Study of Charm Production in Z Decays

TL;DR

This ALEPH study precisely measures charm production in Z decays, decomposing D*+ production into charm hadronisation, b-hadron decays, and gluon splitting. It determines the D*+ fragmentation probability, the average D*+ energy fraction in Z→cc events, and the gluon-splitting rate to charm via a two-step fragmentation fit and a hemisphere-mass analysis. The paper further reports ground-state charm production fractions, the vector-to-total charm-state ratio, Ds*+ and Λc+ production, and a charm-counting Rc that agrees with Standard Model predictions, collectively enhancing our understanding of charm hadronisation and Z-decay dynamics. These results yield a coherent, high-precision picture of charm production in high-energy e+e− annihilation and provide stringent tests of fragmentation models and QCD processes in Z decays.

Abstract

The production rates of D*+-, Ds*+-, D+-, D0 / D0bar, Ds+, and Lambda_c in Z to ccbar decays are measured using the LEP I data sample recorded by the ALEPH detector. The fractional energy spectrum of the D*+- is well described as the sum of three contributions: charm hadronisation, b hadron decays and gluon splitting into a pair of heavy quarks. The probability for a c quark to hadronise into a D*+ is found to be f(c to D*+) = 0.233 +- 0.010 (stat.) +- 0.011 (syst.). The average fraction of the beam energy carried by D*+- mesons in Z to cc events is measured to be < X_E (D*+-) >_cc = 0.4878 +- 0.0046 (stat.) +- 0.0061 (syst.). The D*+- energy and the hemisphere mass imbalance distributions are simultaneously used to measure the fraction of hadronic Z decays in which a gluon splits to a cc pair: n_{gluon to cc} = (3.23 +- 0.48 (stat.) +- 0.53 (syst.) %. The ratio of the Vector/(Vector+Pseudoscalar) production rates in charmed mesons is found to be P_V = 0.595 +- 0.045. The fractional decay width of the Z into cc pairs is determined from the sum of the production rates for various weakly decaying charmed states to be Rc = 0.1738 +- 0.0047 (stat.) +- 0.0116 (syst.).

Figures (13)

• Figure 1: ${\mathrm{K}}^-\pi^+$ invariant mass distribution. The fitted curve is described in section 7.
• Figure 2: Distribution of the ${\mathrm{K}} \pi \pi$ - ${\mathrm{K}} \pi$ mass difference in four different ranges of the ${\mathrm{K}} \pi \pi$ energy, after requiring that the ${\mathrm{K}}\pi$ mass is consistent with the ${\mathrm{D}}^0$ mass (histogram with error bars). The background fit is also shown.
• Figure 3: ${\mathrm{K}}^-\pi^+\pi^+$ invariant mass distribution with the fit result.
• Figure 4: ${\mathrm{K}}^+{\mathrm{K}}^-\pi^+$ invariant mass distribution, with the fit result. The peak at low mass comes from the ${\mathrm{D}}^{+}$
• Figure 5: $M({\mathrm{D}}_{\rm{s}}^{* +})-M({\mathrm{D}}_{\rm{s}}^+)$ distributions with conversions in the ${\mathrm{b}} \bar{{\mathrm{b}}}$-enriched sample (a) and in the ${\mathrm{c}} \bar{{\mathrm{c}}}$-enriched sample (b), and with calorimetric photons (c), (d). The binnings have been chosen to match the different resolutions in the two photon detection methods. The dashed line is the background fit described in Section 6