C. Q. Geng

Axion-photon couplings in invisible axion models.

We reexamine the axion-photon couplings in various invisible axion models motivated by the recent proposal of using optical interferometry at the ASST facility at the SSCL to search for the axion. We illustrate that the assignment of U(1){sub PQ} charges for the fermion fields plays an important role in determining the couplings. Several simple nonminimal invisible axion models with suppressed and enhanced axion-photon couplings are constructed, respectively. We also discuss the implications of possible new experiments to detect solar axions by conversion to x rays in a static magnetic apparatus tracking the Sun.

Testing radiative neutrino mass generation at the LHC

We investigate in detail a model that contains an additional SU(2) singlet and triplet scalar fields than the Standard Model (SM). This allows the radiative generation of Majorana neutrino masses at two-loop order with the help of doubly charged Higgs bosons that arise from the extended Higgs sector. The phenomenology of the Higgs and neutrino sectors of the model is studied. We give the analytical form of the masses of scalar and pseudoscalar bosons and their mixings, and the structure of the active neutrino mass matrix. It is found that the model accommodates only normal neutrino mass hierarchy, and that there is a large parameter space where the doubly charged Higgs can be observed at the Large Hadron Collider (LHC), thereby making it testable at the LHC. Furthermore, the neutrino-less double beta (0νββ) decays arise predominantly from exchange processes involving the doubly charged Higgs, whose existence is thus unmistakable if 0νββ decays are observed. The production and decays of the doubly charged Higgs are analyzed, and distinct and distinguishing signals are discussed.

Unconventional neutrino mass generation, neutrinoless double beta decays, and collider phenomenology

We study a model in which lepton number violation is solely triggered by a dimension 4 hard breaking term in the scalar potential. A minimal model, which contains a SU(2) triplet with hypercharge Y=2 and a pair of singlet doubly charged scalar fields in addition to the standard model (SM) Higgs doublet, is constructed. The model is technically natural in the sense that lepton number is preserved in the limit that the hard term vanishes. SM phenomenology restricts the vacuum expectation value of the triplet scalar field v{sub T}<5.78 GeV. Neutrino masses controlled by v{sub T} are generated at the two loop level and are naturally in the sub-eV range. In general they exhibit normal hierarchy structure. Here the neutrino mass term does not dominate neutrinoless double beta decays of nuclei. Instead the short-distance physics with doubly charged Higgs exchange gives the leading contribution. We expect weak scale singly and doubly charged Higgs bosons to make their appearances at the LHC and the ILC.

Dark Matter in Inert Triplet Models

We study the inert triplet models, in which the standard model is extended to have a new SU(2){sub L} triplet scalar (Y=0 or 2) with an Z{sub 2} symmetry. We show that the neutral component of the triplet can be a good dark matter candidate. In particular, for the hypercharge Y=0 triplet model, the WMAP data favors the region where the dark matter mass is around 5.5 TeV, which is also consistent with the direct detection experiments. In contrast, for the Y=2 model, although dark matter with its mass around 2.8 TeV is allowed by WMAP, it is excluded by the direct detection experiments because the spin-independent cross section is enhanced by the Z-mediated tree-level scattering process.

Probing new physics in B ---> K(*) l+ l- decays

We study the exclusive decays of

Radiative leptonic B decays in the light front model

B\to K^{(*)}\ell^+ \ell^-

Lepton polarization asymmetry in B-->K(*)l+l-

by the results in the perturbative QCD with the heavy quark effective theory and lattice calculations. We obtain the form factors for the

T -violating muon polarization in K + → μ + ν γ

B\to K^{(*)}

Study of B-s,B-d -> l(+)l(-)gamma decays

transitions in the whole allowed region. Our predictions on the branching ratios of

Charmful three-body baryonic B decays

B\to K \ell^{+} \ell^{-}