Measurement of the tau Michel parameters $\bar{\eta }$ and $\xi \kappa$ in the radiative leptonic decay ${\tau }^{-}\to {\ell }^{-}{\nu }_{\tau }{\bar{\nu }}_{\ell }\gamma$

Abstract

We present a measurement of the Michel parameters of the $\tau$ lepton, $\bar{\eta }$ and $\xi \kappa$, in the radiative leptonic decay ${\tau }^{-}\to {\ell }^{-}{\nu }_{\tau }{\bar{\nu }}_{\ell }\gamma$ using 711~f${\mathrm{b}}^{-1}$ of collision data collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The Michel parameters are measured in an unbinned maximum likelihood fit to the kinematic distribution of $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}\to ({\pi }^{+}{\pi }^0{\bar{\nu }}_{\tau })({\ell }^{-}{\nu }_{\tau }{\bar{\nu }}_{\ell }\gamma )$ $(\ell =e$ or $\mu )$. The measured values of the Michel parameters are $\bar{\eta }=-1.3\pm 1.5\pm 0.8$ and $\xi \kappa =0.5\pm 0.4\pm 0.2$, where the first error is statistical and the second is systematic. This is the first measurement of these parameters. These results are consistent with the Standard Model predictions within their uncertainties and constrain the coupling constants of the generalized weak interaction.

References in corpus (5)

• Tau and muon pair production cross-sections in electron-positron annihilations at sqrt{s} = 10.58 GeV
• Radiative $\mu$ and $\tau$ leptonic decays at NLO
• W-propagator corrections to muon and tau leptonic decays
• Michel parameters in radiative muon decay
• Measurement of the branching fractions of the radiative leptonic tau decays tau -> e gamma nu anti-nu and tau -> mu gamma nu anti-nu at BaBar

Cited by in corpus (4)

• Type-X two Higgs doublet model in light of the muon $\mathbf{g}-2$: confronting Higgs and collider data
• Impact of electroweak group representation in models for $B$ and $g-2$ anomalies from Dark Loops
• Study of the muon decay-in-flight in the ${\tau }^{-}\to {\mu }^{-}{\bar{\nu }}_{\mu }{\nu }_{\tau }$ decay to measure the Michel parameter ${\xi }^{\prime }$
• Dirac-Majorana neutrinos distinction in four-body decays