Showing papers by "Claudia Ciocca published in 1998"

Photon and light meson production in hadronic Z\(^0\)decays

Abstract: The inclusive production rates and differential cross-sections of photons and mesons with a final state containing photons have been measured with the OPAL detector at LEP. The light mesons covered by the measurements are the $\pi^0$ , $\eta$ , $\rho(770)^{\pm}$ , $\omega(782)$ , $\eta'(958)$ and a $_0(980)^{\pm}$ . The particle multiplicities per hadronic Z $^0$ decay, extrapolated to the full energy range, are: \begin{eqnarray} \langle n_{\gamma} \rangle & = & 20.97 \pm$ 0.02 \pm1.15 , onumber \\ \langle n_{\pi^0} \rangle & = & 9.55 \pm 0.06 \pm 0.75 , onumber \\ \langle n_{\eta} \rangle & = & 0.97 \pm 0.03 \pm 0.11 , onumber\\ \langle n_{\rho^{\pm}} \rangle & = & 2.40 \pm 0.06 \pm 0.43 , onumber \\ \langle n_{\omega} \rangle & = & 1.04 \pm 0.04 \pm 0.14 , onumber \\ \langle n_{\eta'} \rangle & = & 0.14 \pm 0.01 \pm 0.02 , onumber \\ \langle n_{{\mathrm a}_0^{\pm}} \rangle & = & 0.27 \pm 0.04 \pm 0.10 , onumber \end{eqnarray} where the first errors are statistical and the second systematic. In general, the results are in agreement with the predictions of the JETSET and HERWIG Monte Carlo models.

Bose-Einstein correlations of three charged pions in hadronic Z0 decays

Abstract: Bose-Einstein Correlations (BEC) of three identical charged pions were studied in $4 \times 10^6$ hadronic ${\rm Z}^0$ decays recorded with the OPAL detector at LEP. The genuine three-pion correlations, corrected for the Coulomb effect, were separated from the known two-pion correlations by a new subtraction procedure. A significant genuine three-pion BEC enhancement near threshold was observed having an emitter source radius of $r_3 = 0.580 \pm 0.004 \mbox{ (stat.)} \pm 0.029 \mbox{ (syst.)} \mbox{ fm}$ and a strength of $\lambda_3 = 0.504 \pm 0.010 \mbox{ (stat.)}\pm 0.041 \mbox{ (syst.)}$ . The Coulomb correction was found to increase the $\lambda_3$ value by $\sim $ 9% and to reduce $r_3$ by $\sim $ 6%. The measured $\lambda_3$ corresponds to a value of $0.707 \pm 0.014 \mbox{ (stat.)} \pm 0.078 \mbox{ (syst.)}$ when one takes into account the three-pion sample purity. A relation between the two-pion and the three-pion source parameters is discussed.

An upper limit for the \(\tau\)–neutrino mass from \(\tau\rightarrow 5\pi^{\pm}\nu_{\tau}\) decays

Abstract: An upper limit for the \(\tau\)–neutrino mass has been determined from the decay \(\tau\rightarrow 5\pi^{\pm} u_{\tau}\) using data collected with the OPAL detector from 1991 to 1995 in \(\rm e^{+}e^{-}\) collisions at \(\sqrt{s}\approx M_\mathrm{Z}\). A limit of 43.2 MeV at 95% CL is obtained using a two–dimensional method in the \(5\pi\) invariant mass and energy distribution from 22 selected events. Combining this result with OPAL's previously published measurement using \(\rm \tau^{+}\tau^{-}\rightarrow 3h^{\pm}\bar{ u}_{\tau}+3h^{\mp} u_{\tau}\) decays, a new combined limit of \(m_{ u_{\tau}}<27.6\) MeV (95% CL) is obtained.