Protons accelerated to enormous energies slam into a thick metal plate, like bullets into armor. Inside the metal, a J/ψ particle is born for an instant, which immediately decays into two muons — light relatives of the electron. These muons fly out of the target, and detectors catch their tracks.
The scientists compared the number of caught muons with predictions from a computer model. The model expected more muons with large transverse momentum: that is, those flying at an angle to the beam. In reality, there were noticeably fewer. This is not a mistake but a stroke of luck: in the future SHiP experiment, such muons would be a nuisance, as they are harder to deflect with a magnetic shield.
The measured cross section for J/ψ production per nucleon was 1.18 nanobarns with an uncertainty of about 10%. This is close to the result of the earlier NA50 experiment, but slightly higher. The difference does not exceed the systematic errors, so the scientists conclude: secondary collisions inside the thick target do not enhance J/ψ production by more than 32% (at 90% confidence).
Such a result is good news for SHiP. If secondary particles produced many additional J/ψ, the muon background would be higher. Now it is clear: the thick target does not spoil the picture, and the setup can be built with confidence.
as written by the authors arXiv:2604.03661
Di-muon events emanating from a replica of the SHiP target at the CERN SPS (with a 400 GeV/c proton beam) contain a clear signal of \jpsi production. The target consisted of 13 interaction lengths of molybdenum and tungsten, followed by a 2.4m iron hadron absorber. The di-muon rate is in reasonable agreement with \MC based on Pythia v8. However, the $$J/\psi$$ rate observed in the data is much lower than predicted by the Monte Carlo simulation, which is advantageous for SHiP since muons with a large $$p_T$$ are more difficult to deflect by the active muon shield. For the center of mass rapidity ($$y_{\rm cm}$$) interval with the largest overlap with the NA50 measurement, $$0.3 < y_{\rm cm} < 0.6$$, we obtain the production cross-section per nucleon $$\sigma(J/\psi)/A$$ including the branching ratios of $$J/\psi$$ decays into muons, $$B_{\mu^+\mu^-}$$: $$B_{\mu^+\mu^-}\sigma(J/\psi)/A=(1.18~\pm~0.04~\pm~0.10)~\mathrm{nb}$$/nucleon. This is to be compared to the NA50 extrapolated result (using a much thinner target): $$B_{\mu^+\mu^-}\sigma(J/\psi)/A=(0.99~\pm~0.04)~\mathrm{nb}$$/nucleon. Within the systematic errors, no significant enhancement due to secondary production of \jpsi inside the target is observed. An upper limit at 90\% confidence level of possible contributions from secondary collisions of $$<32\%$$ is obtained.
The J/ψ particle was discovered simultaneously by two groups in 1974 and became the first proof of the existence of charm quarks. The discovery earned a Nobel Prize.
In Liu Cixin's novel 'The Three-Body Problem', aliens use protons as probes — in reality, protons themselves serve as projectiles to create new particles.