New Physics Effects In B And D Meson Decays Within Minimal Flavour Violation

The Standard Model(SM)of particle physics has become a very successful theory after decades of development.It describes the three basic interactions—strong interaction,weak in-teraction and electromagnetic interaction—in nature,while the gravitational interaction hasn't been included in the same framework yet.Although the SM predictions have achieved excellent agreements with most of the experimental measurements,we are still left with a few problems that the SM itself cannot explain,which may indicate the possible existence of New Physics(NP)beyond it.The B-meson decays play a key role in the exploration of possible NP signals.Especially among the quark-level b?cl-vl and b?sl+l-transitions,the latest experimen-tal results from the BaBar,Belle and LHCb collaborations have shown that there exist 2?4?deviations between the experimental data and the SM predictions for the observables RD(*)and RK(*)These anomalous signals indicate that the hypothesis of Lepton Flavour Universality(LFU)might be violated.In this thesis,we have studied the leptonic and semi-leptonic decays of B and D mesons within a model-independent effective theory framework.First of all,we give a brief introduction of the basic knowledge relevant to this thesis,such as the SM,the CKM matrix,and the effective theory.After that,we consider a most general model-independent NP beyond the SM.With the help of the Minimal Flavour Violation(MFV)hypothesis,we redefine the NP parameters and relate the Wilson coefficients describing the NP effects to each other.Then,starting with the most general effective Hamiltonian with NP operators included,we present the branching fractions of some B-meson decays,such as B-?l-?l,B??l-?l,and B??l-vl,as wellas the ones of some L-meson decays,including D(s)+?l+vl,D?Plvl,and D?Vlve.Here we also apply the MFV hypothesis to the leptonic and semi-leptonic B-and D-meson decays considered above,and introduce a series of ratios of different branching fractions.Finally,we use the experimental results provided by the Particle Data Group to constrain the space of the NP parameters.Here,since the leptonic decays play a significant role,we have paid special attention to the(pseudo)-scalar NP parameters.After detailed numerical analyses,we find that the deviation between the NP parameter space allowed by the ratios of D-meson decay branching fractions and the SM is around 1?,while the deviation between the allowed NP parameter space and the SM is only 1?2?.As a consequence,we do not find any signal of the LFU violation in the leptonic and semi-leptonic B-and D-meson decays.We believe,with more and more high-precision experimental data in the future,the model-independent strategy adopted here can be applied to more decay processes,which may provide us with some amazing results.