Search for the t and b′ quarks in hadronic decays of the Z0 boson

[Abstract

EXPERIMENTAL-STUDY OF THE TRIPLE-GLUON VERTEX

In four-jet events from e+e- –> Z0 –> multihadrons one can separate the three principal contributions from the triple-gluon vertex, double gluon-bremsstrahlung and the secondary quark-antiquark production, using the shape of the two-dimensional angular distributions in the generalized Nachtmann-Reiter angle theta-NR* and the opening angle of the secondary jets. Thus one can identify directly the contribution from the triple-gluon vertex without comparison with a specific non-QCD model. Applying this new method to events taken with the DELPHI-detector we get for the ratio of the colour factor N(C) to the fermionic Casimir operator C(F): N(C)/C(F) = 2.55 +/- 0.55(stat.) +/- 0.4(fragm. + models) +/- 0.2(error in bias) in agreement with the value 2.25 expected in QCD from N(C) = 3 and C(F) = 4/3

A STUDY OF THE REACTION E+E--]MU+MU- AROUND THE Z0 POLE

Measurements of the cross section and forward-backward asymmetry for the reaction e+e- –> mu+mu- using the DELPHI detector at LEP are presented. The data come from a scan around the Z0 peak at seven centre of mass energies, giving a sample of 3858 events in the polar angle region 22-degrees < theta < 158-degrees. From a fit to the cross section for 43-degrees < theta < 137-degrees, a polar angle region for which the absolute efficiency has been determined, the square root of the product of the Z0 –> e+e- and Z0 –> mu+mu- partial widths is determined to be (GAMMA-e-GAMMA-mu)1/2 = 85.0 +/- 0.9 (stat.) +/- 0.8 (syst.) MeV. From this measurement of the partial width, the value of the effective weak mixing angle is determined to be sin2(theta-w) = 0.2267 +/- 0.0037. The ratio of the hadronic to muon pair partial widths is found to be GAMMA-h/GAMMA-mu = 19.89 +/- 0.40 (stat.) +/- 0.19 (syst.). The forward-backward asymmetry at the resonance peak energy E(CMS) = 91.22 GeV is found to be A(FB) = 0.028 +/- 0.020 (stat.) +/- 0.005 (syst.). From a combined fit to the cross section and forward-backward asymmetry data, the products of the electron and muon vector and axial-vector coupling constants are determined to be V(e)V-mu = 0.0024 +/- 0.0015 (stat.) +/- 0.0004 (syst.) and A(e)A-mu = 0.253 +/- 0.003 (stat.) +/- 0.003 (syst.). These results are in good agreement with the expectations of the minimal standard model

MEASUREMENT OF THE PARTIAL WIDTH OF THE DECAY OF THE Z0 INTO CHARM QUARK PAIRS

A determination of the partial width gamma-cc-BAR of the Z0 boson into charm quark pairs is presented, based on a total sample of 36 900 Z0 hadronic decays measured with the DELPHI detector at the LEP collider. The production rate of ccBAR events is derived from the inclusive analysis of charged pions coming from the decay of charmed meson D*+ –> D0-pi+ and D*- –> DBAR0-pi- where the pi- +/- is constrained by kinematics to have a low p(T) with respect to the jet axis. The probability to procedure these pi- +/- from D* +/- decay in ccBAR events is taken to be 0.31 +/- 0.05 as measured at square-root s = 10.55 GeV. The measured relative partial width GAMMA-ccBAR/GAMMA-h = 0.162 +/- 0.030 (stat.) +/- 0.050(syst.) is in good agreement with the standard model value of 0.171. Together with our previous measurement of the total hadronic width GAMMA-h this implies GAMMA-ccBAR = 282 +/- 53(stat.) +/- 88(syst.) MeV