The results of particle physics experiments are often expressed as the probability of particles interacting in the detector depending on how much energy the particles have. As the particle energy increases, the interaction probability changes slowly and smoothly, except at certain special energies called "resonances," which appear as bumps on the graph of probability versus energy. At a resonance, the probability of the particles interacting increases significantly. For example, the J/psi meson was discovered when the electron-positron collision energy just equaled the mass of the charm quark and anti-charm quark, creating a huge spike in particle production. Finding resonances is the primary way new particles are discovered in particle accelerator experiments. The mass of the new particle is the resonance energy.