A Pythia8 Tune for Open Charm and Beauty Production in Fixed-Target Collisions

Abstract

We present FTFT (Fixed-Target Fragmentation Tune), a set of Pythia8 parameters optimised for the production of charm and beauty hadrons in fixed-target collisions at centre-of-mass energies $\sqrt{s} \simeq 20-42$ GeV. Accurate heavy-flavour production in this regime is needed to predict neutrino fluxes and hidden-particle yields at beam-dump experiments such as SHiP. The tune is derived in two stages. First, differential distributions in Feynman-$x$ ($x_F$), transverse momentum squared ($p_T^2$), and charge or leading-particle production asymmetry of $D$ mesons are fitted to data from pion- and proton-beam fixed-target experiments. Second, $K$-factors for each beam are extracted from inclusive charm cross-section measurements to scale the simulated cross section to the measured data. The fitted charm-fragmentation and multiparton-interaction parameters depart from standard LHC-tuned defaults, in line with earlier fixed-target studies.