Editing Hadron (section)

{{Short description|Composite subatomic particle}}
[[File:Bosons-Hadrons-Fermions-RGB.svg|thumb|upright=1.5|A hadron is a [[Composite particle|composite subatomic particle]]. Every hadron must fall into one of the two fundamental classes of particle, [[boson]]s and [[fermion]]s.]]
{{Standard model of particle physics}}

In [[particle physics]], a '''hadron''' is a [[composite particle|composite subatomic particle]] made of two or more [[quark]]s [[bound state|held together]] by the [[strong interaction|strong nuclear force]]. Pronounced {{IPAc-en|audio=En-us-hadron.ogg|ˈ|h|æ|d|r|ɒ|n}}, the name is derived {{ety|grc|''{{linktext|ἁδρός}}'' (hadrós)|stout, thick}}. They are analogous to [[molecule]]s, which are held together by the [[electromagnetism|electric force]]. Most of the [[mass]] of ordinary [[matter]] comes from two hadrons: the [[proton]] and the [[neutron]], while most of the mass of the protons and neutrons is in turn due to the [[binding energy]] of their constituent quarks, due to the strong force.

Hadrons are categorized into two broad families: [[baryon]]s, made of an odd number of [[quark]]s (usually three) and [[meson]]s, made of an even number of quarks (usually two: one quark and one [[antiparticle|antiquark]]).<ref name=GellMann-1964/> Protons and neutrons (which make the majority of the mass of an [[atom]]) are examples of baryons; [[pion]]s are an example of a meson. A [[tetraquark]] state (an [[exotic meson]]), named the [[Z(4430)]]{{sup|−}}, was discovered in 2007 by the [[Belle experiment|Belle Collaboration]]<ref name=Choi-etal-2008-Belle/> and confirmed as a resonance in 2014 by the [[LHCb]] collaboration.<ref name="LHCb2014">{{Cite journal |last1=Aaij |first1=R. |display-authors=etal |year=2014 |title=Observation of the Resonant Character of the Z(4430)<sup>−</sup> State |journal=Physical Review Letters |volume=112 |issue=22 |pages=222002 |arxiv=1404.1903 |bibcode=2014PhRvL.112v2002A |doi=10.1103/PhysRevLett.112.222002 |pmid=24949760 |s2cid=904429 |collaboration=[[LHCb|LHCb collaboration]]}}</ref> Two [[pentaquark]] states ([[exotic baryon]]s), named {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, were discovered in 2015 by the [[LHCb]] collaboration.<ref name=Aaij-etal-2015-LHCb-Jψp/> There are several other [[Exotic hadrons|"Exotic" hadron]] candidates and other colour-singlet quark combinations that may also exist.

Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an [[atomic nucleus]]) are believed to be [[particle decay|unstable]] and eventually decay into other particles. The only known possible exception is free protons, which [[Proton decay|appear to be stable]], or at least, take immense amounts of time to decay (order of 10<sup>34+</sup>&nbsp;years). By way of comparison, free neutrons are the [[free neutron decay|longest-lived unstable particle]], and decay with a [[half-life]] of about 611&nbsp;seconds, and have a mean lifetime of 879&nbsp;seconds,{{efn|
The proton and neutrons' respective antiparticles are expected to follow the same pattern, but they are difficult to capture and study, because they immediately annihilate on contact with ordinary matter.
}}<ref name="PDG Live: 2020 Review of Particle Physics">{{cite web |last=Zyla |first=P. A. |date=2020 |title=n MEAN LIFE |url=https://pdglive.lbl.gov/DataBlock.action?node=S017T |access-date=3 February 2022 |website=PDG Live: 2020 Review of Particle Physics |publisher=Particle Data Group}}</ref> see [[free neutron decay]].

Hadron physics is studied by colliding hadrons, e.g. protons, with each other or [[high-energy nuclear physics|the nuclei of dense, heavy elements]], such as [[lead]] (Pb) or [[gold]] (Au), and detecting the debris in the produced [[particle shower]]s. A similar process occurs in the natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions of [[cosmic ray]]s with rarefied gas particles in the outer atmosphere.<ref>{{cite book |last=Martin |first=B. R. |title=Particle physics |date=2017 |isbn=9781118911907 |edition=Fourth |location=Chichester, West Sussex, UK}}</ref>