ALBERT

Notes: Abstract. We study $\pi\pi$ correlations in the exclusive reaction $\mathrm{\bar{p}p} \to 2\pi^+2\pi^-$ at rest measured by the CPLEAR experiment. Avoiding the introduction of an arbitrary reference sample, we analyse differential distributions for equal charge pion pairs removing the phase-space factor event by event. A peak at small relative momenta is most pronounced for large total momentum of the pair. The physical implications of bosonic symmetrization for the properties of the pion source, in particular its radius, are briefly discussed. The two extremes considered are the chaotic Hanbury-Brown–Twiss mechanism and the coherent Skyrmion model.

Notes: Abstract. The WA97 experiment has measured the transverse mass $({m_{\mathrm T}})$ spectra for negative hadrons ( $\mathrm{h}^-$ ) and strange particles produced at mid–rapidity in Pb–Pb collisions. The increased statistics of analysed data samples allowed us to perform a study of the spectra of ${\rm K_S^0}$ , $\mathrm{\Lambda}$ , $\mathrm{\Xi}$ , $\mathrm{\Omega}$ and $\mathrm{h}^-$ as a function of the collision centrality. The data, which correspond to the most central 40% of the total inelastic cross section, have been divided into four centrality classes according to the estimated number of nucleons taking part in the collision. The $m_{\mathrm T}$ spectra, analysed separately for each centrality bin, exhibit only weak ( $\leq$ 15%) centrality dependence. The deviation of the $\mathrm{\Omega}$ inverse slope from the linear dependence on the particle mass is confirmed even for the most central Pb–Pb collisions.

Notes: Abstract. We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993–95. A total luminosity of $103 {\rm pb}^{-1}$ was collected at centre-of-mass energies $\sqrt s \approx m_{\rm Z}$ and $\sqrt s \approx m_{\rm Z} \pm 1.8{\rm GeV}$ which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990–92, we obtain the final results: \begin{eqnarray*} m_{\rm Z} = 91\,189.8 \pm 3.1\,{\rm MeV}\, , & & \Gamma_{\rm Z} = 2\,502.4 \pm 4.2\,{\rm MeV} \, , \Gamma_{\rm had} = 1\,751.1 \pm 3.8\,{\rm MeV} \, , & & \Gamma_{\ell} = 84.14 \pm 0.17\,{\rm MeV} \,. \end{eqnarray*} An invisible width of $\Gamma_{\rm inv} = 499.1\pm 2.9\,{\rm MeV}$ is derived which in the Standard Model yields for the number of light neutrino species $N_{\nu} = 2.978\pm 0.014$ . Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be $\bar g^{\ell}_{\rm V}= -0.0397\pm 0.0017$ and $\bar g^{\ell}_{\rm A} = -0.50153 \pm 0.00053$ . Including our measurements of the ${\rm Z}\rightarrow{\rm b\bar b}$ forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of $\sin^2 \bar \theta_{\rm W} = 0.23093 \pm 0.00066$ is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of $m_{\rm H} 〈 133$ GeV is set at 95% confidence level.

Notes: Abstract. The $e^+$ -jet invariant mass spectrum produced in the reaction $e^+ p \rightarrow e^+ X$ has been studied at a center-of-mass energy of 300 GeV. The data were collected using the ZEUS detector operating at the HERA collider, and correspond to an integrated luminosity of $47.7$ pb $^{-1}$ . The observed mass spectrum is in good agreement with Standard Model expectations up to an $e^+$ -jet mass of $210$ GeV. Above this mass, some excess is seen. The angular distribution of these events is typical of high- $Q^2$ neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state. Limits are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark or R-parity-violating squark production. Specific leptoquark types are ruled out at 95% confidence level for coupling strength $\lambda=0.3$ for masses between 150 and 280 GeV.

Notes: Abstract. This paper describes a measurement of the Michel parameters, $\eta$ , $\rho$ , $\xi$ , $\xi \delta$ , and the average $\nu_{\tau}$ helicity, $h_{\nu_{\tau}}$ , in $\tau$ lepton decays together with the first measurement of the tensor coupling in the weak charged current. The $\tau^{+} \tau^{-}$ pairs were produced at the LEP $\mathrm{e^{+} e^{-}}$ collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the $\tau$ the measured values of the parameters were: $\eta = -0.005 \pm 0.036 \pm 0.037$ , $\rho = 0.775 \pm 0.023 \pm 0.020$ , $\xi = 0.929 \pm 0.070 \pm 0.030$ , $\xi\delta = 0.779 \pm 0.070 \pm 0.028$ , $h_{\nu_{\tau}} = -0.997 \pm 0.027 \pm 0.011$ . The strength of the tensor coupling was measured to be $\kappa^W_\tau = -0.029 \pm 0.036 \pm 0.018$ . The first error is statistical and the second error is systematic in all cases. The results are consistent with the $V\!-\!A$ structure of the weak charged current in decays of the $\tau$ lepton.

