Estimating the production rate of loosely bound hadronic molecules using event generators

Abstract

We examine the use of hadronic event generators, such as Pythia or Herwig, to estimate the production rate of loosely bound hadronic molecules, such as the deuteron and the $X(3872)$. In the case of the deuteron, we point out that there are large uncertainties in the normalization of the predictions using event generators, because baryon pair distributions are not among the inputs used to tune the event generators. Predictions using Pythia for antideuteron production in $\ensuremath{\Upsilon}$ decay are compared to measurements by the CLEO Collaboration. They suggest that Pythia overpredicts the probability of producing pairs of baryons, at least in $\ensuremath{\Upsilon}$ decay into three gluons, and that the standard value of the coalescence parameter underpredicts the probability for formation of a deuteron from a neutron and proton with small relative momentum. In the case of the $X(3872)$, we discuss a proposed upper bound on the prompt cross section at the Tevatron that has been used as an argument against the $X(3872)$ being a loosely bound charm-meson molecule. We demonstrate that this proposed upper bound is invalid by showing that the analogous upper bound for the antideuteron would be smaller than the observed antideuteron cross section.