Two-photon decays of neutral B mesons within a model featuring one universal extra dimension
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EMENTARY PARTICLES AND FIELDS Theory
Two-Photon Decays of Neutral B Mesons within a Model Featuring One Universal Extra Dimension G. G. Devidze and A. G. Liparteliani Institute of High Energy Physics and Informatization, Tbilisi State University, Universitetskaya ul. 9, 0186 Tbilisi, Georgia Received May 31, 2006; in final form, November 15, 2006
Abstract—Contributions to the Bs(d) → γγ decay amplitude beyond the Standard Model are estimated within a model featuring one universal extra dimension. It is found that these additional contributions may amount to 6 or 7% of the branching ratios evaluated within the Standard Model. PACS numbers: 13.20.He, 13.25.Hw, 11.10.Kk DOI: 10.1134/S1063778807060178
INTRODUCTION In the Standard Model, the leading contribution to the two-photon decays of neutral Bs(d) mesons comes from one-loop diagrams, up-type quarks and W bosons being particles exchanged in the loop [1]. The branching ratio of these decays is about 10−7(9) [1]. The rate of these decays may become greater in some extensions of the Standard Model. For example, it was shown in [2] that, in supersymmetric extensions of the Standard Model, the branching ratio for the decay Bs → γγ may reach Br(Bs → γγ) ∼ 10−6 for a proper choice of supersymmetric parameters. Such an enhancement is due primarily to the exchange of charged Higgs particles in the loop. The rate of the decay under consideration can also be increased by contributions from the exchange of charged scalar particles predicted by some exotic models, such as the Appelquist–Cheng–Dobrescu (ACD) model [3], which involves one universal extra dimension. In that model, charged Higgs particles forming “towers” are real physical objects of specific mass rather than fictitious particles (ghosts). In this study, we aim at calculating the contributions of the above physical scalar particles to the Bs(d) → γγ decay widths. The ensuing exposition is organized as follows. In Section 1, we give an outline of the ACD model with an emphasis on details needed for our analytic calculations. In Section 2, we describe the procedure for calculating the Bs(d) → γγ decay widths within the ACD model, which involves one extra dimension. The respective branching ratios are estimated numerically in Section 3. Some details of the calculations are given in the Appendix.
1. STRUCTURE OF THE ACD MODEL In models featuring universal extra dimensions, all fields that appear in the Standard Model “live” in extra dimensions. This means that these fields depend on all spacetime coordinates. Therefore, all fourdimensional derivatives of boson fields in the Standard Model Lagrangian are replaced by the respective five-dimensional derivatives. The boson fields in question are exemplified by U (1)Y and SU (2)L gauge fields and SU (3)c gauge fields from the QCD sector. The Higgs doublet is chosen in such a way that it is P5 -even (P5 is the parity transformation in five dimensions) and has zero modes. All zero modes remain massless before the application of the Higgs mechanism. In addition, we note that, after the
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