Measurement of f(c -> D^*+ X), f(b -> D^*+ X) and Gamma_cc(bar)/Gamma_had using D^*+/- Mesons

TL;DR

This work measures heavy-quark hadronisation into D^{*±}D^{*±} and the charm-partial width in Z0 decays using the OPAL detector. A robust double-tagging strategy combines exclusive D^{*+}D^{*+} reconstruction, inclusive slow-pion tagging, and lepton-based bottom tagging to extract f(c→D^{*+}D^{*+}X)f(ccD^{*+}D^{*+}X), f(b→D^{*+}D^{*+}X)f(bbD^{*+}D^{*+}X), and Γ_{cȼ}/Γ_{had}. The analysis yields precise values for fragmentation and production rates, including a charm partial width Γ_{cȼ}/Γ_{had}=0.180±0.011±0.012±0.006, and a mean D^{*+}D^{*+} scaled-energy ⟨x⟩_c≈0.515, consistent with Standard Model predictions and prior LEP results. The results provide key constraints on heavy-flavour fragmentation and Z-boson couplings at the Z pole.

Abstract

The production rates of D^*+/- mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary cc(bar) pairs in hadronic Z^0 decays have been measured at LEP using almost 4.4 million hadronic Z^0 decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed D^*+/- mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into D^*+/- mesons have been found to be: f(b -> D^*+ X) = 0.173 +/- 0.016 +/- 0.012 and f(c -> D^*+ X) = 0.222 +/- 0.014 +/- 0.014 The fraction of cc(bar) events in hadronic Z^0 decays, Gamma_cc(bar)/Gamma_had = Gamma(Z^0 -> cc(bar))/Gamma(Z^0 -> hadrons), is determined to be Gamma_cc(bar)/Gamma_had = 0.180 +/- 0.011 +/- 0.012 +/- 0.006 In all cases the first error is statistical, and the second one systematic. The last error quoted for Gamma_cc(bar)/Gamma_had is due to external branching ratios.

Figures (5)

• Figure 1: Distributions of the difference $\Delta M=M_{*}-M_{0}$ reconstructed in the four different $\mathrm{D}^{*{ +}}$D^* +$$ channels. The arrows indicate the selected signal region. (a) 3-prong decay mode, (b) the two semileptonic modes combined, (c) the satellite decay mode, and (d) the 5-prong decay mode. The points with error bars are the signal candidates. Superimposed in each case (line histogram) are the background estimator distributions, normalised to the upper sidebands in $\Delta M$.
• Figure 2: Distributions of the different tag-variables used in the flavour separation. Shown are the data distribution after background subtraction (points with error bars), the equivalent Monte Carlo distribution for all candidates (open histogram) and the predicted charm component (hatched histogram). Shown are the (a) decay length significance in the jet with the exclusive $\mathrm{D}^{*{ +}}$D^* +$$ candidate; (b) decay length significance in the jet opposite the exclusive $\mathrm{D}^{*{ +}}$D^* +$$ candidate; (c) distribution of the neural network based on jet-shape variables, and (d) distribution of the hemisphere charge.
• Figure 3: (a) Distribution of the mass difference $\Delta M=M_{*}-M_{0}$ reconstructed in the decay $\mathrm{D}^{*{ +}}$D^* +$\to\PDz\pi^+$π^+$$, $\PDz\to\mathrm{K}^-$K^-$\pi^+$π^+$$. Superimposed is the background distribution obtained from the background estimator discussed in the text, and the result of the fit to this background estimator. The arrows indicate the selected signal region. (b) $M_{0}$ spectrum of $\mathrm{D}^{*{ +}}$D^* +$$ candidates, with $M_0$ cut removed, and an additional $\Delta M$ cut applied. Shown are the data (points with error bars), the result of the fit as described in the text, and the two components from satellite and from fully reconstructed $\mathrm{D}^{*{ +}}$D^* +$$ mesons, as obtained in the fit. The arrows indicate the selected signal region.
• Figure 4: Efficiency corrected yield of $\mathrm{D}^{*{ +}}$D^* +$$ mesons as a function of the scaled energy $x_{\mathrm{D}^{*{ +}}$D^* +$}$, $1/N_{\rm had} {\mathrm d}N_{\mathrm{D}^{*{ +}}$D^* +$}/{\mathrm d}x_{\mathrm{D}^{*{ +}}$D^* +$}$, for all candidates (filled points with error bars) reconstructed in the decay $\mathrm{D}^{*{ +}}$D^* +$\to\PDz\pi^+$π^+$, \PDz\to\mathrm{K}^-$K^-$\pi^+$π^+$$, and the charm component after flavour separation (solid line). The open points are the bottom component, and the dashed line represents the result of the fit. Also shown is the predicted contribution from gluon splitting events (hatched area). The reconstruction is only done for $x_{\mathrm{D}^{*{ +}}$D^* +$}>0.2$, as indicated by the solid vertical line.
• Figure 5: Spectrum of the squared transverse momentum of double tagged candidates after applying all cuts of the (a) signal candidates, reconstructed with opposite charges in both jets. The non-charm component is indicated by the hatched area; (b) lepton-tagged candidates. The points are the data and the lines the results of the fit; (c) background candidates, reconstructed in $\Delta M$ sidebands.