Phenomenological Studies Of New Physics In Semileptonic B Decays

The Standard Model(SM)of particle physics describes the fundamental particles of the universe and the forces through which they interact.The SM can describe the strong,magnetic and weak interactions,and its predictions withstand the tests of almost all high-energy physics experiments until now.However,hints of Lepton Flavour Universality(LFU)violation have been observed in b→cτv and b→sl+l-transitions during the past few years.The devia-tion of RD(*),RK(*)from the expected value in the SM imply a challenge to the idea of lepton universality within the SM and points towards a possible evidence of New Physics(NP).In this thesis,we focus on the NP effects on B mesons and B baryons decays.In the chapter 2,we briefly introduce the SM and basic theoretical tool for analytic computation.The following chapters are the main content of the thesis,which includes:Systematic investigations of b→cτvτ,decay(FCCC process)The Flavour-Changing Charged Current(FCCC)process is a tree level process by exchanging W± in SM.Some anoma-lous effects of FCCC process have been found in experiments.Recently,the D*and τ po-larization fractions pLD*and PLτ in B→ D*τv decay have also been measured by the Belle collaboration.Motivated by these intriguing results,we revisit the RD(*)and RK(*)anoma-lies in a scalar S1+S3 leptoquark(LQ)model in chapter 3,in which two scalar LQs,one be-ing SU(2)L singlet and the other SU(2)L triplet,are introduced simultaneously.We consider five b→cτv mediated decays,B→D(*)τv,Bc→ηxτv,Bc→J/ψτv,and Λb→Λcτv,and focus on the S1+S3 LQ effects on the q2 distributions of the branching fractions,the LFU ra-tios,and the various angular observables in these decays.Under the combined constraints by the available data on RD(*),RJ/φ,PLτ(D*),and PLD*,we perform scans for the S1+S3 LQ couplings and make predictions for a number of observables.It is found numerically that both the differen-tial branching fractions and the LFU ratios are largely enhanced by the S1+S3 LQ effects,with the latter being expected to provide testable signatures at the SuperKEKB and High-Luminosity LHC experiments.Systematic investigations of b→sl+l-decay(FCNC process)The branching ratio of the standard model is extremely small in the the Flavour-Change Neutral Current(FCNC)pro-cess.However,these processes may be significantly enhanced by NP.If these phenomena are observed experimentally,it will be a clear signal of the existence of NP.Based on these consider-ations,we investigate three b→sl+l-transitions,B→K(*)l+l-and Λb→Λ(→Nπ)l+l-in chapter 4 and 5.In the model-independent approach,the current b→sμ+μ-anomalies can be explained by the C9NP,22,C10NP,22 contribution,which provides in turn a constraint on λ22L*λ32L.We make predictions for a number of observables in our numerical analysis.It can be found that RK(*),RA complete separation from its SM predictions in the S1+S3 LQ model.These results indicates that the accurate measurement of R can be used to further study the LFU anomaly in FCNC process.