Search for neutral heavy leptons produced in ZZ decays

Weak isosinglet Neutral Heavy Leptons (νm) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived νm production giving monojet or acollinear jet topologies, and for long-lived νm giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 → νmν̄) of about 1.3 × 10-6 at 95% confidence level for νm masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the νm mass. The results are also interpreted in terms of limits for the single production of excited neutrinos. © Springer-Verlag 1997

Measurement of the quark and gluon fragmentation functions in Z0 hadronic decays

The transverse, longitudinal and asymmetric components of the fragmentation function are measured from the inclusive charged particles produced in e(+)e(-) collisions at LEP. As in deep inelastic scattering, these data are important for tests of QCD. The transverse sigma(T) and longitudinal sigma(L) components of the total hadronic cross section sigma(tot) are evaluated from the measured fragmentation functions. They are found to be a sigma(T)/sigma(tot) = 0.949 +/- 0.001(stat.) +/- 0.007(syst.) and sigma(L)/sigma(tot) = 0.051 +/- 0.001(stat.) +/- 0.007(syst.) respectively. The strong coupling constant is calculated from sigma(L)/sigma(tot) in next-to-leading order of perturbative QCD, giving alpha(s)(M-Z) = 0.120 +/- 0.002(stat.) +/- 0.013(syst.) +/- 0.007(scale). Including non-perturbative power corrections :leads to alpha(s)(M-Z) = 0.101 +/- 0.002(stat.) +/- 0.013(syst.) +/- 0.007(scale). The measured transverse and longitudinal components of the fragmentation function are used to estimate the mean charged multiplicity, [n(ch)] = 21.21 +/- 0.01(stat.)+/- 0.20(syst.) The fragmentation functions and multiplicities in bb and light quark events are compared. The measured transverse and longitudinal components of the fragmentation function allow the gluon fragmentation function to be evaluated