Notes: Abstract. We have studied hadronic events from $\mathrm{e^+e^-}$ annihilation data at centre-of-mass energies of $\sqrt{s}=$ 172, 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb $^{-1}$ . We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling $\alpha_s $ , the mean charged particle multiplicity $\langle n_{\mathrm{ch}}\rangle$ and the peak position $\xi_0$ in the $\xi_p=\ln(1/x_p)$ distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of $\alpha_s $ , $\langle n_{\mathrm{ch}}\rangle$ and $\xi_0$ are consistent with previous determinations at $\sqrt{s}=M_{\mathrm{Z^0}}$ .

Notes: Abstract. Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z $^0$ hadronic decays are studied as a function of the three components of the momentum difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r $_{t_{side}}$ , and longitudinal, r $_l$ , dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y $_{cut}$ = 0.04, are r $_{t_{side}}$ = (0.809 $\pm$ 0.009 (stat) $^{+0.019}_{-0.032}$ (syst)) fm, r $_l$ = (0.989 $\pm$ 0.011 (stat) $^{+0.030}_{-0.015}$ ({\it syst})) fm and r $_l$ /r $_{t_{side}}$ = 1.222 $\pm$ 0.027 (stat) $^{+0.075}_{-0.012}$ (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.

Notes: Abstract. The production of single photons has been studied in the reaction $e^+e^-\rightarrow\gamma+invisible particles$ at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multi-photon final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb $^{-1}$ and 158 pb $^{-1}$ at the two energies. The number of light neutrino families is measured to be $2.84\pm0.15(stat)\pm0.14(syst)$ . The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and composite models. A limit on the gravitational scale is also determined.

Notes: Abstract. The data recorded at a centre-of-mass energy of 189 \GeV by the OPAL detector at LEP are used to search for trilinear couplings of the neutral gauge bosons in the process ${\rm e}^+{\rm e}^-\rightarrow{\rm Z\gamma}$ . The cross-sections for multihadronic events with an energetic isolated photon, and for events with a high energy photon accompanied by missing energy are measured. These cross-sections and the photon energy, polar angle and isolation angle distributions are compared to the Standard Model predictions and to the theoretical expectations of a model allowing for ${\rm Z\gamma Z}$ and ${\rm Z\gamma\gamma}$ vertices. Since no significant deviations with respect to the Standard Model expectations are found, constraints are derived on the strength of neutral trilinear gauge couplings.

Notes: Abstract. We have determined branching ratios for antiproton annihilations at rest on protons or neutrons in liquid deuterium which we compare to frequencies of isospin-related processes in antiproton-proton annihilations. Using the annihilation rates into $\pi^0\pi^0$ and $\pi^-\pi^0$ where the annihilation took place on the proton or neutron, respectively, we discuss the fraction of S-wave and P-wave annihilation in liquid D $_2$ . The frequencies for $\pi^-\omega$ and $\rho^-\pi^0$ , and $\pi^-\eta$ and $\pi^-\eta'$ and the corresponding frequencies for $\overline{\mbox p}$ p annihilations are used to determine isoscalar and isovector contributions to the protonium wave function. The isospin decomposition of the annihilating $\overline{\mbox p}$ p system in the $^3$ S $_1$ or $^1$ S $_0$ state is consistent with both, pure $\overline{\mbox p}$ p initial wave function and with the predictions of $\overline{\mbox N}$ N potential models. For the $^3$ P $_0$ state of the $\overline{\mbox p}$ p atom we find consistency with a pure $\overline{\mbox p}$ p system at annihilation while $\overline{\mbox N}$ N potential models predict large $\overline{\mbox n}$ n contributions. We observe $\rho$ - $\omega$ interference in $\overline{\mbox p}$ p $\rightarrow \pi^+\pi^-\eta$ and $\pi^+\pi^-\pi^0$ annihilation which we compare to $\rho$ - $\omega$ interference in $e^+e^-$ annihilation. The interference patterns show striking similarities due to similar phase relations; the interference magnitude depends on the $\omega$ - $\rho$ production ratio. The similarity of the phase in all 3 data sets demonstrates that isovector and isoscalar parts of the protonium ( $\overline{\mbox p}$ p atomic) wave function are relatively real, again in conflict with $\overline{\mbox N}$ N potential models. The annihilation rate for $\overline{\mbox p}$ d $\rightarrow$ K $^-$ K $^0$ p confirms the dominance of the isovector contribution to $\overline{\mbox N}$ N $\rightarrow$ K $\overline{\mbox K}$ annihilations. No complications due to initial state interactions are required by the data. Furthermore, we searched for narrow quasinuclear bound states close to the $\overline{\mbox N}$ N threshold, also predicted by $\overline{\mbox N}$ N potential models, but with negative outcome. We conclude that $\overline{\mbox N}$ N potential models are not suited to provide insight into the dynamics of the annihilation process.