Electroweak parameters of the z0 resonance and the standard model

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The reaction e+ e- ---> gamma gamma (gamma) at Z0 energies

The total and differential cross-sections for the reaction e+e- → γγ(γ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb-1. The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ+ > 113 GeV and Λ- > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z0 decay into π0γ, ηψ and γγγ are below 1.5 × 10-4, 2.8 × 10-4 and 1.4 × 10-4 respectively.0info:eu-repo/semantics/publishe

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

DETERMINATION OF Z0 RESONANCE PARAMETERS AND COUPLINGS FROM ITS HADRONIC AND LEPTONIC DECAYS

From measurements of the cross sections for e+ e - –> hadrons and the cross sections and forward-backward charge-asymmetries for e+ e- –> e+ e-, mu+ mu- and tau+ tau- 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 total width M(Z) = 91.177 +/- 0.022 GeV, GAMMA(Z) = 2.465 +/- 0.020 GeV, the hadronic and leptonic partial widths GAMMA(h) = 1.726 +/- 0.019 GeV, GAMMA(t) = 83.4 +/- 0.8 MeV, the invisible width GAMMA(inv) = 488 +/- 17 MeV, the ratio of hadronic over leptonic partial widths R(Z)z = 20.70 +/- 0.29 and the Born level hadronic peak cross section sigma-0 = 41.84 +/- 0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (GAMMA(t) = 83.7 +/- 0.8 MeV). From these results the number of light neutrino species is determined to be N(nu) = 2.94 +/0. 10. The individual leptonic widths obtained are: GAMMA(e) = 82.4 +/- 1.2 MeV, GAMMA(mu) = 86.9 +/- 2.1 MeV and GAMMA(tau) = 82.7 +/- 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: VBAR(t)2 = 0.0003 +/- 0.0010 and ABAR(l)2 = 0.2508 +/- 0.0027. These values correspond to the electroweak parameters: rho(eff) = 1.003 +/- 0.011 and sin2-theta(W)eff = 0.241 +/- 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin2-theta(W)MSBAR = 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 GAMMA(Z) and GAMMA(inv), are used to derive lower mass bounds for possible new particles

MEASUREMENT OF THE PARTIAL WIDTH OF THE Z0 INTO B(B)OVER-BAR FINAL-STATES USING THEIR SEMILEPTONIC DECAYS

The spectra of prompt electrons and muons from the semi-leptonic decays of heavy hadrons produced in Z0 decays have been used to measure the coupling of the Z0 to b quarks weighted by the B hadrons mean semi-leptonic branching fraction, giving a value: BR(sl)b*GAMMA(bbBAR)/GAMMA(H)=0.0221+/-0.0015. The data has also been used to measure the value of the fragmentation parameter, defined in the context of the LUND PS Model, version 7.2, giving: epsilon(b)=(8(+5/-3)+/-2) 10(-3). The corresponding value of the mean fraction of the beam energy taken by a B hadron in the fragmentation of a b quark is: X(E)bBAR=0.69(+0.02/-0.03)+/-0.01. If the values of GAMMA(bbBAR) and GAMMA(H) are taken from the Standard Model, the following value is obtained for the mean semi-leptonic braching fraction of B hadrons: BR(sl)b=(10.1+/-0.7)%. Taking the value of GAMMA(bbBAR)/GAMMA(H) from an independent analysis of DELPHI data based on the use of the boosted sphericity product, a value: BR(sl)b=(10.1+/-1.3)% is obtained

MEASUREMENT OF THE TRIPLE-GLUON VERTEX FROM 4-JET EVENTS AT LEP

From the combined data of 1990 and 1991 of the DELPHI experiment at LEP, 13057 4-jet events are obtained and used for determining the contribution of the triple-gluon vertex. The relevant variables are the generalized Nachtmann Reiter angle theta(NR)* and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields C(A)/C(F)=2.12+/-0.35 and N(C)/N(A)=0.46+/-0.19, where C(A)/C(F) is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and N(C)/N(A), the ratio of the number of quark colours to the number of gluons. This constitutes a convincing model-independent proof of the existence of the triple-gluon vertex, since its contribution is directly proportional to C(A)/C(F). The results are in agreement with the values expected from QCD: C(A)/C(F)=2.25, and N(C)/N(A)=3/8

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