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

First evidence of hard scattering processes in single tagged gamma-gamma-collisions

For the first time, multihadronic production from single tagged gamma gamma collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared ([Q(2)] = 0.06 (GeV/c(2))(2)). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (rho, omega or pi), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these structure functions were used and the predictions compared with the data. This study confirms recent results from no-tag experiments in requiring a QCD-based component to successfully describe the data, indicating that the photon has a significant partonic content

Production characteristics of K-0 and light meson resonances in hadronic decays of the Z(0)

An analysis of inclusive production of KO and the meson resonances K*(+/-)(892), rho(0)(770), f(0)(975) and f(2)(1270) in hadronic decays of the Z(0) is presented, based on about 973,000 multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. Overall multiplicities have been determined as 1.962 +/- 0.060 K-0 mesons, 0.712 +/- 0.067 K*(+/-)(892) and 1.21 +/- 0.15 rho(0)(770) per hadronic Z(0) decay. The average multiplicities of f(0)(975) for scaled momentum, x(p) = p/p(beam), in the range 0.05 less than or equal to x(p) less than or equal to 0.6 and of f(2)(1270) for 0.05 less than or equal to x(p) less than or equal to 1.0 are 0.098 +/- 0.016 and 0.170 +/- 0.043 respectively. The f(0)(975) and rho(0)(770) x(p)-spectra have p similar shapes. The f(2)(1270)/rho(0)(770) ratio increases with x(p). The average multiplicities and the differential cross sections are compared with the JETSET Parton Shower model. The model with default parameters fails to reproduce the experimental K-0 momentum spectrum at low momentum, describes the K*(+/-)(892) and rho(0)(770) x(p)-spectrum shapes, but significantly overestimates their production rates

MEASUREMENT OF THE AVERAGE LIFETIME OF B-HADRONS

The average lifetime of B hadrons produced in hadronic Z0 decays has been measured with the DELPHI detector at LEP. The measurement is based on the analysis of the impact parameter distribution of high p(t) muons and hadrons. The resulting mean B lifetimes are tau(B) = (1.30 +/- 0.10 +/- 0.08) ps and tau(B) = (1.27 +/- 0.04 +/- 0.12) ps respectively, giving a combined value of tau(B) = (1.28 +/- 0.10) ps. The hadronic sample was also used to measure the partial Z0 width GAMMA(bbBAR)/GAMMA(h) and gave a value of 0.222(-0.031)+0.033 +/- 0.017

STUDY OF ORIENTATION OF 3-JET EVENTS IN Z(0) HADRONIC DECAYS USING THE DELPHI DETECTOR

The study of the orientation of three-jet events from e+ e- –> Z0 –> multi-hadrons is presented, in particular the polar angle distributions of the thrust axis and of the normal to the three-jet plane, and the azimuthal correlations between the hadron plane and the one defined by the beam and thrust axes. The data are compared with results at lower energy and with QCD predictions. Good agreement with QCD predictions is observed. The scalar gluon theory is excluded by the data

ENERGY ENERGY CORRELATIONS IN HADRONIC FINAL-STATES FROM Z0 DECAYS

We have studied the energy-energy angular correlations in hadronic final states from Z0 decay using the DELPHI detector at LEP. From a comparison with Monte Carlo calculations based on the exact second order QCD matrix element and string fragmentation we find that Lambda-MS(5) = 104(-20)+25(stat.)-20(+25)(syst.)-00(+30)(theor.) MeV, which corresponds to alpha-s(91 GeV) = 0.106 +/- 0.003(stat.) +/- 0.003(syst.)-0.000(+0.003)(theor.). The theoretical error stems from different choices for the renormalization scale of alpha-s. In the Monte Carlo simulation the scale of alphA-s as well as the fragmentation parameters have been optimized to described reasonably well as aspects of multihadron production