Determination of Z 0 resonance parameters and couplings from its hadronic and leptonic decays

• Published in: Nuclear physics. B 1991, Vol.367 (3), p.511-574
• Main Authors: Adye, T., Allen, P., Almehed, S., Alted, F., Alvsvaag, S.J., Amaldi, U., Anassontzis, E., Apel, W.-D., Apsimon, R.J., Astier, P., Barao, F., Barring, O., Begalli, M., Blyth, S., Booth, P.S.L., Bugge, L., Buran, T., Burmeister, H., Caccia, M., Campagne, J.-E., Cavallo, F.R., Cerrito, L., Charpentier, P., Couchot, F., Crawley, H.B., Crosetti, G., Crosland, N., Dalmagne, B., Darbo, G., Daubie, E., David, P., De Beer, M., De Boeck, H., Di Ciaccio, L., Dijkstra, H., Dulinski, W., Ekspong, G., Elliot Peisert, A., Frodesen, A.G., Fulda-Quenzer, F., Fuster, J., Gavillet, P., Gracco, V., Guy, J., Hahn, M., Holmes, R., Holthuizen, D., Honore, P.F., Hooper, J.E., Hultqvist, K., Jonker, M., Katsoufis, E.C., Keranen, R., Khovanski, N.N., Klein, H., Klempt, W., Kopf, M., Lamsa, J.W., Lemonne, J., Loerstad, B., Lozano, J.J., Marin, J.-C., Mathis, L., Mitaroff, W.A., Moenig, K., Morettini, P., Muller, H., Nau-Korzen, U., Nielsen, B.S., Ostankov, A., Pape, L., Pyyhtia, J., Range, W.H., Ratoff, P.N., Redaelli, N.G., Ronchese, P., Rybicki, K., Sanchez, J., Smith, G.R., Stavropoulos, G., Stocchi, A., Strub, R., Tchikilev, O., Tilquin, A., Trischuk, W., Tsirou, A., Turluer, M.-L., Tyapkin, I.A., Ueberschaer, S., Uvarov, V.A., Valls Ferrer, J.A., Van Doninck, W.K., Vegni, G., Velasco, J., Walck, C., Waldner, F., Wetherell, A.M., Yepes, P., Zevgolatakos, E., Zimin, N.I.
• Format: Article
• Language: English
• Subjects: High Energy Physics - Experiment; Physics
• ISSN: 0550-3213; 1873-1562
• DOI: 10.1016/0550-3213(91)90007-K

From measurements of the cross sections for e + e −→ hadrons and the cross sections and forward-backward charge-asymmetries for e e −→ e + e −, μ + μ − and π +π − at several centre-of-mass energies around the Z 0 pole with the DELPHI apparatus, using approximately 150 000 hadronic and leptonic events from 1989 and 1990, one determines the following Z 0 parameters: the mass and total width M Z = 91.177 ± 0.022 GeV, Γ Z = 2.465 ± 0.020 GeV, the hadronic and leptonic partial widths Γ h = 1.726 ± 0.019 GeV, Γ l = 83.4 ± 0.8 MeV, the invisible width Γ inv = 488 ± 17 MeV, the ratio of hadronic over leptonic partial widths R Z = 20.70 ± 0.29 and the Born level hadronic peak cross section σ 0 = 41.84±0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above ( Γ l = 83.7 ± 0.8 rmMeV). From these results the number of light neutrino species is determined to be N v = 2.94 ±0.10. The individual leptonic widths obtained are: Γ e = 82.4±_1.2 MeV, Γ u = 86.9±2.1 MeV and Γ τ = 82.7 ± 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z 0 to charged leptons are: V ̄ l 2 = 0.0003±0.0010 and A ̄ l 2 = 0.2508±0.0027 . These values correspond to the electroweak parameters: ϱ eff = 1.003 ± 0.011 and sin 2 θ W eff = 0.241 ± 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin 2θ W M ̄ S = 0.2338 ± 0.0027 . All these values are in good agreement with the predictions of the MSM. Fits yield 43< m top < 215 GeV at the 95% level. Finally, the measured values of Γ Z and Γ inv are used to derived lower mass bounds for possible new particles.