Hye-Sung Lee — Page 5

arXiv2004

Neutralino Relic Density in a Supersymmetric U(1)' Model

We study properties of the lightest neutralino (χ) 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< 100 GeV in the limit that the U(1)' gaugino mass is large compared to the electroweak scale. We find that the Z-χ-χcoupling 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.

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arXiv2004

Solution to the B -> pi K Puzzle in a Flavor-Changing Z' Model

Recent experiments suggest that certain B -> pi 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 -> phi K_S decay.

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arXiv2004

Big Bang Nucleosynthesis Constraints on Z' Properties

In models involving new TeV-scale Z' gauge bosons, the new U(1)' symmetry often prevents the generation of Majorana masses needed for a conventional neutrino seesaw, leading to three super-weakly interacting right-handed neutrinos, the Dirac partners of the ordinary neutrinos. These can be produced prior to Big Bang Nucleosynthesis (BBN) by the Z' interactions, leading to a faster expansion rate and too much ^4He. We quantify the constraints on the Z' properties from nucleosynthesis for Z' couplings motivated by a class of E6 models parametrized by an angle theta_E6. The decoupling temperature, which is higher than that of ordinary left-handed neutrinos due to the large Z' mass, is calculated, and the equivalent number of extra weakly interacting neutrinos, Delta N_nu, is obtained numerically as a function of the Z' mass, couplings, and the Z-Z' mixing angle. The ^4He abundance from BBN gives the most stringent limit on M_Z' unless Z' coupling to the right-handed neutrinos are small.

hep-ph↗

arXiv2003

Z' Mediated Flavor Changing Neutral Currents in B Meson Decays

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 B_d -> ϕK, B_d -> η' K and B_s -> μ^+ μ^- 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 S_{ϕK_S} but is not sufficient to explain large branching ratio of B_d -> η' K with the same parameter value. S_{ϕK_S} and S_{η' K_S} are seen to be the dominant observables that constrain the extra weak phase in the model.

hep-ph↗

arXiv2003

WMAP and Inflation

We assay how inflationary models whose properties are dominated by the dynamics of a single scalar field are constrained by cosmic microwave background (CMB) data from the Wilkinson Microwave Anisotropy Probe (WMAP). We classify inflationary models in a plane defined by the horizon-flow parameters. Our approach differs from that of the WMAP collaboration in that we analyze only WMAP data and take the spectral shapes from slow-roll inflation rather than power-law parameterizations of the spectra. The only other information we use is the measurement of $h$ from the Hubble Space Telescope (HST) Key Project. We find that the spectral index of primordial density perturbations lies in the 1σrange 0.94<n_s<1.04 with no evidence of running. The ratio of the amplitudes of tensor and scalar perturbations is smaller than 0.61 and the inflationary scale is below 2.8\times 10^{16} GeV, both at the 2σC.L. No class of inflation or ekpyrotic/cyclic models is excluded. The λϕ^4 potential is excluded at 3σonly if the number of e-folds is assumed to be less than 45.

hep-ph↗

arXiv2003

Effective number of neutrinos and baryon asymmetry from BBN and WMAP

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 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 2σranges for the effective number of neutrinos and for the baryon asymmetry (baryon to photon number ratio η) to be 1.7-3.0 and 5.53-6.76 \times 10^{-10}, respectively.

hep-ph↗

arXiv2003

Primordial Nucleosynthesis Constraints on Z' Properties

In models involving new TeV-scale Z' gauge bosons, the new U(1)' symmetry often prevents the generation of Majorana masses needed for a conventional neutrino seesaw, leading to three superweakly interacting ``right-handed'' neutrinos nu_R, the Dirac partners of the ordinary neutrinos. These can be produced prior to big bang nucleosynthesis by the Z' interactions, leading to a faster expansion rate and too much ^4He. We quantify the constraints on the Z' properties from nucleosynthesis for Z' couplings motivated by a class of E_6 models parametrized by an angle theta_E6. The rate for the annihilation of three approximately massless right-handed neutrinos into other particle pairs through the Z' channel is calculated. The decoupling temperature, which is higher than that of ordinary left-handed neutrinos due to the large Z' mass, is evaluated, and the equivalent number of new doublet neutrinos Delta N_nu is obtained numerically as a function of the Z' mass and couplings for a variety of assumptions concerning the Z-Z' mixing angle and the quark-hadron transition temperature T_c. Except near the values of theta_E6 for which the Z' decouples from the right-handed neutrinos, the Z' mass and mixing constraints from nucleosynthesis are much more stringent than the existing laboratory limits from searches for direct production or from precision electroweak data, and are comparable to the ranges that may ultimately be probed at proposed colliders. For the case T_c = 150 MeV with the theoretically favored range of Z-Z' mixings, Delta N_nu < 0.3 for M_Z' > 4.3 TeV for any value of theta_E6. Larger mixing or larger T_c often lead to unacceptably large Delta N_nu except near the nu_R decoupling limit.

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