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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 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]
There are six leptons in total; the three charged leptons are called "electron-like leptons", while the neutral leptons are called "neutrinos". Neutrinos are known to oscillate, so that neutrinos of definite flavor do not have definite mass: Instead, they exist in a superposition of mass eigenstates.
Half of the fermions are leptons, three of which have an electric charge of −1 e, called the electron (e −), the muon (μ −), and the tau (τ −); the other three leptons are neutrinos (ν e, ν μ, ν τ), which are the only elementary fermions with neither electric nor color charge. The remaining six particles are quarks (discussed below).
The temperature of the universe had fallen sufficiently to allow the quarks from the preceding quark epoch to bind together into hadrons. Initially, the temperature was high enough to allow the formation of hadron/anti-hadron pairs, which kept matter and anti-matter in thermal equilibrium. Following the annihilation of matter and antimatter, a ...
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 −
These particles include all quarks and leptons and all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose–Einstein statistics. Some fermions are elementary particles (such as electrons), and some are composite particles (such as protons).