Estimating the Production Rate of Loosely-bound Hadronic Molecules using Event Generators

TL;DR

This paper evaluates the use of event generators like Pythia and Herwig to estimate production rates of loosely-bound hadronic molecules, highlighting uncertainties and comparing predictions with experimental data for deuterons and the X(3872).

Contribution

It demonstrates the limitations of current event generators in predicting hadronic molecule production and challenges previous bounds used to argue about the nature of X(3872).

Findings

Pythia overpredicts baryon pair production in Upsilon decay.

Standard coalescence parameters underestimate deuteron formation.

Upper bounds on X(3872) production are invalid when compared to anti-deuteron data.

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 anti-deuteron production in 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 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 anti-deuteron would be smaller than the observed anti-deuteron cross section.