Phenomenological Studies Of Lepton Flavor Universality Violation

The Standard Model of particle physics is the most successful theory for describing the interactions among elementary particles.Except for the gravitational interaction,it can uniformly describe the strong,weak and electromagnetic interactions.However,the Standard Model is not a perfect theory.Although the discovery of the Higgs boson is a compelling evidence of the Higgs mechanism,it is still uncertain whether there is only one Higgs doublet that is responsible for the electroweak symmetry breaking.In addition,there exist some observables in the Standard Model whose theoretical predictions deviate from the corresponding experimental measurements,such as the R（D（*））and（g-2）e,μ anomalies,both of which imply the violation of lepton-flavor universality.These deviations can be explained neither theoretically nor experimentally in a reasonable and self-consistent way,and might be therefore indicating the existence of new physics beyond the Standard Model.The Large Hadron Collider located at the European Organization for Nuclear Research is currently devoted to the seeking for new physics signals beyond the Standard Model,such as the direct searches for heavy new particles.Although residing at a higher energy scale,these new particles can still influence the low-energy flavor physics processes by quantum effects.Consequently,we can also search for the new physics signals indirectly through low-energy flavor physics processes.Motivated by these possible new physics signals,we can perform a series of phenomenological studies.For example,we can fit the unkown parameters characterizing the new physics models by comparising the experimental measurements and the theoretical predictions of the observables considered.In addition,based on the best-fit values of the new physics parameters,we can perform a phenomenological study of the related flavor physics processes,and then make a discrimination among different new physics models.Finally,we can extend the Standard Model or any known new physics models by constructing specific models to explain more observed anomalies.In this thesis,motivated by the R（D（*））anomalies,we will firstly perform a phenomenological study of the inclusive semileptonic B→X_cτντ decay,in the frameworks of model-independent low-energy effective field theory,leptoquark models,as well as the standard model effective field theory,respectively.Then,we will construct a specfic new physics model,the power-aligned two Higgs doublet model,and perform a simultaneous and correlative study of（g-2）e,μ anomalies.Motivated by R（D（*））anomalies,we will perform a detailed phenomenological analysis of the new physics effects on the inclusive B →X_cτντ decay.It is well-known that,using the heavy-quark expansion and the Heavy Quark Effective Theory,we can write the decay as a double series expansion,both perturbatively and non-perturbatively,giving therefore a high-precision theoretical prediction.Including the leading-order perturbative O（αs）and nonperturbative O(Λ_QCD²/m_b²)corrections,we will perform a complete analysis of the decay,both model-independently and also in the frameworks of leptoquark models and the the standard model effective field theory.For the model-independent analysis,we firstly consider the case with one new physics operator at a time.It is found that the new physics is not sensitive to the leading-order perturbative and non-perturbative corrections,but,compared to the Standard Model,the new physics effect is still significant.Then,given that there are two sets of best-fit solutions after removing the lepton-flavor universal right-handed vector operator,we also perform a phenomenological analysis of the effects of these two sets on the inclusive B→X_cτντdecay.It is found that these two sets of best-fit solutions can be completely distinguished from each other,with one of them having considerable new physics effect,while the other one only marginal effect on the observables.Finally,we study phenomenologically this decay in the R2,U1 and S1 leptoquark models,respectively.It is found that the observables have nearly the same behaviors in the U1 and S1 leptoquark models,being enhanced to some extent with respect to the Standard Model predictions,while in the R2 leptoquark model,the behaviors of the observables remain almost the same as in the Standard Model,except for the lepton invariant-mass spectrum,（1/Γ0）dΓ/dq2,which shows only a marginal enhancement.In the framework of the standard model effective field theory,on the other hand,we consider mainly the case with the combination of two new physics operators at a time.It is found that the observable（1/Γ0）dΓ/dq2 has almost the same behavior in each new physics scenario,with a consistent enhancement with respect to the Standard Model predictions,while for the lepton energy spectrum,（1/Γ0）dΓ/dET,each scenario behaves quite differently.In addition,for the observables constructed from the ratios,the new physics effects are cancelled significantly and hence have almost the same behavior as in the Standard Model.Consequently,we could distinguish these new physics scenarios from the behaviors of（1/Γ0）dΓ/dEΓ naively but not in an exact way.Motivated by（g-2）e.μ anomalies,we will construct a specific new physics model,the power-aligned two Higgs doublet model,and illustrate that it can make a simultaneous and correlative explanation of the Δa_e,μ.In the context of minimal flavor violation hypothesis,there exists a power-aligned relation between the Hermitian Yukawa matrices.Within such a poweraligned framework,we find that the charged-Higgs mass is pushed up to a few TeV by the current constraints from the B-physics observables.It is found that a simultaneous explanation ofΔae.μ can be reached with TeV-scale quasi-degenerate Higgs masses,and the resulting parameter space is also phenomenologically safer under the Z-boson and τ-lepton decay data,as well as the current Large Hadron Collider bounds.Furthermore,the flavor-universal power that enhances the charged-lepton Yukawa couplings prompts a quasi-linear correlation between Δa_e,μ,which makes the model distinguishable from the other two Higgs doublet model candidates and hence testable by future precise measurements.With the improvement of the precision of experimental measurements and the development of the relevant theories,more hints of new physics signals will be discovered in the future.