Narrow Results

It is shown that the ratio of branching ratios in Higgs boson decay r = Γ(H → τ⁺τ^-)/Γ(H → μ⁺μ^-) in a simplified (T' × Z₂) model is (r)_{T' × Z2} ≃ 0.001. This prediction is on a footing with the successful one in (s, d) mixing, . This value for r differs from that of the minimal standard model by a very large factor (over 250,000) and can provide a smoking gun for (T' × Z₂) flavor symmetry.

The BaBar and Belle experiments together recorded a total sample of some 3 ×10⁹ tau lepton decays between the years 2000 and 2010. This enabled many new and improved measurements of tau hadronic decays to be made, and resulted in lower limits for branching fractions to rare and forbidden hadronic modes, such as those mediated by second-class weak currents which are sensitive to the u–d quark mass difference. Better measurements of strange decays have contributed to more precise data on the CKM matrix element, |V_us|. However, only limited progress has been made in measurements that feed into knowledge of the value of the strong coupling constant, α_s, and the strange quark mass. This is largely due to the dominance of systematic uncertainties on branching fractions and shapes of mass spectra (spectral functions) for some of the important hadronic tau decay modes.

Searches for Lepton Flavor Violation (LFV) stand at the forefront of experimental particle physics research, offering a sensitive probe to many scenarios of physics beyond the Standard Model. The high proton–proton collision energy and luminosity provided by the CERN Large Hadron Collider (LHC) and the excellent CMS detector performance allow for an extensive program of LFV searches. This paper reviews a broad range of LFV searches conducted at the CMS experiment using data collected in LHC Run 2, including $\tau \to 3\mu$  decays, Higgs boson decays, and top quark production and decays. In each analysis, the online and offline event selections, signal modeling, background suppression and estimation, and statistical interpretation are elucidated. These searches involve various final state particles in a large transverse momentum range, showcasing the capability of the CMS experiment in exploring fundamental questions in particle physics.

Precise measurements of the τ lepton properties provide stringent tests of the Standard Model structure and accurate determinations of its parameters. We overview the present status of a few selected topics: lepton universality, QCD tests and the determination of α_s, m_s and |V_us| from hadronic τ decays, and lepton flavor violation phenomena.

Some recent τ-physics results are presented from the BaBar and Belle experiments at the SLAC and KEK B factories, which produce copious numbers of τ-lepton pairs. Measurements of the tau mass and lifetime allow to test lepton universality and CPT invariance, while searches for lepton-flavour violation in tau decays are powerful ways to look for physics beyond the Standard Model. In semihadronic, non-strange tau decays, the vector hadronic final state is particularly important in helping determine the hadronic corrections to the anomalous magnetic moment of the muon, while studies of strange final states are the best available ways to measure the CKM matrix element V_us and the mass of the strange quark.

(τ⁺τ^-) is an atom of simple hydrogenlike structure similar to positronium (e^-e⁺) and (μ⁺μ^-). In this paper energy levels and decay widths of different decay channels of (τ⁺τ^-) are given. Cross-section of production of this atomic system in e^-e⁺ annihilation taking into account radiative corrections is calculated. According to our estimates 31 (τ⁺τ^-) atoms are produced at VEPP-4M and 943 (τ⁺τ^-) atoms may be produced at BEPCII under the present experimental conditions.

Using the massive helicity formalism, we calculate the five-body average square amplitude of the decays ${\tau }^{-}\to {\ell }_i^{-}{\ell }_i^{+}{\ell }_j^{-}{\bar{\nu }}_j{\nu }_{\tau }$$(\ell =e,\mu )$ within the Standard Model (SM), we then introduce a dimension-five effective vertex ${\mathrm{\Gamma }}^{\tau \tau \gamma }$ in order to determine the feasibility of imposing limits on the tau anomalous magnetic dipole moment $(a_{\tau })$ via the current or future experimental measurements of the branching ratio for the decay ${\tau }^{-}\to e^{-}{e}^{+}{e}^{-}{\bar{\nu }}_e{\nu }_{\tau }$.

In this paper, we present the major results obtained in $\tau$ physics by the Belle and Belle II experiments. For the Belle II experiment, we also discuss prospects for improved measurements of the $\tau$ lepton properties, new results in Michel parameters determination, and searches for CP and lepton flavor violation in $\tau$ decays.

We present here the CP asymmetries in the decay rate and angular distributions of $\tau \to K_S\pi {\nu }_{\tau }$ decays in the Standard Model (SM) and beyond (BSM). The CP asymmetries in the SM are induced by the CP violation in $K^0-{\bar{K}}^0$ mixing. To investigate the BSM CP-violating (CPV) effects, a model-independent analysis is performed by using the low-energy effective field theory (LEFT) framework at $\mu =2$ GeV. If one further assumes the BSM physics to stem from above the electroweak scale, the LEFT shall then be matched onto the SM effective field theory (SMEFT), the operators of which contributing to $\tau \to K_S\pi {\nu }_{\tau }$ decays will also contribute to the neutron electric dipole moment (EDM) and $D^0-{\bar{D}}^0$ mixing. The stringent bounds from the latter suggest that no remarkable CPV effects can be observed in either the decay rate or the angular distributions. The prospects for future measurements of these observables are also mentioned.

