Measurement of di-muons from 400 GeV/c protons interacting in a thick molybdenum/tungsten target

Di-muon events emanating from a replica of the SHiP target at the CERN SPS contain a clear signal of $J/ψ$~production. The production rate is in reasonable agreement with Monte Carlo simulations based on Pythia v8. For the interval in rapidity defined in the center of mass, $y_{cm}$, with the largest overlap with the NA50 measurement, $0.3 < y_{cm} < 0.6$, we obtain the production cross-section per nucleon $σ(J/ψ)/A$ including the branching ratios of $J/ψ$ decays into muons, $B_{μμ}$: $B_{μμ}σ(J/ψ)/A=(1.18~\pm~0.04~\pm~0.10)~\rm{nb}$/nucleon. This is to be compared to the NA50 extrapolated result (using a much thinner target): $B_{μμ}σ(J/ψ)/A=(0.99~\pm~0.04)~\rm{nb}$/nucleon. Within the systematic errors, no significant enhancement due to secondary production of $J/ψ$ inside the target is observed. An upper limit of possible contributions from cascade collisions of $<32\%$ is obtained.