic density of particles that are semi-relativistic at decoupling is yet known. We propose a new ansatz for the thermal average of the annihilation cross sections for such particles, and find a semi-analytical treatment for calculating their relic densities. As examples, we consider Majorana- and Dirac-type neutrinos. We show that such semi-relativistic relics cannot be good cold Dark Matter candidates. However, late decays of meta-stable semi-relativistic relics might have released a large amount of entropy, thereby diluting the density of other, unwanted relics.

hep-ph↗

arXiv2007

Constraints on the Very Early Universe from Thermal WIMP Dark Matter

We investigate the relic density n_χof non-relativistic long-lived or stable particles χin non-standard cosmological scenarios. We calculate the relic abundance starting from arbitrary initial temperatures of the radiation-dominated epoch, and derive the lower bound on the initial temperature T_0 \geq m_χ/23, assuming that thermally produced χparticles account for the dark matter energy density in the universe; this bound holds for all χannihilation cross sections. We also investigate cosmological scenarios with modified expansion rate. Even in this case an approximate formula similar to the standard one is capable of predicting the final relic abundance correctly. Choosing the χannihilation cross section such that the observed cold dark matter abundance is reproduced in standard cosmology, we constrain possible modifications of the expansion rate at T \sim m_χ/20, well before Big Bang nucleosynthesis.

hep-ph↗

arXiv2006

Relic abundance of dark matter in the minimal universal extra dimension model

We investigate the relic abundance of dark matter in the minimal universal extra dimension model including resonance processes by second Kaluza-Klein (KK) particles in all coannihilation processes. After including second KK resonance processes, the relic abundance of dark matter is reduced by about 30%. Thus, the compactification scale 1/R of the extra dimension consistent with the WMAP observation is increased by a few hundred GeV. As a result, the cosmologically allowed compactification scale is 600 GeV < 1/R < 1400 GeV for Lambda R = 20.

hep-ph↗

arXiv2006

Abundance of Cosmological Relics in Low-Temperature Scenarios

We investigate the relic density n_χof non-relativistic long-lived or stable particles χin cosmological scenarios in which the temperature T is too low for χto achieve full chemical equilibrium. The case with a heavier particle decaying into χis also investigated. We derive approximate solutions for n_χ(T) which accurately reproduce numerical results when full thermal equilibrium is not achieved. If full equilibrium is reached, our ansatz no longer reproduces the correct temperature dependence of the χnumber density. However, it does give the correct final relic density, to an accuracy of about 3% or better, for all cross sections and initial temperatures.

hep-ph↗

arXiv2005

Hadronic EDM Constraints on Orbifold GUTs

We point out that the null results of the hadronic electric dipole moment (EDM) searches constrain orbifold grand unified theories (GUTs), where the GUT symmetry and supersymmetry (SUSY) are both broken by boundary conditions in extra dimensions and it leads to rich fermion and sfermion flavor structures. A marginal chromoelectric dipole moment (CEDM) of the up quark is induced by the misalignment between the CP violating left- and right-handed up-type squark mixings, in contrast to the conventional four-dimensional SUSY GUTs. The up quark CEDM constraint is found to be as strong as those from charged lepton flavor violation (LFV) searches. The interplay between future EDM and LFV experiments will probe the structures of the GUTs and the SUSY breaking mediation mechanism.

hep-ph↗

arXiv2005

Relic Abundance of LKP Dark Matter in UED model including Effects of Second KK Resonances

We reevaluate the thermal relic density of the Kaluza-Klein (KK) dark matter in universal extra dimension models. In particular, we consider the effect of the resonance caused by second KK particles on the density. We find that the annihilation cross sections relevant to the density are significantly enhanced due to the resonance when the Higgs boson mass is large enough (m_h \gtrsim 200 GeV). As a result, the mass of the dark matter particle consistent with the WMAP observation is increased compared to the result which does not include any resonance.

hep-ph↗

arXiv2005

Significant effects of second KK particles on LKP dark matter physics

We point out that Kaluza-Klein (KK) dark matter physics is drastically affected by second KK particles. In this work various interesting phenomena caused by the second KK modes are discussed. In particular, we reevaluate the annihilation cross section and thermal relic density of the KK dark matter quantitatively in universal extra dimensions, in which all the standard model particles propagate. In these models, the first KK mode of $B$ boson is a viable dark matter candidate by virtue of KK-parity. We demonstrate that the KK dark matter annihilation cross section can be enhanced, compared with the tree level cross section mediated only by first KK particles. The dark matter mass consistent with the WMAP observation is increased.

hep-ph↗

arXiv2004

Hadronic EDMs in SUSY SU(5) GUTs with Right-handed Neutrinos

We discuss hadronic EDM constraints on the neutrino sector in the SUSY SU(5) GUT with the right-handed neutrinos. The hadronic EDMs are sensitive to the right-handed down-type squark mixings, especially between the second and third generations and between the first and third ones, compared with the other low-energy observables, and the flavor mixings are induced by the neutrino Yukawa interaction. The current experimental bound of the neutron EDM may imply that the right-handed tau neutrino mass is smaller than about 10^{14} GeV in the minimal supergravity scenario, and it may be improved furthermore in future experiments, such as the deuteron EDM measurement.

