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In cosmological models of the Big Bang, the lepton epoch was the period in the evolution of the early universe in which the leptons dominated the mass of the Universe.It started roughly 1 second after the Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch. [1]
A hadron is a composite subatomic particle.Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions. In particle physics, a hadron (/ ˈ h æ d r ɒ n / ⓘ; from Ancient Greek ἁδρός (hadrós) 'stout, thick') is a composite subatomic particle made of two or more quarks held together by the strong interaction.
In particle physics, a lepton is an elementary particle of half-integer spin (spin 1 / 2 ) that does not undergo strong interactions. [1] Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), including the electron, muon, and tauon, and neutral leptons, better known as neutrinos.
Leptons, on the other hand, are not composed of quarks and as such do not participate in the strong interaction. The best known baryons are protons and neutrons, which make up most of the mass of the visible matter in the universe, whereas electrons, the other major component of atoms, are leptons.
The hadrons are divided by number of quarks (including antiquarks) into the baryons containing an odd number of quarks (almost always 3), of which the proton and neutron (the two nucleons) are by far the best known; and the mesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which the pions and kaons ...
About 10 seconds after the Big Bang the temperature of the universe falls to the point at which new lepton–antilepton pairs are no longer created and most remaining leptons and antileptons quickly annihilated each other, giving rise to pairs of high-energy photons, and leaving a small residue of non-annihilated leptons. [31] [32] [33]
In physical cosmology, the hadron era [1]: 209 is said to have begun at a time of 10-44 seconds, or at 10-8 seconds [2], and ended at 10-4 seconds. The temperature was high enough to allow the formation of hadron/anti-hadron pairs, which kept matter and anti-matter in thermal equilibrium.
In particle physics the semileptonic decay of a hadron is a decay caused by the weak force in which one lepton (and the corresponding neutrino) is produced in addition to one or more hadrons. An example for this can be K 0 → e − + ν e + π + This is to be contrasted with purely hadronic decays, such as K 0 → π + + π −