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Physicists measured how particle pairs are born alongside the J/ψ particle and, for the first time, peeked into a process involving protons.
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At Japan's KEK laboratory, electrons and their antimatter counterparts, positrons, collide inside the Belle II detector. In such collisions, new particles are born for a fleeting moment. Among them is J/ψ, a bound state of a charm quark and its antiquark. Physicists measured how often pairs of other particles (pions, kaons, or protons) appear alongside J/ψ.
The measurements were taken at collision energies from 3.8 to 7 gigaelectronvolts. For pions and kaons, the results matched previous data from the Belle experiment. But the process producing a proton-antiproton pair together with J/ψ was studied for the first time. The number of events was small, and the cross-section plot shows no noticeable peaks or dips.
The physicists also searched for exotic particles, so-called charmonium-like states. These are hypothetical objects that do not fit the simple picture of a quark and an antiquark. No traces of them were found in the Belle II data. But the very fact of a precise cross-section measurement is an important step: now theorists can test their models.