The short period from about 10−6 s to 10−5 s after the Big Bang when heavy atomic particles such as protons, neutrons, pions, and kaons were formed. Before the start of the hadron era, quarks behaved as free particles. The process by which hadrons formed from these quarks is called the quark–hadron phase transition. By the end of the hadron era, all the other hadron species had either decayed or annihilated, leaving only protons and neutrons. Immediately after this the Universe entered the lepton era; both are subdivisions of the radiation era.