Burgard, Dissertation, Fachbereich Physik der Universität Hamburg, 1995 :

"Electroweak Charm Physics on and near the Z0 Peak and a Search for D0D0


An analysis is presented to measure the forward-backward asymmetries of the processes e+e- → cc and e+e- → bb at energies on and near the Z0 peak. A total of 3.7 million hadronic decays of the Z0 boson were recorded with the OPAL detector at LEP between 1990 to 1994. In this sample, Z0 decays to heavy quarks are identified by reconstructing D* mesons. About 19400 D* mesons are found in five different decay modes. Using jetshape variables, decay length information and the scaled energy of the D* mesons, the flavour composition of the selected sample is determined. From the angular distribution of the charge weighted thrust axis, the charm and bottom asymmetries are measured in three different energy ranges on and around the Z0 peak. The results obtained are:

AcFB=-0.018 ±0.075 ± 0.012 AbFB=-0.015 ±0.192 ± 0.035 ⟨Ecms> = 89.55 GeV
AcFB= 0.077 ±0.016 ± 0.006 AbFB= 0.089 ±0.039 ± 0.017 ⟨Ecms> = 91.18 GeV
AcFB= 0.170 ±0.064 ± 0.018 AbFB=-0.274 ±0.160 ± 0.045 ⟨Ecms> = 92.91 GeV

where in each case the first quoted error represents the statistical and the second the systematic uncertainty.

This result is combined with other OPAL measurements of the heavy flavour asymmetries and the ratio between the effective left-handed and right-handed weak coupling constants of the charm quark is extracted. It is found that

|ccL| / |ccR| = 2.1 ± 0.3 .

A subsample of the reconstructed D* mesons is then used to search for D0D0 mixing. No evidence for the existence of this process is found and from the accuracy of this search a limit on the mixing parameter of

rM < 2.2% (90% c.l.)

is derived. Prospects for an improvement of this analysis are dicussed.

Finally, a new technique to search for D0D0 mixing is proposed, which does not rely on the exclusive reconstruction of D0 mesons. A study on Monte Carlo and OPAL data events is presented and the statistical power as well as the systematic uncertainties of the method are examined.