arXiv subjects

Yuhsin Tsai

Publications and source records attributed to Yuhsin Tsai.

80 records · Page 5Linked to original sources

arXiv2012

KeV Warm Dark Matter and Composite Neutrinos

Elementary keV sterile Dirac neutrinos can be a natural ingredient of the composite neutrino scenario. For a certain class of composite neutrino theories, these sterile neutrinos naturally have the appropriate mixing angles to be resonantly produced warm dark matter (WDM). Alternatively, we show these sterile neutrinos can be WDM produced by an entropy-diluted thermal freeze-out, with the necessary entropy production arising not from an out-of-equilibrium decay, but rather from the confinement of the composite neutrino sector, provided there is sufficient supercooling.

hep-ph↗

arXiv2012

The birds and the Bs in RS: the b to s gamma penguin in a warped extra dimension

We calculate contributions to the photon and gluon magnetic dipole operators that mediate b -> s gamma and b -> d gamma transitions in the Randall-Sundrum model of a warped extra dimension with anarchic bulk fermions and a brane localized Higgs. Unlike the Standard Model, there are large contributions to the left-handed b quark decays, parameterized by the Wilson coefficient C'_7, due to the pattern of bulk fermion localization, and sizable contributions from the gluonic penguins, C(')_8, through renormalization group mixing. Further, unlike the Randall-Sundrum result for mu -> e gamma, the unprimed Wilson coefficients receive non-negligible contributions from the misalignment of the bulk fermion spectrum with the Standard Model flavor sector. We compare the size of effects and the constraints imposed by the branching ratios Br(B -> X_s gamma) and X_d gamma)> within the minimal and the custodial model. Within the custodial framework, we study the effect on a number of benchmark observables and find that Br(B -> X_s mu^+ mu^-) and the forward-backward asymmetry in B -> K^* mu^+ mu^- remain close to their Standard Model predictions. On the other hand, there can be large enhancements of the time-dependent CP asymmetry in B -> K^* gamma and the transverse asymmetry A_T^(2).

hep-ph↗

arXiv2012

Electroweak Baryogenesis in R-symmetric Supersymmetry

We demonstrate that electroweak baryogenesis can occur in a supersymmetric model with an exact R-symmetry. The minimal R-symmetric supersymmetric model contains chiral superfields in the adjoint representation, giving Dirac gaugino masses, and an additional set of "R-partner" Higgs superfields, giving R-symmetric μ-terms. New superpotential couplings between the adjoints and the Higgs fields can simultaneously increase the strength of the electroweak phase transition and provide additional tree-level contributions to the lightest Higgs mass. Notably, no light stop is present in this framework, and in fact, we require both stops to be above a few TeV to provide sufficient radiative corrections to the lightest Higgs mass to bring it up to 125 GeV. Large CP-violating phases in the gaugino/higgsino sector allow us to match the baryon asymmetry of the Universe with no constraints from electric dipole moments due to R-symmetry. We briefly discuss some of the more interesting phenomenology, particularly of the of the lightest CP-odd scalar.

hep-ph↗

arXiv2012

Warped Penguins

We present an analysis of the loop-induced magnetic dipole operator in the Randall-Sundrum model of a warped extra dimension with anarchic bulk fermions and an IR brane localized Higgs. These operators are finite at one-loop order and we explicitly calculate the branching ratio for mu to e gamma using the mixed position/momentum space formalism. The particular bound on the anarchic Yukawa and KK scales can depend on the flavor structure of the anarchic matrices. It is possible for a generic model to either be ruled out or unaffected by these bounds without any fine tuning. We quantify how these models realize this surprising behavior. We also review tree-level lepton flavor bounds in these models and show that these are are in mild tension with the penguin bounds from typical models with a 3 TeV Kaluza-Klein scale. Further, we illuminate the nature of the one-loop finiteness of these diagrams and show how to accurately determine the degree of divergence of a 5D loop diagram using both the 5D and KK formalism. This power counting can be obfuscated in the 4D Kaluza-Klein formalism and we explicitly point out subtleties that ensure that the two formalisms agree. Finally, we remark on the existence of a perturbative regime in which these one-loop results give the dominant contribution.

hep-ph↗

arXiv2012

Spontaneous R-symmetry Breaking with Multiple Pseudomoduli

We examine generalized O'Raifeartaigh models that feature multiple tree-level flat directions and only contain fields with R-charges 0 or 2. We show that spontaneous R-breaking at up to one-loop order is impossible in such theories. Specifically, we prove that the R-symmetric origin of field space is always a local minimum of the one-loop Coleman-Weinberg potential, generalizing an earlier result for the case of a single flat direction. This result has consequences for phenomenology and helps elucidate the behavior of various models of dynamical SUSY breaking.

hep-th↗

arXiv2011

Singlet-Stabilized Minimal Gauge Mediation

We propose Singlet Stabilized Minimal Gauge Mediation as a simple ISS-based model of Direct Gauge Mediation which avoids both light gauginos and Landau poles. The hidden sector is a massive s-confining SQCD that is distinguished by a minimal SU(5) flavor group. The uplifted vacuum is stabilized by coupling the meson to an additional singlet sector with its own U(1) gauge symmetry via non-renormalizable interactions suppressed by a higher scale Lambda_UV in the electric theory. This generates a nonzero VEV for the singlet meson via the inverted hierarchy mechanism, but requires tuning to a precision ~ (Lambda/Lambda_UV)^2, which is ~ 10^{-4}. In the course of this analysis we also outline some simple model-building rules for stabilizing uplifted ISS models, which lead us to conclude that meson deformations are required (or at least heavily favored) to stabilize the adjoint component of the magnetic meson.

hep-th↗

arXiv2011

LEP Shines Light on Dark Matter

Dark matter pair production at high energy colliders may leave observable signatures in the energy and momentum spectra of the objects recoiling against the dark matter. We use LEP data on mono-photon events with large missing energy to constrain the coupling of dark matter to electrons. Within a large class of models, our limits are complementary to and competitive with limits on dark matter annihilation and on WIMP-nucleon scattering from indirect and direct searches. Our limits, however, do not suffer from systematic and astrophysical uncertainties associated with direct and indirect limits. For example, we are able to rule out light (< 10 GeV) thermal relic dark matter with universal couplings exclusively to charged leptons. In addition, for dark matter mass below about 80 GeV, LEP limits are stronger than Fermi constraints on annihilation into charged leptons in dwarf spheroidal galaxies. Within its kinematic reach, LEP also provides the strongest constraints on the spin-dependent direct detection cross section in models with universal couplings to both quarks and leptons. In such models the strongest limit is also set on spin independent scattering for dark matter masses below ~4 GeV. Throughout our discussion, we consider both low energy effective theories of dark matter, as well as several motivated renormalizable scenarios involving light mediators.

hep-ph↗

arXiv2008

Leptogenesis with Composite Neutrinos

Models with composite singlet neutrinos can give small Majorana or Dirac masses to the active neutrinos. The mechanism is based on the fact that conserved chiral symmetries give massless neutrinos at the renormalizable level. Thus, they acquire very small masses due to non-renormalizable terms. We investigate such models in two aspects. First, we find UV completions for them and then we investigate the possibility of giving leptogenesis. We find that these models offer new possibilities for leptogenesis. Models with Majorana masses can exhibit standard leptogenesis. Models with Dirac masses can provide a realization of Dirac type leptogenesis with mass scale that can be as low as 10 TeV.

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