Production mechanisms of charm hadrons in the string model

Abstract

In the hadroproduction of charm in the context of string fragmentation, the pull of a beam remnant at the other end of a string may give a charm hadron more energy than the perturbatively produced charm quark. The collapse of a low-mass string to a single hadron is the extreme case in this direction, and gives rise to asymmetries between charm and anticharm hadron spectra. We study these phenomena, and develop models that describe the characteristics not only of the charm hadrons but also of the associated event.

Figures (7)

• Figure 1: Examples of different string configurations in a $\pi^-\mathrm{p}$ collision: (a) $\mathrm{u}\overline{\mathrm{u}} \to \mathrm{c}\overline{\mathrm{c}}$ has a unique colour flow; (b,c) $\mathrm{g}\mathrm{g} \to \mathrm{c}\overline{\mathrm{c}}$ with the two possible colour flows.
• Figure 2: Strings (dashed) in a $\pi^-\mathrm{p}$ collision corresponding to the colour flows in Fig. \ref{['fig.strings']}a, b and c respectively; (a) $\mathrm{u}\overline{\mathrm{u}} \to \mathrm{c}\overline{\mathrm{c}}$ and (b,c) $\mathrm{g}\mathrm{g} \to \mathrm{c}\overline{\mathrm{c}}$. If e.g. the colour-singlet system $\mathrm{c}$-$\overline{\mathrm{u}}$ in (b) has a small invariant mass we will call it a cluster and hadronize it by the procedure described in the text.
• Figure 3: $\mathrm{D}^{\mp}$ meson production in a $\pi^-\mathrm{p}$ collision at a $\pi^-$ beam momentum of 500 GeV, using the default Lund Model. $x_{\mathrm{F}}$ distribution (normalized per $\mathrm{c}\overline{\mathrm{c}}$ event) of (a) $\mathrm{D}^-$ and (b) $\mathrm{D}^+$ for different production channels: (i) Cluster collapse, light quark from p end, (ii) Cluster collapse, light quark from $\pi^-$ end, (iii) Cluster decay, light quark from p end, (iv) Cluster decay, light quark from $\pi^-$ end and (v) String fragmentation. (c) The resulting asymmetry, Eq (\ref{['eq.asymmetry']}). Also shown is data from WA82E769E791.
• Figure 4: $x_{\mathrm{F}}$ distribution of (a) $\mathrm{D}^-$ and (b) $\mathrm{D}^+$ for different production channels (cf. Fig. \ref{['fig.default']}, which corresponds to the default of extreme uneven sharing) in a $\pi^-\mathrm{p}$ collision at a $\pi^-$ beam momentum of 500 GeV using an even sharing of energy between the quarks in a beam remnant. (c) the resulting asymmetry using three different choices of BRDF's: $\propto (1-\chi)^k/\sqrt{\chi^{2}+c_{\mathrm{min}}^2}$, $\propto (1-\chi)^k/\sqrt[4]{\chi^{2}+c_{\mathrm{min}}^2}$ and $\propto (1-\chi)^l$ respectively, with $(k,l)=(1,0)$ for the pion and $(3,1)$ for the proton remnant, and $c_{\mathrm{min}}=0.6$ GeV/$\mathrm{E}_{\mathrm{cm}}$. They correspond to an uneven (full), intermediate (dashed) and even (dotted) sharing of energy. For the $\pi^-$ the three cases correspond to $<\chi>$ = 0.14, 0.23 and 0.5 respectively. Also shown is data from WA82E769E791.
• Figure 5: (a) Distribution of cluster (full) and meson (dashed) masses in the string model. Clusters within the gray area collapse to $\mathrm{D}^-$ or $\mathrm{D}^{*-}$. (b) Dependence of the parton-level mass distribution on some parameters of the model.