hep-ph↗

arXiv2004

Proton Decay, Fermion Masses and Texture from Extra Dimensions in SUSY GUTs

In supersymmetry, there are gauge invariant dimension 5 proton decay operators which must be suppressed by a mass scale much larger than the Planck mass. It is natural to expect that this suppression should be explained by a mechanism that explains the hierarchical structure of the fermion mass matrices. We apply this argument to the case where wave functions of chiral multiplets are localized under a kink background along an extra spatial dimension and the Yukawa couplings as well as the coefficients of the proton decay operators are determined by the overlap of the relevant wave functions. A configuration is found in the context of SU(5) supersymmetric grand unified theory that yields realistic values of quark masses, mixing angles, CP phase and charged lepton masses and sufficiently small genuine dimension 5 proton decay operators. Inclusion of SU(5) breaking effects is essential in order to obtain non-vanishing CP phase as well as correct lepton masses. The resulting mass matrix has a texture structure in which texture zeros are a consequence of extremely small overlap of the wave functions. Our approach requires explicit breaking of supersymmetry in the extra dimension, which can be realized in (de)constructing extra dimension.

hep-ph↗

arXiv2004

Large Supersymmetric Contribution to CP Asymmetry of B_d \to ϕK_S from Left-Handed Squark Mixing

Supersymmetric contribution to the CP asymmetry of B_d \to ϕK_S is re-examined. It is emphasized that our analysis takes into account the recently found constraint from the mercury electric dipole moment, in addition to the well-known constraint from the branching ratio of b \to s γ. We show that, despite these constraints, the CP asymmetry can considerably deviate from the Standard Model prediction, if the CP-odd flavor mixing comes from left-handed squarks. Alignment mechanism of sfermion masses and mixing may imprint the required flavor mixing at high energy scale.

hep-ph↗

arXiv2003

New Constraint on Squark Flavor Mixing from 199Hg Electric Dipole Moment

A new constraint is obtained on the CP-violating flavor mixing between the left-handed top scalar quark (\tilde{t}_L) and charm scalar quark (\tilde{c}_L), by considering a chargino loop contribution to chromo-electric dipole moment of strange quark, which is limited by the electric dipole moment of the ^{199}Hg atom. It is found that the flavor mixing should be suppressed to the level of {\cal O}(0.1) for the CP phase of order unity, when \tan βis relatively large and sparticle masses lie in a few hundred GeV range. Although it is much stronger than the known constraint from the chargino loop contribution to b \to s γ, the moderate constraint we obtain here is argued to leave room for sizable supersymmetric contribution to the CP asymmetry in B_d^0 \to ϕK_s.

hep-ph↗

arXiv2003

Lepton flavor violation in the triplet Higgs model

The triplet Higgs model, which is an extension of the standard model with a weak-scale triplet Higgs boson, is capable of generating small neutrino masses naturally. We investigate lepton flavor violation mediated by the triplet Higgs boson. We stress that various neutrino mass patterns could be distinguished by measuring the lepton flavor violating processes. μ\to eee decay is significantly enhanced in the case of the degenerate masses or the inverted-hierarchical masses compared with that in the case of the hierarchical masses. On the other hand, the μ\to e γrate and the μ- e conversion ratio in nuclei is almost insensitive to the mass spectra. We also emphasize that these decay rates tend to increase as the magnitude of U_{e3} increases. Lepton flavor nonconserving τdecay modes are expected to be unobservable at planned experiments in the light of the current upper bounds of flavor violating muonic decay.

hep-ph↗

arXiv2003

Singular Kahler Potential and Heavy Top Quark in A Democratic Mass Matrix Model

It is pointed out that heavy top quark mass can be attributed to a singular normalization of its kinetic term, in which rescaling into canonical one yields large top Yukawa coupling. We pursue this novel possibility in a democratic mass matrix model where only the normalization of the third generation can be different from that of the other two generations. With diagonal breaking of democratic S_3 symmetry, we show that the singular normalization for the top quark is essential to reproduce observed quark masses and mixing angles. We also briefly argue other applications of this mechanism.

hep-ph↗

arXiv2002

Democratic (S)fermions and Lepton Flavor Violation

The democratic approach to account for fermion masses and mixing is known to be successful not only in the quark sector but also in the lepton sector. Here we extend this ansatz to supersymmetric standard models, in which the Kähler potential obeys underlying S_3 flavor symmetries. The requirement of neutrino bi-large mixing angles constrains the form of the Kähler potential for left-handed lepton multiplets. We find that right-handed sleptons can have non-degenerate masses and flavor mixing, while left-handed sleptons are argued to have universal and hence flavor-blind masses. This mass pattern is testable in future collider experiments when superparticle masses will be measured precisely. Lepton flavor violation arises in this scenario. In particular, μ\to e γis expected to be observed in a planning future experiment if supersymmetry breaking scale is close to the weak scale.

hep-ph↗

arXiv2002

U(1) Flavor Symmetry and Proton Decay in Supersymmetric Standard Model

Issues of proton decay are examined in supersymmetric standard model with U(1) flavor symmetry. Dimension five proton-decay operators which arise generically are controlled by the flavor symmetry. We show that unlike the minimal supersymmetric SU(5) case the proton decay modes containing charged lepton can have large branching ratios if the dimension five operators of left-handed type dominate. Measuring the branching ratio of the electron mode to the muon mode may reveal the mechanism of the neutrino mass generation. The case with vanishing charges for Higgs doublets marginally survives the present experimental bound on proton life time.

hep-ph↗

arXiv2001

Splitting Triplet and Doublet in Extra Dimensions

A novel mechanism to realize the triplet-doublet splitting in supersymmetric SU(5) grand unified theories is proposed in the framework of higher dimensional theories where chiral multiplets are localized due to kink configuration of a SU(5) singlet. An adjoint Higgs field which spontaneously breaks the SU(5) gauge symmetry is assumed to be involved with the localization process, splitting the wave functions of the color-triplet Higgs from its doublet counterpart. The resulting effective four-dimensional theory does not possess manifest SU(5) invariance. By adjusting couplings, the doublet mass can be exponentially suppressed. We also show that dimension 5 proton decay from triplet Higgs exchange can be suppressed to a negligible level.

hep-ph↗