Many models for physics beyond the Standard Model predict lepton-flavour violating decays of charged leptons at a level which may become observable very soon. We investigate the decays of a Tau-lepton into three charged leptons (Tau -> lll', l(') = e,μin a generic way. Using effective-field-theory methods, the relevant operators are classified according to their chirality structure. For each case, we work out the Dalitz plots for the decay distributions, including interference terms which arise from four-lepton operators and radiatively induced processes. We discuss phenomenological implications, in particular the potential to distinguish different new physics models.
Model independent analysis of the simultaneous mixing of gauge bosons and mixing of fermions
我们讨论的情况下，同时混合的规范玻色子和混合的费米子在一个模型独立的方式和各种额外的费米子表示。本文分析了一类轻子族违反过程，即在存在一个额外的中性规范玻色子Z '的情况下，具有普适、非普适或族变耦合的轻子族违反过程Z --> etau，Z--> mu，Z --> e mu，mu--> e e e，tau--> e e e e，tau --> mu mu，tau --> e mu mu和tau --> mu e e.我们导出了Z-Z'混合和普通-奇异轻子混合的联合效应的界。
We conduce a study to probe the sensitivity of the process $e^+e^-\rightarrow (γ, Z) \to ν_τ\bar ν_τγ$ to the total cross section, the magnetic moment and the electric dipole moment of the tau-neutrino in a model-independent way. For this study, the beam polarization facility at the Compact Linear Collider (CLIC) along with the typical center-of-mass energies $\sqrt{s}=380-3000\hspace{0.8mm}GeV$ and integrated luminosities ${\cal L}=10-3000\hspace{0.8mm}fb^{-1}$ is considered. We estimate the sensitivity at the $95\%\hspace{1mm}$ Confidence Level (C.L.) and systematic uncertainties $δ_{sys}=0,…
Testing CP, T and (V-A) Symmetry through Tau Leptons
τ衰变对称性破坏的模型无关检验对于确定τ轻子是基本的还是宏观的很重要。这样的测试也是解决悬而未决的“e - mu - tau”难题的重要步骤。这次演讲回顾了这样的测试的背景下，一般治疗的两体τ衰变，只假设洛伦兹不变性和树状结构的τ-antitau生产衰变序列。直接测量极化部分宽度和相关的“横向W交换”干涉强度，将为区分希格斯玻色子、τ中微子和τ中微子的基本/宏观理论提供重要的探针。类似的目标和技术也适用于顶夸克衰变。
We establish model independent bounds on the anomalous magnetic and electric dipole moments of the tau-lepton using the two-photon processes $γγ\to τ^+τ^-$ and $γγ\to τ^+τ^-γ$. We use ${\cal L}=10, 50, 100, 300, 500, 1000, 1500, 2000, 3000 \hspace{0.8mm}fb^{-1}$ of data collected with the future $e^+e^-$ linear collider such as the CLIC at $\sqrt{s}=380, 1500, 3000 \hspace{0.8mm}GeV$ and we consider systematic uncertainties of $δ_{sys}=0, 3, 5\hspace{1mm}\%$. Precise bounds at $95\%$ C. L. on the anomalous dipole moments to the tau-lepton $-0.00015\leq \tilde{a}_τ\leq 0.00017$ and $|\tilde{d}_…
The integral observables for model-independent detections of Abelian Z' gauge boson in e^+ e^- -> mu^+ mu^- (tau^+ τ^-) process with unpolarized beams at the ILC energies are proposed. They are based on the differential cross-section of deviations from the standard model predictions calculated with a low energy effective Lagrangian and taking into consideration the relations between the Z' couplings to the fermions derived already. Due to these relations, the cross-section exhibits angular distribution giving a possibility for introducing one- or two parameter observables which effectively fit…
Generalized structure functions in semileptonic tau decays
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional（墨西哥国立理工学院高级研究中心）；Universidad de Castilla-La Mancha（卡斯蒂利亚-拉曼恰大学）；Universitat de València — Consejo Superior de Investigaciones Científicas（瓦伦西亚大学—西班牙国家研究委员会）
The QCD coupling, $α_s$, is not a physical observable since it depends on conventions related to the renormalization procedure. Here we discuss a redefinition of the coupling where changes of scheme are parametrised by a single parameter $C$. The new coupling is denoted $\hat α_s$ and its running is scheme independent. Moreover, scheme variations become completely analogous to renormalization scale variations. We discuss how the coupling $\hat α_s$ can be used in order to optimize predictions for the inclusive hadronic decays of the tau lepton. Preliminary investigations of the $C$-scheme in t…
Radiative muon and tau lepton decays are described within the model-independent approach with the help of generalized Michel parameters. The exact dependence on charged lepton masses is taken into account. The results are relevant for modern and future experiments on muon and tau lepton decays.
