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 $J\mathrm{/}\psi$~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 $\sigma (J\mathrm{/}\psi )\mathrm{/}A$ including the branching ratios of $J\mathrm{/}\psi$ decays into muons, $B_{\mu \mu }$: $B_{\mu \mu }\sigma (J\mathrm{/}\psi )\mathrm{/}A=(1.18\pm 0.04\pm 0.10)\mathrm{n}\mathrm{b}$/nucleon. This is to be compared to the NA50 extrapolated result (using a much thinner target): $B_{\mu \mu }\sigma (J\mathrm{/}\psi )\mathrm{/}A=(0.99\pm 0.04)\mathrm{n}\mathrm{b}$/nucleon. Within the systematic errors, no significant enhancement due to secondary production of $J\mathrm{/}\psi$ inside the target is observed. An upper limit of possible contributions from cascade collisions of $<32\mathrm{\%}$ is obtained.