【作者】 李媛媛；

【导师】 李新强；

【作者基本信息】 华中师范大学 ， 粒子物理与原子核物理， 2021， 博士

【摘要】 粒子物理标准模型是描述基本粒子之间相互作用规律最成功的理论;除引力相互作用外,它可以统一地描述强相互作用、弱相互作用和电磁相互作用。然而,标准模型并不是一个完美的理论。虽然希格斯粒子的发现是希格斯机制的有力证据,但是负责电弱对称性破缺的希格斯二重态是否只有一个,目前仍无法完全确定。另外,在标准模型中,一些观测量的理论预言值与实验测量值之间存在一定的偏离,如意味着轻子味普适性被破坏的R（D（*））和（g-2）_e,μ反常;无论从理论还是从实验上,这些偏离都很难得到合理且自洽的解释,从而预示着可能存在超出标准模型的新物理。当前,位于欧洲核子研究中心的大型强子对撞机实验正致力于寻找超出标准模型的新物理信号,如对很重的新粒子的直接寻找;虽然这些粒子存在于很高的能标,但它们可以通过量子效应对低能味物理过程产生影响。因此,我们也可以通过低能味物理过程间接寻找新物理存在的迹象。基于目前这些可能的新物理信号,我们可以做一系列唯象方面的研究。例如,根据观测量的实验测量值与理论预言值,我们可以对新物理模型中的未知参数进行拟合;另外,利用拟合得到的新物理参数,我们还可以对相关的味物理过程进行唯象研究,进而对不同的新物理模型进行区分和辨别;最后,我们还可以对标准模型或者已知的新物理模型进行扩展,通过构建更具体的新物理模型来解释更多的反常现象。在本论文中,我们将首先基于R（D（*））反常,利用拟合得到的新物理参数,分别在模型无关的低能有效理论、轻子夸克模型以及标准模型有效场理论框架下,对单举半轻衰变过程B→X_cτντ进行系统的唯象研究,从而对不同的新物理模型进行区分。接着,我们将构建一个具体的新物理模型——幂次对齐的双希格斯二重态模型,并在该模型下对（g-2）_e,μ反常进行同时且关联的解释。我们首先基于R（D（*））反常,对B→X_cτντ过程中的新物理效应进行系统的唯象研究。我们知道,采用重夸克展开和重夸克有效理论,我们可以对该过程进行微扰和非微扰的双重展开,进而对该过程给出高精度的理论预言。在考虑了对领头幂次贡献的微扰O（αs）和非微扰O(Λ_QCD²/m_b²)修正后,我们将分别在模型无关的低能有效理论、轻子夸克模型以及标准模型有效场理论框架下,对该过程进行全面的唯象分析。首先,在模型无关的低能有效理论框架下,对于一次仅考虑一个新物理算符的情况,我们发现,新物理对领头幂次贡献的微扰和非微扰修正并不十分敏感;但是,相对于标准模型,新物理的影响仍十分显著。其次,已有文献研究结果表明,在剔除掉轻子味普适的右手矢量算符、并通过对其它的新物理算符进行全局拟合后,存在两组最佳拟合值;通过分析这两组解对B→X_cτντ过程的影响,我们发现这两组解是可以得到区分的,其中一组解的新物理效应并不明显,对应的观测量与标准模型中的行为基本一致,而另一组解的新物理效应则十分显著。最后,我们分别在R2、U1以及S1轻子夸克模型中对该过程进行了详细的唯象研究。结果表明,U1和S1轻子夸克模型给出的观测量的行为基本保持一致,相对于标准模型的预言有一定的增强;而在R2轻子夸克模型中,除轻子不变质量谱（1/Γ0）dΓ/dq2相对于标准模型的预言存在很小的增强之外,其它观测量与标准模型的预言基本一致。在标准模型有效场理论中,我们主要对一次考虑两个新物理算符的组合进行了讨论。结果发现,各个组合对轻子不变质量谱的新物理效应几乎一致,相对于标准模型的预言都有一定的增强;而对于轻子能谱（1/Γ0）dΓ/dEτ,各个组合下的新物理效应明显不同。另外,在比值类观测量中,由于新物理效应被抵消,各个组合下的结果与标准模型的预言几乎一致。因此,我们可以通过轻子能谱来对这些组合进行初步的辨别,但仍无法进行完全区分。基于（g-2）e,μ反常,我们将构建一个具体的新物理模型,即幂次对齐的双希格斯二重态模型,并在该模型下对Δa_e,μ进行同时且关联的解释。在最小味破坏的假设下,通过厄米的汤川矩阵,我们将两个汤川耦合构造成了幂次对齐的形式。在该模型框架下,B物理过程将带电希格斯粒子的质量限制在了 TeV的标度。而且,我们发现,在TeV标度简并的希格斯粒子质量谱可以同时解释Δa_e,μ,并且所允许的参数空间可以满足Z玻色子和τ轻子衰变中轻子味普适性的测量以及目前大型强子对撞机上的实验限制。另外,通过味普适的幂次对齐参数,Δa_e,μ之间存在近似线性的关联,这使得幂次对齐的双希格斯二重态模型明显有别于其他的双希格斯二重态模型。因此,在将来的精确测量中,该模型具有很强的分辨性以及可测量性。随着实验测量精度的不断提高以及相关理论的发展,我们期待更多的、与新物理有关的信号被挖掘出来。更多还原

【Abstract】 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.更多还原