Precision Tau Physics (1310.7922v2)

Abstract: Precise measurements of the lepton properties provide stringent tests of the Standard Model and accurate determinations of its parameters. We overview the present status of tau physics, highlighting the most recent developments, and discuss the prospects for future improvements. The leptonic decays of the tau lepton probe the structure of the weak currents and the universality of their couplings to the W boson. The universality of the leptonic Z couplings has also been tested through leptonic Z decays. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interaction in very clean conditions. Accurate determinations of the QCD coupling and the Cabibbo mixing have been obtained with tau data. The large mass of the tau opens the possibility to study many kinematically-allowed exclusive decay modes and extract relevant dynamical information. Violations of flavour and CP conservation laws can also be searched for with tau decays. Related subjects such as muon decays, the electron and muon anomalous magnetic moments, neutrino mixing and B-meson decays into tau leptons are briefly covered. Being one the fermions most strongly coupled to the scalar sector, the tau lepton is playing now a very important role at the LHC as a tool to test the Higgs properties and search for new physics at higher scales.

• The paper demonstrates how tau decay measurements offer high-precision tests of the Standard Model's electroweak interactions and lepton universality.
• The paper employs tau hadronic decay analyses to precisely determine the QCD coupling constant αₛ, confirming asymptotic freedom and its energy running.
• The paper outlines how tau decay channels are used to constrain new physics, setting stringent limits on lepton-flavor-violating processes.

Precision Tau Physics

The paper of tau lepton physics serves as a significant testing ground for the Standard Model (SM) and offers insights into QCD dynamics and potential new physics scenarios. The tau lepton, with its large mass, offers unique opportunities to probe electroweak interactions, QCD effects at low energies, and potential violations of fundamental symmetries. This essay provides an overview of the key areas of research and findings in tau lepton physics.

Overview of Tau Interactions

Tau leptons interact through both weak and electromagnetic forces, with significant decays into both leptonic and hadronic channels. Their decay modes allow for a detailed examination of the weak interaction's structure and the universality of lepton couplings to gauge bosons. Leptonic decays of taus have been used to precisely test the universality of the $W^\pm$ boson couplings, with measurements consistent with the predictions of the SM at the $0.15\%$ level.

Hadronic Tau Decays and QCD

The tau lepton is particularly interesting for QCD studies due to its ability to decay into hadronic final states, offering clean experimental conditions to investigate the strong interaction. Tau decays into hadrons enable precise measurements of the QCD coupling constant $\alpha_s$. The inclusive hadronic tau decay width is employed for $\alpha_s$ determinations, which confirm asymptotic freedom and the running of the QCD coupling with energy scale. The agreement between tau-based determinations of $\alpha_s$ and those obtained from other methods provides a robust validation of this key QCD property.

Electroweak Precision Tests

Tau leptons play a vital role in precision electroweak measurements. Analyses of tau decay modes and tau production in $e^+e^-$ collisions offer constraints on the weak mixing angle $\sin^2\theta_W$ and the $Z$ boson couplings. These measurements are consistent with SM predictions and provide important constraints on potential new physics.

New Physics Search

The tau is a promising candidate for searches of physics beyond the SM due to its heavy mass and the potential for enhanced couplings to new heavy sectors. Tau-based searches for lepton-flavor-violating processes, such as $\tau \to \mu\gamma$ and $\tau \to 3\mu$, have set stringent limits on such processes, providing constraints on models with extended Higgs sectors or lepton-flavor violation unrelated to neutrino masses.

Challenges and Future Prospects

The comprehensive paper of tau physics continues at facilities like the LHC, Belle II, and potential future tau-charm factories. These programs aim to improve the precision of current measurements and search for rare decay modes with higher sensitivity, facilitating tests of models predicting deviations from the SM. Future tau experiments will continue to refine our understanding of electroweak processes, QCD, and the quest for undiscovered phenomena.

In summary, tau lepton physics encapsulates an intersection of various fundamental aspects of particle physics, providing key tests of the SM and fertile ground for exploration of new physics. The ongoing experimental programs hold promise for enhanced precision tests and potential discoveries that could reshape our understanding of fundamental particles and interactions.