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

A measurement of B+ and B-0 lifetimes using (D)over-bar-l(+) events

A measurement of B meson lifetimes is presented using data collected from 1991 to 1993 by the DELPHI detector at the LEP collider, Samples of events with a D meson and a lepton in the same jet are selected where ($) over bar D(0)l(+) and D*(-)l(+) events originate mainly from the semileptonic decays of B+ and B-0 mesons, respectively. From the reconstructed B decay length and the estimated B momentum, taking into account the dilution due to B decays into ($) over bar D**l(+)v, the following B meson lifetimes and lifetime ratio are measured: tau(B+) = 1.61(-0.16)(+0.16) (stat.) +/- 0.12 (syst.) ps tau(B-0) = 1.61(-0.13)(+0.14) (stat.) +/- 0.08 (syst.) ps tau (B+)/tau(B-0) = 1.00(-0.15)(+0.17) (stat.) +/- 0.10 (syst.) and an average lifetime of B+ and B-0 mesons is obtained: tau(B) = 1.61(-0.07)(+0.08) (stat.) +/- 0.05 (syst.) p

Measurement of the partial decay width R(b)(0)=Gamma(b(b)over-bar)/Gamma(had) of the Z with the DELPHI detector at LEP

The partial decay width of the Z to quark pairs has been measured by the DELPHI detector at LEP. B-hadrons, containing b-quarks, were tagged by several methods using tracks with large impact parameters to the primary vertex complemented sometimes by event shape variables or using leptons with high transverse momentum relative to the hadron. In order to reduce the systematic uncertainties, in all methods the b-tagging efficiency has been extracted directly from the data. Combining all methods, the value: Gamma()/Gamma(had) = 0.2216 +/- 0.0016(stat.) +/- 0.0021(syst.) was found, where the production fraction was fixed to its Standard Model value