【作者】 袁兴博；

【导师】 杨亚东；

【作者基本信息】 华中师范大学 ， 粒子物理与原子核物理， 2014， 博士

【摘要】 粒子物理的标准模型描述了宇宙中基本的粒子及它们间的相互作用。从低能标一直到电弱能标,它的理论预言与所有的实验数据相符合。然而,无论从唯象还是美学的角度来看,标准模型并不是一个完全令人满意的模型。目前,CERN的大型强子对撞机(Large Hadron Collider, LHC)正在运行一个长期的寻找超出模准模型的新物理的计划,例如对新的重基本粒子的寻找。另一方面,间接寻找新物理的信号也是可行的,而且对稀有夸克味转变的精确测量已经被证明为是一个有效的手段。味物理实验的一个重要特点是它可以探测很高能标下的物理,在许多情况下甚至高于LHC所能达到的能标。这是因为未知的粒子可以通过量子效应来影响被精确测量的物理过程,从而显现出它存在的信号。因此,味物理对检验标准模型和探测新物理都是十分重要的。本学位论文着重研究新物理对B和K介子稀有衰变的影响。在第2和第3章,我们简要的介绍标准模型和味物理的一些理论背景。之后是论文的主题部分,包括如下两个方面：系统的研究B和K介子衰变对顶夸克味改变中性流的限制味改变中性流(Flavour-Changing Neutral Current, FCNC)衰变t→qZ和t→qγ(q=u, c)的分支比在标准模型下是非常小的。然而,这些过程可能会被一些新物理所显著的增强。如果在LHC上观测到这些过程,这将成为新物理存在的明确信号。然而,当在高能对撞机上研究t→qZ和t→qγ这些过程(或者说一个有效的tqZ和tqγ耦合)时,我们也应该考虑包含顶夸克圈的低能味物理过程。在第4和第5章中,我们系统的研究了反常耦合tqZ和tqγ在B和K介子稀有衰变中的效应,如B→Xsγ和K→πvv过程等。在考虑了这些低能味物理过程的限制后,我们发现,对于tqZ和tqγ耦合的耦合常数为复数的情况,我们给出的对B(t→qZ)和B(t→qγ)上限的预言与目前实验直接观测的结果在一个量级上。而对于耦合为实数的情况,我们给出的限制远强于目前的实验结果,但仍在高亮度LHC(High-Luminosity run of LHC)的探测范围内。唯象研究各种双Higgs二重态模型双Higgs二重态模型(Two-Higgs-Doublet Model,2HDM)是对标准模型Higg部分最简单的扩展。然而,一般的2HDM会导致过大的味改变中性流。为了解决这个问题,自然味守恒(Natural Flavor Conservation, NFC)和最小味破坏(Minimal Flavor Violation, MFV)这两个假设被提了出来。在自然味守恒假设下,2HDM被附加了一个分立的Z2对称性,相应的存在四种2HDM的类型：type-Ⅰ, type-Ⅱ, type-X和type-Y2HDM。在第6章中,我们系统的研究了利用最新的实验测量和理论限制来区分这四种2HDM类型的可能性。我们发现,Bs→μ+μ-衰变的质量本征态衰变率不对称(mass eigenstate rate asymmetry)A△r和比值R=B(Bs→μ+μ-)exp/B(Bs→μ+μ-)sM的关联可以很好的用来区分这四种模型。在最小味破坏的假设下,存在两类的2HDM模型：type-Ⅲ和type-C模型,其中第2个Higgs二重态分别味色单态或色八重态。然而,这两个模型对Wilson系数C8的修正大小相似但符号相反,但对C7的修正基本相同。单举衰变B→Xs γ的分支比不能用来区分这两种模型。在第7章中,我们转向了B介子的遍举辐射衰变,如B→K*γ过程。我们发现,这些遍举衰变的直接CP和同位旋破缺可以给出与分支比互补的限制,并用来区分这两种2HDM的类型。我们也得到了MFV2HDM下遍举衰变中各可观测量间的关联。更多还原

【Abstract】 The Standard Model (SM) of particle physics describes the fundamental particles of the universe and the forces through which they interact. Its predictions are consistent with all ex-perimental data up to energies of the electroweak scale. However, the SM is not completely satisfactory from the view point of both aesthetics and phenomenology. The Large Hadron Col-lider (LHC) at CERN is performing a long program to search for direct signal of New Physics (NP) beyond the SM, such as evidence of new heavy particles.Alternatively, indirect search for NP is also possible and precise measurements of extremely rare quark flavour transitions have been proven to be powerful tools. An essential feature of flavour physics experiments is their ability to probe very high energy scales, in many cases be-yond the energies directly accessible in the LHC. This is because unknown virtual particles can manifest itself by altering the results of precise measurements through quantum effects. There-fore, flavour physics plays an important role in testing the SM and probing NP beyond it.In this thesis, we focus on the NP effects on rare B and K decays. In chapter2and3, we briefly introduce the SM and the basic concepts about the flavour physics that constitute the background on which the original work is based. Then, we present the main body of this thesis, which includes the following two parts:Systematic investigations of the bounds on anomalous top-quark FCNC couplings from B and K decays The Flavour-Changing Neutral Current (FCNC) decays t→qZ andt→qγ (q=u, c) are exceedingly rare within the SM. However, these processes could be significantly enhanced by some NP candidates. Any positive signal of these processes at LHC would therefore imply NP beyond the SM. However, when studying the phenomena of the t→qZ and t→qγ transitions, or equivalently the effective anomalous tqZ and tqγ couplings, at high-energy colliders, the low-energy processes involving top-quark loops should also be taken into account. In chapter4and5, we systematically study the anomalous tqZ and tqγ coupling effects in the rare B-and K-meson decays, such as the B→Xg-y and K→πvv modes, etc. With these low-energy constraints taken into account, it is found that, for general complex anomalous couplings tqZ and tq-y, the predicted upper bounds on B(t→qZ) and B(t→qγ) are about of the same order as the current experimental bounds. In particular, for the case of real couplings, our bounds are much lower than the current direct limits but still accessible at the High-Luminosity run of LHC. Phenomenological studies of the Two-Higgs Doublet Models The simplest extension to the SM Higgs sector is the so-called Two-Higgs-Doublet Model (2HDM). However, a generic2HDM would induce excessively large FCNCs. To address this problem, two different hypothe-ses, natural flavor conservation (NFC) and minimal flavor violation (MFV), have been proposed. In the NFC, a discrete Z2symmetry is imposed, and there are four types of2HDMs, referred respectively as the type-I, type-Ⅱ, type-X and type-Y2HDMs. In chapter6, we systematically investigate the possibility to discriminate the four models in the light of recent experimental mea-surements and theoretical constraints. It is found that the correlation between the mass eigenstate rate asymmetry AΔr of Bs→μ+μ-and the ratio R=B(Bs→μ+μ-)exp/B(Bs→μ+μ-)SM could be sensitive probe to discriminate the four models. Under the criterion of MFV, there are two classes of2HDMs referred as the type-Ⅲ and the type-C model, in which the second Higgs doublet is color-singlet and color-octet respectively. However, as the two models predict similar corrections to the coefficient C7whereas similar magnitudes but with opposite signs to C8, the branching ratios of inclusive B→Xsγ decay cannot discriminate the two models. In chapter7, we resort to the exclusive radiative B decays, such as B→K*γ. It is found that, the direct CP and isospin asymmetries of these exclusive decays provide complementary bounds on the2HDM parameters, and are suitable for discriminating the two models. We also obtained interesting correlations among the exclusive and the inclusive radiative B decays.更多还原