We present a new method for direct measurement of all Michel parameters in the ${\tau }^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }{\nu }_{\tau }$ decay related to the polarization of the daughter muon: ${\xi }^{\prime }$, ${\xi }^{\prime \prime }$, ${\eta }^{\prime \prime }$, ${\alpha }^{\prime }/A$, and ${\beta }^{\prime }/A$. The method is based on the reconstruction of the muon decays-in-flight in the tracker of the $e^{+}{e}^{-}$ collider experiments and relies on the correlation between muon spin and its daughter electron momentum. The low probability of decay-in-flight in the tracker volume is compensated by the unprecedented statistics of modern flavor factories, which ultimately leads to the possibility of achieving high accuracy. We discuss the application of the suggested method in the experiments at $e^{+}{e}^{-}$ colliders and present a feasibility study for the future Super Charm-Tau Factory experiment. We also discuss the result of the first use of the proposed method in the Belle experiment, confirming its applicability.

Ultraperipheral collisions of heavy ions give physicists an opportunity for studies of two-photon-induced processes in a clean environment with strongly suppressed hadronic interactions. In particular, measurements of cross-section of the process can be used to set new constraints on poorly known anomalous magnetic moment of tau lepton. Obtaining a precise experimental value for tau $g-2$ becomes especially important in view of new data from the Muon $g-2$ experiment and also for verification of Standard Model extensions, including supersymmetry and theories with composite leptons. With the ALICE experiment in Run 3 it will be possible to address low transverse momenta for reconstructed charged particles to increase the number of available events. We have carried out realistic simulations of the ALICE detector to develop event selection strategy for tau decay events. In this paper, we discuss the results of the simulations and feasibility of tau $g-2$ measurement with the ALICE experiment.

The Sun albedo of Cosmic Rays (CRs) at GeVs energy has been discovered recently by the FERMI satellite. They are traces of atmospheric CRs hitting solar atmosphere and reflecting skimming gamma photons. Even if relevant for astrophysics, as being a trace of atmospheric solar CR noises they cannot offer any signal of neutrino astronomy. On the contrary, the Moon with no atmosphere, may become soon a novel filtering calorimeter and an amplifier of energetic muon astronomical neutrinos (at TeV up to hundred TeVs energy); these lepton tracks leave an imprint in their beta decay while in flight to Earth. Their TeV electron air-shower are among the main signals. Also, a more energetic, but more rare, PeV up to EeV tau lunar neutrino events may be escaping as a tau lepton from the Moon: $\tau$ PeV secondaries, then, may be shining on Earth’s atmosphere in lunar shadows in a surprising way. One or a few gamma air-shower events inside the Moon shadows may occur each year in near future Cherenkov telescope array (CTA) or large high altitude air shower observatory (LHAASO) TeV gamma array detector, assuming a nonnegligible astrophysical TeV up to hundred TeV neutrino component (with respect to our terrestrial ruling atmospheric ones); these signals will open a new wonderful passe-partout keyhole for neutrino, been seen along the Moon. The lunar solid angle is small and the muon or tau expected rate is rare, but with the future largest tau radio array as the giant radio array for neutrino detection (GRAND), one might well discover such neutrino imprint.

We report recent results on τ lepton physics obtained with the Belle detector. They include a search for lepton-flavor-violating decays τ → lhh′, a high-precision measurement of the τ lepton lifetime and a study of the hadronic decays with the in the final state.

Since Run-1 of the LHC, CMS has taken the opportunity to improve further particle reconstruction. A number of improvements were made to the hadronic tau reconstruction and identification algorithms. In particular, the reconstruction of the tau decay products leaving deposits in the electromagnetic calorimeter was improved to better model signal of ${\pi }^0$ from $\tau$ decays. This modification improves energy response and removes the tau footprint from isolation area. In addition to this, improvements were made to discriminators that combine isolation and tau lifetime variables, and the rejection of electrons misidentified as hadronic taus was improved using multivariate techniques. The results of these improvements using 13 TeV data at LHC Run-2 are presented and validation of tau identification using a variety of techniques has been highlighted.

To test the Lorentz structure of the charged weak interaction, we study leptonic decays of tau lepton in the Belle experiment. We perform this test through the measurement of Michel parameters which appear in the differential decay widths of leptonic tau decays. In this paper, we discuss the preliminary results of the measurement of Michel parameters at Belle.

The use of the τ leptons in the Standard Model processes and in channels probing for Physics beyond the Standard Model is very important at the LHC. These processes include Higgs production, heavy mass resonances and decays of supersymmetric particles. In the selection of such rare events of interest, the hadronic τ trigger plays a fundamental role. It allows efficient collection of the desired signal events, while keeping the rate of background events within the allowed bandwidth. This contribution summarises the status and performance of the ATLAS tau trigger system during 2011 data taking period and shows the trigger efficiency curves obtained from data.