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In particle physics, a hadron ((ギリシア語:ἁδρός), ''hadrós'', "stout, thick") is a composite particle made of quarks held together by the strong force (in a similar way as molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons, made of three quarks, and mesons, made of one quark and one antiquark. Protons and neutrons are examples of baryons; pions are an example of a meson. Hadrons containing more than three valence quarks (exotic hadrons) have been discovered in recent years. A tetraquark state (an exotic meson), named the Z(4430)−, was discovered in 2007 by the Belle Collaboration 〔 〕 and confirmed as a resonance in 2014 by the LHCb collaboration.〔LHCb collaboration (2014): (Observation of the resonant character of the Z(4430)− state )〕 Two pentaquark states (exotic baryons), named and , were discovered in 2015 by the LHCb collaboration.〔 decays |journal=Physical Review Letters |volume=115 |issue=7 |doi=10.1103/PhysRevLett.115.072001 }}〕 There are several more exotic hadron candidates, and other colour-singlet quark combinations may also exist. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions; free neutrons decay with a half-life of about 611 seconds. Experimentally, hadron physics is studied by colliding protons or nuclei of heavy elements such as lead, and detecting the debris in the produced particle showers. ==Etymology== The term "hadron" was introduced by Lev B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics.〔 〕 In this talk he said: Not withstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called non-leptonic. This definition is not exact because "non-leptonic" may also signify "photonic". In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ''ἁδρός'' signifies "large", "massive", in contrast to ''λεπτός'' which means "small", "light"). I hope that this terminology will prove to be 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hadron」の詳細全文を読む スポンサード リンク
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