Probing new physics through semileptonic flavor changing neutral current transition

Abstract

In this thesis, the exclusive semileptonic rare B-mesons decays which are induced by the flavor changing neutral current transition (FCNC) have been studied within and beyond the Standard Model (SM). To probe physics beyond the SM, B → K ∗ + − , B → K 1 + − level, these FCNC transitions arises as b → s B → K 2 ∗ + − + − and B c → D ∗ s + − decays are considered. At quark and, in the SM, are forbidden at tree level but are allowed at loop level through Glashow-Iliopoulos-Maiani (GIM) mechanism. Moreover, FCNC transitions are further suppressed due to the fact that they are directly proportional to the off diagonal elements of the Cabibo Kobayashi Maskawa (CKM) matrix. Because of these two conditions FCNC transitions are relatively rare and become important to investigate the physics beyond the SM, usually known as new physics (NP). The important points of the thesis are in order: • We study the exclusive channel of flavor changing neutral current transition (FCNC) i.e. B → K ∗ + − in the frame work of a family of non-universal Z model. In this model, the Z boson couplings to the fermions could lead to FCNC transition at tree level. In addition, the off-diagonal elements of these effective chiral Z couplings can contain new weak phases that provide a new source of CP violation and, therefore, could explain the CP asymmetries in the current high energy colliders. In this context, we have studied the polarized and unpolarized CP violation asymmetries for the said decay. These asymmetries are highly suppressed in the standard model but significantly enhanced in the Z model. In addition to the CP violation asymmetries, the single lepton polarization asymmetries are also studied and found them sensitive to the couplings of the Z boson. Finally, it is analyzed that all these asymmetries which will hopefully be tested at LHC can serve to probe the non-universal Z model, particularly, the accurate measurements of these asymmetries may play a crucial role to extract the precise values of the coupling parameters of Z boson. • The sensitivity of the zero position of the forward backward asymmetry A FB for the exclusive B → K 1 (1270)μ + μ − decay is examined by using most general non-standard 4-fermion interactions. Our anal-ysis shows that the zero position of the forward backward asymmetry is very sensitive to the sign and size of the Wilson coefficients corresponding to the new vector type interactions, which are the counter partners of the usual Standard Model operators but have opposite chirality. In addition to these, the other significant effect comes from the interference of Scalar-Psudoscalar and Tensor type operators. These results will not only enhance our theoretical understanding about the axial vector mesons but will also serve as a good tool to look for physics beyond the SM. • A detailed study of the impact of New Physics (NP) operators with different Lorentz structures, which are absent in the Standard Model Hamiltonian, on the B → K 2 ∗ (1430)μ + μ − decay is performed. In this context, the various observables such as branching ratio, forward-backward asymmetry of leptons, lepton polarization asymmetries and the helicity fractions of the final state K 2 ∗ (1430) meson have been studied. We have examined the effects of new vector-axial vector, scalar-pseudoscalar and tensor type interactions for this decay B → K 2 ∗ (1430)μ + μ − by using the constraints on different NP couplings which come from the B s → μ + μ − , B → X s μ + μ − and B ̄ → K ̄ ∗ μ + μ − decays. It is found that the effects of V A, S P and T operators are significant on the zero position of A FB (q 2 ) as well as on its magnitude. In addition to this these NP operators also give significant effects on the differential decay rate, lepton polarization asymmetries and helicities fractions of final state K 2 ∗ (1430) meson. • The semileptonic B c → D ∗ s + − ( = μ, τ) decays have been studied in the Standard Model (SM) and in the Universal Extra Dimension (UED) model. In addition to the contribution from the Flavor Changing Neutral Current (FCNC) transitions the weak annihilation (WA) contribution is also important for this decay because ∗ of enhanced CKM matrix elements V cb V cs . It is found that the WA gives 6.7 times larger branching ratio than the penguin contribution for the decay B c → D ∗ s μ + μ − . The contribution from the WA and FCNC transitions are parameterized in terms of the form factors. In this work we first relate the form factors through Ward identities and then express them in terms of g + (0) which is extracted from the decay B c → D ∗ s γ through QCD sum rules approach. These form factors are then used to analyze the physical observables like branching ratio and helicity fractions of the final state D ∗ s meson in the SM. This analysis is then extended to the UED model where the dependency of above mentioned physical observables depend on the compactification radius R. It is shown that the helicity fractions of D ∗ s are sensitive to the UED model especially when we have muons as the final state lepton. This sensitivity is marked up at low q 2 region, irrespective of the choice of the form factors. It is hoped that in the next couple of years LHC will provide enough data on the B c → D ∗ s + − channel, and then , these helicity fractions would serve as a useful tool to establish new physics predicted by the UED model.