When a fast proton slams into an atomic nucleus, new particles are born for an instant. One of them is the phi meson: a short-lived clump of matter that decays into an electron-positron pair. Catching this decay is like hearing the echo of a distant explosion: the blast itself is over, but its trace is still flying.

At the J-PARC laboratory in Japan, physicists have for the first time measured how often phi mesons are produced when 30 GeV protons strike carbon and copper targets. They used a new spectrometer, E16, which catches the electrons and positrons from phi meson decays. The experiment ran on a just-commissioned beam line: it was the line's first working session.

By measuring the number of phi mesons produced, the scientists calculated the total cross section of the reaction — a measure of how readily colliding particles trigger a given process. For carbon, the cross section was 2.0 ± 0.9 (statistical uncertainty) ± 1.0 (systematic) millibarns; for copper, it was 10.3 ± 4.4 ± 4.4 millibarns. They then checked how the cross section depends on the size of the nucleus. It turned out to grow almost linearly with the number of nucleons: the exponent α = 0.99 ± 0.38 ± 0.34. This means that each proton or neutron in the nucleus seems not to notice its neighbors — the phi meson is born on a single nucleon, not on the nucleus as a whole.

Extrapolating to a nucleus consisting of a single proton gives a cross section that matches well with previous measurements of proton-proton collisions at similar energies. Thus the new instrument confirmed that the picture of phi meson production is the same for all nuclei — from a lone proton to heavy copper.