Guy Wilkinson

Abstract:

A study of the fragmentation properties of charm and bottom quarks into D mesons is presented. From 263 700 Z0 hadronic decays collected in 1991 with the DELPHI detector at the LEP collider, D0, D+ and D*+ are reconstructed in the modes K-π+, K-π+K+ and D0π+ followed by D0→K-π+, respectively. The fractional decay widths {Mathematical expression} are determined, and first results are presented for the production of D mesons from {Mathematical expression} and {Mathematical expression} events separately. The average energy fraction of D*± in charm quark fragmentation is found to be 〈XE(D*)〉c=0.487± 0.015 (stat)±0.005 (sys.). Assuming that the fraction of Ds and charm-baryons produced at LEP is similar to that around 10 GeV, the Z0 partial width into charm quark pairs is determined to be Γc/Γh=0.187±0.031 (stat)±0.023 (sys). The probability for a b quark to fragment into {Mathematical expression} or b-baryons is inferred to be 0.268±0.094 (stat)±0.100 (sys) from the measured probability that it fragments into a {Mathematical expression} or B-. © 1993 Springer-Verlag.

Abstract:

An analysis of the production of the Λ baryon in the hadronic decays of the Z0 is presented, based on about 993K multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. The differencial cross section of the Λ and the correlations between Λ and Λ produced in the same event are compared to current models, based both on string fragmentation and on cluster decay. The predictions of the string fragmentation model are found to give satisfactory agreements with the data, clearly better than those of the cluster model. © 1993.

Abstract:

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 θNR* and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields {Mathematical expression} where CA/CF is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and NC/NA, 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 CA/CF. The results are in agreement with the values expected from QCD:CA/CF=2.25, and NC/NA=3/8. © 1993 Springer-Verlag.

Abstract:

Based on a sample equivalent to 365 000 hadronic Z0 decays, the search in DELPHI data for pairs of leptons accompanied by a pair of charged particles is described. A total of 11 events were found in the electron channel, 9 in the muon channel and 7 in the tau channel. Results on lepton pairs with a radiated photon are also presented. The data from all channels are compatible with the expectations from standard processes. However, one event was found in the tau channel with an unusually high mass of the charged particle pair. © 1993.