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.
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.