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Register a new userLimits on excited tau leptons masses from leptonic tau decays
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Jorge Isidro Aranda-Sanchez
Publication date
2000
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and research's language is
English
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We study the effects induced by excited leptons on the leptonic tau decay at one loop level. Using a general effective lagrangian approach to describe the couplings of the excited leptons, we compute their contributions to the leptonic decays and use the current experimental values of the branching ratios to put limits on the mass of excited states and the substructure scale.
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The tau -> (omega, phi)P^- u decays, where P^-=pi^-, K^-, are considered within a phenomenological model with dominance of meson intermediate states. We assume SU(3) flavor symmetry to fix some of the unknown strong interaction couplings. Our predictions for the tau -> phi (pi^-, K^-) u branching fractions are in good agreement with recent measurements of the BABAR and BELLE Collaborations.
We derive the corrections induced by the W-boson propagator to the differential rates of the leptonic decay of a polarized muon and tau lepton. Results are presented both for decays inclusive of inner bremsstrahlung as well as for radiative ones, when a photon emitted in the decay process is measured. The numerical effect of these corrections is discussed. The definition of the Fermi constant is briefly reviewed.
The branching fractions of radiative leptonic $tau$ decays $(tau to l u bar{ u} gamma$, $l=e,mu)$ were recently measured by the Babar collaboration with a relative error of about 3%. The measurement of the branching ratio $mathcal{B} (tau to e bar{ u} u gamma)$, for a minimum photon energy of 10 MeV in the $tau$ rest frame, differs from our recent SM prediction by 3.5 standard deviations, whereas our result agrees with Babars value for $mathcal{B} (tau to mu bar{ u} u gamma)$. Our predictions also agree with the measurement of $mathcal{B} (mu to e bar{ u} u gamma)$ by the MEG collaboration. We also report on a recent proposal to test the $tau$ dipole moments via precise measurements of radiative leptonic $tau$ decays at high-luminosity $B$ factories.
The cross sections and polarization components of the $tau$ leptons produced in the charged current induced quasielastic $ u_tau~(bar u_tau) - N$ scattering have been studied. The theoretical uncertainties arising due to the use of different vector form factors and the axial dipole mass in the axial vector form factor have been investigated. Due to the high mass of $tau$ lepton, the contributions from the term containing pseudoscalar and second class current form factors are non-negligible and contribute to the uncertainty in the cross section and polarization observables as these form factors are not well known. In view of the currently proposed experiments by DUNE, SHiP and DsTau collaborations to study the production of $tau$ lepton, an updated calculation of the cross sections and polarizations of tau leptons in the case of quasielastic production have been done and the numerical results have been presented along with a discussion of the theoretical uncertainties.
Hadronic tau decays offer the possibility of determining the strong coupling alpha_s at relatively low energy. Precisely for this reason, however, good control over the perturbative QCD corrections, the non-perturbative condensate contributions in the framework of the operator product expansion (OPE), as well as the corrections going beyond the OPE, the duality violations (DVs), is required. On the perturbative QCD side, the contour-improved versus fixed-order resummation of the series is still an issue, and will be discussed. Regarding the analysis, self-consistent fits to the data including all theory parameters have to be performed, and this is also explained in some detail. The fit quantities are moment integrals of the tau spectral function data in a certain energy window and care should be taken to have acceptable perturbative behaviour of those moments as well as control over higher-dimensional operator corrections in the OPE.
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