Production & decay of a J/psi particle in the H1 detector at DESY, the German particle physics laboratory in Hamburg. The computer display shows the detector side-on in dark & pale blue, green, & orange. An electron enters from left and a proton from right, both unseen, accelerated in DESY's HERA collider. They meet at centre right to create a J/psi particle, which consists of a charm quark & a charm antiquark. The J/psi lives too briefly to be seen, but is revealed by its decay into a pair of penetrating muons (tracks leading to top & bottom left). The energy recorded for the two muons is equivalent to the mass of the J/psi.