Hye-Sung Lee

Z′ mediated flavor changing neutral currents in B meson decays

Abstract We study the effects of an extra U(1)′ gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on Bd→φK and Bd→η′K decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z′ contribution can readily account for the anomaly in SφKS but is not sufficient to explain large branching ratio of Bd→η′K with the same parameter value. SφKS and Sη′KS are seen to be the dominant observables that constrain the extra weak phase in the model.

Effective number of neutrinos and baryon asymmetry from BBN and WMAP

Abstract We place constraints on the number of relativistic degrees of freedom and on the baryon asymmetry at the epoch of Big Bang Nucleosynthesis (BBN) and at recombination, using cosmic background radiation (CBR) data from the Wilkinson Microwave Anisotropy Probe (WMAP), complemented by the Hubble Space Telescope (HST) Key Project measurement of the Hubble constant, along with the latest compilation of deuterium abundances and Hii region measurements of the primordial helium abundance. The agreement between the derived values of these key cosmological and particle physics parameters at these widely separated (in time or redshift) epochs is remarkable. From the combination of CBR and BBN data, we find the 2 σ ranges for the effective number of neutrinos N ν and for the baryon asymmetry (baryon to photon number ratio η ) to be 1.7–3.0 and 5.53–6.76×10 −10 , respectively.

Solution to the B → π K puzzle in a flavor-changing Z' model

Abstract Recent experiments suggest that certain B → π K branching ratios are inconsistent with the standard model expectations. We show that a flavor-changing Z ′ provides a solution to the problem. Electroweak penguin amplitudes are enhanced by the Z ′ boson for select parameters. We discuss implications for the Z ′ mass and its couplings to the standard model fermions. We also show that the solution is consistent with constraints from the CP asymmetries of the B → ϕ K S decay.

Lightest neutralino in extensions of the MSSM

Abstract We study neutralino sectors in extensions of the MSSM that dynamically generate the μ-term. The extra neutralino states are superpartners of the Higgs singlets and/or additional gauge bosons. The extended models may have distinct lightest neutralino properties which can have important influences on their phenomenology. We consider constraints on the lightest neutralino from LEP, Tevatron, and ( g − 2 ) μ measurements and the relic density of the dark matter. The lightest neutralino can be extremely light and/or dominated by its singlino component which does not couple directly to SM particles except Higgs doublets.

Revival of the Thermal Sneutrino Dark Matter

Studies of the rotation curves of galaxies, large scale structures, and recent measurements of the cosmic microwave background radiation, have confirmed that about 23% of the energy in the Universe is in the form of cold dark matter (CDM) [1]. The origin and the nature of CDM is one of the biggest puzzles in both particle physics and cosmology. Since all known particles are ruled out as dark matter candidates, dark matter provides the strongest phenomenological motivation for new physics beyond the Standard Model (SM). The Minimal Supersymmetric Standard Model (MSSM), supplemented with an exact discrete symmetry (R-parity), possesses two natural CDM candidates: the lightest neutralino and the lightest scalar neutrino (sneutrino). The former is a generic mixture of the superpartners of the neutral gauge and Higgs bosons, and its phenomenology has been the subject of extensive studies over the last 20 years [2]. In contrast, the left-handed (LH) sneutrinos of the MSSM have been ruled out as a major component of the dark matter in the Universe, by the combination of cosmological and experimental constraints. More precisely, LH sneutrinos are weakly charged, and typically annihilate too rapidly via Z-mediated s-channel diagrams, resulting in a relic density too small to account for all of the dark matter. To suppress the annihilation rate it was proposed that the sneutrinos should be either very light (O(GeV)) [3] or very heavy (O(TeV)) [4]. However, a very light sneutrino is excluded by the measurement of the invisible width of the Z gauge boson, while a very heavy sneutrino is excluded by direct dark matter searches [4]. Therefore, the LH sneutrinos of the MSSM are now disfavored as dark matter candidates. On the other hand, the recent evidence of neutrino masses provides strong impetus for extending the particle content of the MSSM with right-handed (RH) neutri� ∗

Lightest U-parity particle (LUP) dark matter

Abstract We suggest a U ( 1 ) ′ gauge symmetry as an alternative to the usual R-parity of supersymmetric standard models, showing that it can also work as a common source of stabilities of proton and dark matter in addition to other attractive features. The residual discrete symmetries of a single U ( 1 ) ′ can provide stabilities to both the MSSM sector (proton) and the hidden sector (new dark matter candidate, LUP). The LUP can expand the viability of many models such as R-parity violating models and gauge mediation models regarding dark matter issue.

Neutralino relic density in a supersymmetric U(1)′ model

Abstract We study properties of the lightest neutralino ( χ 0 ) and calculate its cosmological relic density in a supersymmetric U ( 1 ) ′ model with a secluded U ( 1 ) ′ breaking sector (the S -model). The lightest neutralino mass is smaller than in the minimal supersymmetric standard model; for instance, m χ 0 ≲ 100  GeV in the limit that the U ( 1 ) ′ gaugino mass is large compared to the electroweak scale. We find that the Z χ 0 χ 0 coupling can be enhanced due to the singlino components in the extended neutralino sector. Neutralino annihilation through the Z -resonance then reproduces the measured cold dark matter density over broad regions of the model parameter space.

Muon g − 2 anomaly and dark leptonic gauge boson

One of the major motivations to search for a dark gauge boson of MeV-GeV scale is the long-standing muon g-2 anomaly. Because of active searches such as fixed target experiments and rare meson decays, the muon g-2 favored parameter region has been rapidly reduced. With the most recent data, it is practically excluded now in the popular dark photon model. We overview the issue and investigate a potentially alternative model based on the gauged lepton number or U(1)_L, which is under different experimental constraints.

Fourth Generation Parity

We present a very simple fourth generation (4G) model with an abelian gauge interaction under which only the 4G fermions have nonzero charge. The U(1) gauge symmetry can have a Z2 residual discrete symmetry (4G parity), which can stabilize the lightest 4G particle (L4P). When the 4G neutrino is the L4P, it would be a neutral and stable particle and the other 4G fermions would decay into the L4P, leaving the trace of missing energy plus the standard model fermions. Because of the new symmetry, the 4G particle creation and decay modes are different from those of the sequential 4G model, and the 4G particles can be appreciably lighter than typical experimental bounds.

Muon anomalous magnetic moment in a supersymmetric U(1)′ model

Abstract We study the muon anomalous magnetic moment a μ = ( g μ − 2 ) / 2 in a supersymmetric U ( 1 ) ′ model. The neutralino sector has extra components from the superpartners of the U ( 1 ) ′ gauge boson and the extra Higgs singlets that break the U ( 1 ) ′ symmetry. The theoretical maximum bound on the lightest neutralino mass is much smaller than that of the Minimal Supersymmetric Standard Model (MSSM) because of the mixing pattern of the extra components. In a U ( 1 ) ′ model where the U ( 1 ) ′ symmetry is broken by a secluded sector (the S -model), tan β is required to be ≲3 to have realistic electroweak symmetry breaking. These facts suggest that the a μ prediction may be meaningfully different from that of the MSSM. We evaluate and compare the muon anomalous magnetic moment in this model and the MSSM and discuss the constraints on tan β and relevant soft breaking terms. There are regions of the parameter space that can explain the experimental deviation of a μ from the Standard Model calculation and yield an acceptable cold dark matter relic density without conflict with collider experimental constraints.