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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.
The name lepton comes from the Greek λεπτός leptós, "fine, small, thin" (neuter nominative/accusative singular form: λεπτόν leptón); [14] [15] the earliest attested form of the word is the Mycenaean Greek 𐀩𐀡𐀵, re-po-to, written in Linear B syllabic script. [16] Lepton was first used by physicist Léon Rosenfeld in 1948: [17]
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 −
LEP was a circular lepton collider – the most powerful such ever built. For context, modern colliders can be generally categorized based on their shape (circular or linear) and on what types of particles they accelerate and collide (leptons or hadrons). Leptons are point particles and are relatively light. Because they are point particles ...
The set of particles believed today to be elementary is known as the Standard Model and includes quarks, bosons and leptons. The term "subnuclear zoo" was coined or popularized by Robert Oppenheimer in 1956 at the VI Rochester International Conference on High Energy Physics. [3]
The leptons do not carry color charge, and do not respond to strong interaction. The charged leptons carry an electric charge of −1 e , while the three neutrinos carry zero electric charge. Thus, the neutrinos' motions are influenced by only the weak interaction and gravity , making them difficult to observe.