We analyze the sensitivity of hadronic tau decays to non-standard interactions within the model-independent framework of the Standard Model Effective Field Theory (SMEFT). Both exclusive and inclusive decays are studied, using the latest lattice data and QCD dispersion relations. We show that there are enough theoretically clean channels to disentangle all the effective couplings contributing to these decays, with the $τ\to ππν_τ$ channel representing an unexpected powerful New Physics probe. We find that the ratios of non-standard couplings to the Fermi constant are bound at the sub-percent l…
The deviation between the standard model prediction and the measurement of the muon g-2 is currently at 3-4 sigma (can be up to 7 sigma in the upcoming experiment E989). If new physics is responsible for such discrepancy, it is expected that the new contributions to tau g-2 are even larger than that for muon due to its large mass. In spite of that, the tau g-2 is much more difficult to be directly measured because of its short lifetime. In this paper, we consider the effect of the tau g-2 at $e^-e^+$ colliders in a model independent way. Using the tau pair production channel at the Large Elect…
Hadronic $τ$ decays are studied as probe of new physics. We determine the dependence of several inclusive and exclusive $τ$ observables on the Wilson coefficients of the low-energy effective theory describing charged-current interactions between light quarks and leptons. The analysis includes both strange and non-strange decay channels. The main result is the likelihood function for the Wilson coefficients in the tau sector, based on the up-to-date experimental measurements and state-of-the-art theoretical techniques. The likelihood can be readily combined with inputs from other low-energy pre…
We present model independent high resolution amplitude analyses of CERN data on pi(-)p->pi(-)pi(+)n at 17.2 GeV/c for dipion masses 580-1080 MeV at small momentum transfers on polarized target (Analysis I) and unpolarized target (Analysis II). The results for S- and P-wave transversity amplitudes from the Analysis I are used in a model independent determination of helicity amplitudes. All three analyses provide similar evidence for rho^0(770)-f_0(980) mixing in S- and P-wave amplitudes. In Analyses I and II the S-wave amplitudes |S_d|^2 peak at rho^0(770) mass while the P-wave amplitudes |L_d|…
Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and $\ell$--$τ$ conversion in nuclei, with $\ell = e,μ$. Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. Our work paves the way to extract the related information from Belle II and foreseen future experiments.
Within the Standard Model Effective Field Theory framework, with operators up to dimension 6, we perform a model-independent analysis of the lepton-flavour-violating processes involving tau leptons. Namely, we study hadronic tau decays and $\ell$-$τ$ conversion in nuclei ($\ell=e,μ$). Based on available experimental limits, we establish constraints on the Wilson coefficients of the operators contributing to these processes. The related information from Belle II and foreseen future experiments can be easily incorporated into the resulting framework.
