An example of strange particle production is when a negative pion collides with proton, giving rise to a neutral lambda particle and a neutral kaon. Intermediary particles are those which transfer energy, momentum, charge, and other properties from one particle to another in association with one of the four fundamental forces. Gravitons are thought to transmit the gravitational force, though they have not been directly observed. The graviton is postulated to have zero mass and spin 2. We will discover additional intermediary particles in our discussion of the standard model.
Antiparticles exist for all particles.
These have the same mass and spin but opposite values of the electric charge and various other quantum numbers such as lepton number or baryon number. The lepton number is the number of leptons minus the number of antileptons, with a similar definition for baryon number. Thus, a lepton has lepton number and a baryon has baryon number . Their antiparticles have lepton number and baryon number . As far as we know, baryon number and lepton number are absolutely conserved, which means that baryons and leptons can only be created or destroyed in particle-antiparticle pairs.