Tau Decays Beyond the Standard Model

In a recent paper, 8 semileptonic parameters were defined to specify the most general Lorentz-invariant spin correlation functions for 2-body τ decays. These parameters can be used to search for anomalous ΓL, ΓT polarized-partial widths, for non-CKM-type leptonic CP violation, and for leptonic T̃FS violation. They can also be used to bound the effective-mass scales Λ for “new physics” arising from additional Lorentz structures, e.g. from lepton compositeness, tau weak magnetism, weak electricity, or second-class currents. It is emphasized that (i) for these tests “different modes have different merits” and that (ii) the parameters can be measured either by using spin-correlation techniques without polarized beams, or with longitudinally polarized beams.