New Physics Effects In Rare B And K Meson Decays

The Standard Model (SM) of particle physics describes the fundamental particles of the universe and the forces through which they interact. Its predictions are consistent with all ex-perimental data up to energies of the electroweak scale. However, the SM is not completely satisfactory from the view point of both aesthetics and phenomenology. The Large Hadron Col-lider (LHC) at CERN is performing a long program to search for direct signal of New Physics (NP) beyond the SM, such as evidence of new heavy particles.Alternatively, indirect search for NP is also possible and precise measurements of extremely rare quark flavour transitions have been proven to be powerful tools. An essential feature of flavour physics experiments is their ability to probe very high energy scales, in many cases be-yond the energies directly accessible in the LHC. This is because unknown virtual particles can manifest itself by altering the results of precise measurements through quantum effects. There-fore, flavour physics plays an important role in testing the SM and probing NP beyond it.In this thesis, we focus on the NP effects on rare B and K decays. In chapter2and3, we briefly introduce the SM and the basic concepts about the flavour physics that constitute the background on which the original work is based. Then, we present the main body of this thesis, which includes the following two parts:Systematic investigations of the bounds on anomalous top-quark FCNC couplings from B and K decays The Flavour-Changing Neutral Current (FCNC) decays tâ†'qZ andtâ†'qγ (q=u, c) are exceedingly rare within the SM. However, these processes could be significantly enhanced by some NP candidates. Any positive signal of these processes at LHC would therefore imply NP beyond the SM. However, when studying the phenomena of the tâ†'qZ and tâ†'qγ transitions, or equivalently the effective anomalous tqZ and tqγ couplings, at high-energy colliders, the low-energy processes involving top-quark loops should also be taken into account. In chapter4and5, we systematically study the anomalous tqZ and tqγ coupling effects in the rare B-and K-meson decays, such as the Bâ†'Xg-y and Kâ†'Ï€vv modes, etc. With these low-energy constraints taken into account, it is found that, for general complex anomalous couplings tqZ and tq-y, the predicted upper bounds on B(tâ†'qZ) and B(tâ†'qγ) are about of the same order as the current experimental bounds. In particular, for the case of real couplings, our bounds are much lower than the current direct limits but still accessible at the High-Luminosity run of LHC. Phenomenological studies of the Two-Higgs Doublet Models The simplest extension to the SM Higgs sector is the so-called Two-Higgs-Doublet Model (2HDM). However, a generic2HDM would induce excessively large FCNCs. To address this problem, two different hypothe-ses, natural flavor conservation (NFC) and minimal flavor violation (MFV), have been proposed. In the NFC, a discrete Z2symmetry is imposed, and there are four types of2HDMs, referred respectively as the type-I, type-â…¡, type-X and type-Y2HDMs. In chapter6, we systematically investigate the possibility to discriminate the four models in the light of recent experimental mea-surements and theoretical constraints. It is found that the correlation between the mass eigenstate rate asymmetry AΔr of Bsâ†'μ+μ-and the ratio R=B(Bsâ†'μ+μ-)exp/B(Bsâ†'μ+μ-)SM could be sensitive probe to discriminate the four models. Under the criterion of MFV, there are two classes of2HDMs referred as the type-â…¢ and the type-C model, in which the second Higgs doublet is color-singlet and color-octet respectively. However, as the two models predict similar corrections to the coefficient C7whereas similar magnitudes but with opposite signs to C8, the branching ratios of inclusive Bâ†'Xsγ decay cannot discriminate the two models. In chapter7, we resort to the exclusive radiative B decays, such as Bâ†'K*γ. It is found that, the direct CP and isospin asymmetries of these exclusive decays provide complementary bounds on the2HDM parameters, and are suitable for discriminating the two models. We also obtained interesting correlations among the exclusive and the inclusive radiative B decays.