Twenty five years of J/psi particles

17 November 1999



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Twenty-five years after its discovery, the J/psi particle still figures prominently in modern experiments in particle physics. One of the reasons for this is the particle's clean signature in a detector. Here we see the decay of a J/psi in the H1 detector at the DESY laboratory in Hamburg. H1 detects particles produced in head-on collisions between electrons and protons in the Hadron Electron Ring Accelerator (HERA). This computer reconstruction shows a side-view of the multilayered detector, in which the electron beam enters from the left, and the proton beam from the right. In this instance, an electron has emitted a photon, which has in turn interacted with a proton in the other beam. After the interaction, both the electron and the proton have continued unseen along the beam pipe, which threads the centre of the detector. Two clear tracks have, however, emerged, which penetrate through the thick iron layer (the outer square shape) that forms the return yoke of H1's magnet. The only particles that leave tracks and can penetrate so far are muons and antimuons, and in this case their total energies add together to give the mass of the parent J/psi particle.

Credit: DESY / H1