2022 Impact factor 4.4
Particles and Fields

Eur. Phys. J. C 21, 1-21 (2001)
DOI: 10.1007/s100520100714

Precision neutral current asymmetry parameter measurements from the Tau polarization at LEP

The OPAL Collaboration G. Abbiendi and et al.

Dipartimento di Fisica dell' Università di Bologna and INFN, 40126 Bologna, Italy

(Received: 5 March 2001 / Published online: 6 July 2001 -© Springer-Verlag / Società Italiana di Fisica 2001)

Measurements of the $\tau$ lepton polarization and forward-backward polarization asymmetry near the Z0 resonance using the OPAL detector are described. The measurements are based on analyses of $\tau \rightarrow {\rm e} \nu_e\nu_{\tau}$, $\tau\rightarrow \mu\nu_{\mu}\nu_{\tau}$, $\tau\rightarrow \pi\nu_{\tau}$, $\tau \rightarrow \rho\nu_{\tau}$ and $\tau\rightarrow {\rm a}_1\nu_{\tau}$ decays from a sample of 144,810 $\rm e^+e^-\rightarrow \tau^+\tau^-$ candidates corresponding to an integrated luminosity of 151 pb-1. Assuming that the $\tau$ lepton decays according to V-A theory, we measure the average $\tau$ polarization near $\sqrt{s} = {\rm M}_{\mathrm{Z}}$ to be $\langle P_{\tau}\rangle= (-14.10 \pm 0.73 \pm 0.55)\%$ and the $\tau$ polarization forward-backward asymmetry to be $\rm A_{\mathrm{pol}}^{\mathrm{FB}} = (-10.55 \pm 0.76 \pm 0.25)\%$, where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z0 interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters:

\begin{displaymath}{A}_{\tau} = 0.1456 \pm 0.0076 \pm 0.0057, {A}_{\mathrm e} = 0.1454 \pm 0.0108 \pm 0.0036.

These measurements are consistent with the hypothesis of lepton universality and combine to give ${\cal A}_{\ell} = 0.1455 \pm 0.0073$. Within the context of the Standard Model this combined result corresponds to $=0.23172 \pm 0.00092$. Combing these results with those from the other OPAL neutral current measurements yields a value of $=0.23211 \pm 0.00068$.

© Società Italiana di Fisica, Springer-Verlag 2001