The Large Hadron Collider accelerates two beams of protons, which travel around the collider in opposite directions, to a total energy of 6.5 TeV per proton. (1 TeV = 1 teraelectron volt = 10¹² eV.) The beams cross at several points, and a few protons undergo headon collisions. Such collisions usually produce many subatomic particles, but in principle the colliding protons could produce a single subatomic particle at rest. (It would be unstable and would almost instantly decay into other subatomic particles.) What would be the mass, as a multiple of the proton’s mass, of such a particle?