CHIGEN: Monte Carlo Event Generator for inclusive χ_cJ production

The enclosed Fortran code provides public access to the CHIGEN Monte Carlo generator for inclusive χ_cJ production within the LO colour singlet model, via the radiative decay

χ_c → J/ψ γ → l⁺ l^- γ

In the region of low p_t(χ), the 2 → 1 gluon-fusion matrix elements are used for the χ_c2 (and χ_c0) predictions, with the intrinsic partonic k_t modelled by a Gaussian distribution, while at high p_t(χ) the 2 → 2 matrix elements are used. In the case of the χ_c1, where the 2 → 1 process does not occur for on-shell initial-state gluons, the 2 → 2 matrix element is used throughout, with the same Gaussian intrinsic partonic k_t taken in the low p_t(χ) region. A smooth phenomenological interpolation between the regions of low and high p_t(χ) is then implemented, with the precise position of the transition determined by a fit to existing prompt J/ψ data.

Requires MSTW08 fortran and grid files in the same directory to run.