We propose a {\em unitarity method} for determining $τ$ neutrino appearance probability $P(ν_μ \rightarrow ν_τ)$ in long-baseline (LBL) accelerator experiments and atmospheric neutrino observations. When simultaneous in situ measurements of $P(ν_μ \rightarrow ν_μ)$ and $P(ν_μ \rightarrow ν_{e})$ proceed, as is typical in the LBL experiments, one can use unitarity to "measure" $P(ν_μ \rightarrow ν_τ)$. A theorists' toy analysis for the model-independent determination of $P(ν_μ \rightarrow ν_μ)$ and $P(ν_μ \rightarrow ν_{e})$ is presented by using the NOvA data. It is shown in our analysis that …
Model independent constraints on leptoquarks from MU and TAU lepton rare processes
我们进行了一个模型独立的分析，以限制leptoquark（LQ）模型的负搜索$μ\to e γ$，$μ\to 3e$衰变（和类似的过程中的$τ$部门），并在核中的相干$μ-e$转换。我们大大提高了一些限制，通过分析已知的文献中，分析表明，我们远远低估了LQ的贡献的$μ\to 3e$。特别是，我们发现，相干$μ-e$转换介导的光子转换机制和$μ\to 3e$衰变的核是黄金板的味道改变leptoquark耦合，涉及第二和第三代夸克，可以强烈约束。这是由于这些过程通过由所谓的"光子-企鹅“图诱导的大$\log(m_q^2/m^2_{LQ})$项得到增强。这些增强在振幅中产生轻微的GIM抑制，在先前的分析中没有考虑到。我们发现，$μ\to e γ$衰减可以设置较弱的约束的LQ模型，这是因为它的振幅是强烈的GIM抑制的条款的顺序$O(m_q^2/m^2_{LQ})$。我们还提出了相应的限制在$τ$部门的结果。最后，对未来μ子实验对改进现有束缚态的前景进行了分析和讨论。
Exclusive hadronic Tau decays in the resonance effective action of QCD
到目前为止，强子弱流的矢量和轴矢量分量的形状因子的研究已经进行了大量的基础强相互作用的模型化。虽然了解所涉及的动力学非常重要，但实验数据的数量和质量开始显示出与分析模型的一些差异。此外，从理论的角度来看，这些模型中的大多数与量子色动力学（QCD）不一致。我们提出了一个基于QCD的模型无关的程序来分析这些衰变，通过使用共振手征理论，低能量有效作用的QCD在相关的共振区。在这个框架下，我们研究了Tau（-）-> Pi（-）Pi（0）nu_tau中Rho介子的强子离壳层宽度和π介子的矢量形状因子.我们还讨论了Tau（-）->（Pi Pi Pi）（-）nu_tau衰变.
The singly charged $SU(2)_L$ singlet scalar, with its necessarily flavour violating couplings to leptons, lends itself particularly well for an explanation of the Cabibbo Angle Anomaly and of hints for lepton flavour universality violation in $τ\to μ\bar νν$. In a setup addressing both anomalies, we predict loop-induced effects in $τ\to eγ$ and in $τ\to eμμ$. A recast of ATLAS selectron and smuon searches allows us to derive a coupling-independent lower limit of $\approx 200$ GeV on the mass of the singly charged singlet scalar. At a future $e^+e^-$ collider, dark matter mono-photon searches c…
Leptoquarks are ubiquitous in several extensions of the Standard Model and seem to be able to accommodate the universality-violation-driven $B$-meson-decay anomalies and the $(g-2)_μ$ discrepancy interpreted as deviations from the Standard Model predictions. In addition, the search for lepton-flavour violation in the charged sector is, at present, a major research program that could also be facilitated by the dynamics generated by leptoquarks. In this article, we consider a rather wide framework of both scalar and vector leptoquarks as the generators of lepton-flavour violation in processes in…
We present an overview of searches fo violation of lepton flavor universality with focus on low energy precision probes using pions, kaons, tau leptons, and nuclear beta decays. The current experimental results are reviewed, the theoretical status within the context of the Standard Model is summarized, and future prospects (both experimental and theoretical) are discussed. We review the implications of these measurements for physics beyond the Standard Model by performing a global model-independent fit to modified $W$ couplings to leptons and four-fermion operators. We also discuss new physics…
