Chronology of Milestone Events in Particle Physics - ABREU 1991B
Chronology of Milestone Events in Particle Physics


DELPHI collaboration; Abreu, P. et al.
Determination of Zo Resonance Parameters and Couplings from its Hadronic and Leptonic Decays
Nucl. Phys. B367 (1991) 511;

From measurements of the cross section for e+ e- hadrons and the cross sections and forward-backward charge asymmetries for e+ e- e+ e-, + - and + - at several centre-of-mass energies around the Z0 pole with the DELPHI apparatus, using approximately 150000 hadronic and leptonic events from 1989 and 1990, one determines the following Z0 parameters: the mass and the total width MZ = 91.177 0.022 GeV, Z = 2.465 0.020 GeV, the hadronic and leptonic partial widths h = 1.726 0.019 GeV, l = 83.4 0.8 MeV, the invisible width inv = 488 17 MeV, the ratio of hadronic over leptonic partial widths Rz = 20.70 0.29 and the Born level hadronic peak cross section 0 = 41.84 0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (l = 83.7 0.8 MeV). From these results the number of light neutrinos species is determined to be N = 2.94 0.10. The individual leptonic width obtained is are: e = 82.4 1.2 MeV, = 86.9 2.1 MeV and = 82.7 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: V2l = 0.0003 0.0010 and A2l = 0.2508 0.0027. These values correspond to the electroweak parameters: eff = 1.003 0.011 and sin 2Weff = 0.241 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin 2Woverline MS = 0.2338 0.0027. All these values are in good agreement with the predictions of the MSM. Fits yield 43 < mt < 215 GeV at the 95% level. Finally, the measured values of Z and inv are used to derive lower mass bounds for possible new particles.

Accelerator CERN-LEP Detectors DELPHI

Related references
More (earlier) information appears in
DELPHI Coll., P. Aarnio et al., Phys. Lett. 241B (1990) 425;
S. Jadach et al., Phys. Lett. 260B (1991) 240;
DELPHI Coll., P. Abreu et al., Phys. Lett. 241B (1990) 435;

  e+ e 2hadron (hadrons) 88.22-95.35 GeV (Ecm) cs, const
  e+ e l+ l 88.22-94.23 GeV (Ecm) cs, asym, const
  e+ e e e+ 88.22-94.22 GeV (Ecm) cs, asym, const
  e+ e + 88.22-94.22 GeV (Ecm) cs, asym, const
  e+ e + 88.22-94.22 GeV (Ecm) cs, asym, const
  Zo e e+
  Zo +
  Zo +
  Zo 2hadron (hadrons)
  Zo l+ l

Particles studied Particles studied
  ex, mass, w, pw

Record comments
Confirmation of the number of light neutrinos = 3. DELPHI Collaboration.
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