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

[Abstract

A MEASUREMENT OF SIN2-THETA-W FROM THE CHARGE ASYMMETRY OF HADRONIC EVENTS AT THE Z0 PEAK

The weak mixing angle has been measured from the charge asymmetry of hadronic events with two different approaches using the DELPHI detector at LEP. Both methods are based on a momentum-weighted charge sum to determine the jet charge in both event hemispheres. In a data sample of 247 300 multihadronic Z0 decays a charge asymmetry of [Q(F)] - [Q(B)] = -0.0076 +/- 0.0012(stat.) +/- O.0005(exp. syst.) +/- 0.0014(frag.) and a raw forward-backward asymmetry of A(FB)raw = 0.0109 +/- 0.0020(stat.) +/- 0.0010(exp.syst.) +/- 0.0017(frag.) have been measured. This result corresponds to a value of sin2-theta(eff) = 0.2345 +/- 0.0030(exp.) +/- 0.0027(frag.) +/- sin2-theta(MSBAR) = 0.2341 +/- 0.0030(exp.) +/- 0.0027(frag.) and to sin2-theta(w) = 1 -m(w)2/m(z)2 = 0.2299 +/- 0.0030(exp.) +/- 0.0027(frag.) +/- 0.0028 (theor.). The experimental error is the quadratic sum of the statistical and the experimental systematic error and the theoretical error originates from a value of m(t) = 130 +/- 40 GeV/c2 and a range of m(H) from 45 GeV/c2 to 100 GeV/c2

A Measurement of sin**2 theta(W) from the charge asymmetry of hadronic events at the Z0 peak

Abstract View references (24)The weak mixing angle has been measured from the charge asymmetry of hadronic events with two different approaches using the DELPHI detector at LEP. Both methods are based on a momentum-weighted charge sum to determine the jet charge in both event hemispheres. In a data sample of 247 300 multihadronic Z0 decays a charge asymmetry of 〈QF〉 - 〈QB〉 = -0.0076±0.0012(stat.)±0.0005(exp. syst.)±0.0014(frag.) and a raw forward-backward asymmetry of Araw FB = -0.0109±0.0020(stat.)±0.0010(exp. syst.)±0.0017(frag.) have been measured. This result corresponds to a value of sinθeff=0.2345±0.0030(exp.)±0.0027(frag.) ,sin2θMS=0.2341±0.0030(exp.)±0. 0027(frag.) and to sin2θW=1-m2 W/m2 Z=0.2299± 0.0030(exp.)±0.0027(frag.)±0.0028(theor.). The experimental error is the quadratic sum of the statistical and the experimental systematic error and the theoretical error originates from a value of mt=130±40 GeV/c2 and a range of mH from 45 GeV/c2 to 1000 GeV/c2.0info:eu-repo/semantics/publishe

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

A SEARCH FOR NEUTRAL HIGGS PARTICLES IN Z(0) DECAYS

The search in DELPHI data for neutral Higgs bosons is described. No candidate for the Standard Model Higgs is seen in Z0 decays to H-0-nu-nuBAR, H-0-mu+-mu- or H-0-tau+-tau- after selections that proved efficient for finding simulated H-0. One remaining candidate for Z0 –> H-0e+ e- is consistent with background. Together with our earlier studies, these results restrict the H-0 mass to be above 38 GeV/c2 at the 95% confidence level. No signal is found for decays of Minimal Supersymmetric Standard Model neutral Higgs bosons to tau+-tau-. Limits are obtained for their decays to produce four jets

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

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