Editing Subatomic particle (section)

=== By composition ===
Subatomic particles are either "elementary", i.e. not made of multiple other particles, or "composite" and made of more than one elementary particle bound together. 
 
The elementary particles of the [[Standard Model]] are:<ref name="IntroSM1">
{{cite book |last1=Cottingham |first1=W. N. |url=https://books.google.com/books?id=Dm36BYq9iu0C |title=An introduction to the standard model of particle physics |last2=Greenwood |first2=D.A. |date=2007 |publisher=[[Cambridge University Press]] |isbn=978-0-521-85249-4 |page=1}}</ref>
* Six "[[Flavour (particle physics)|flavors]]" of [[quark]]s: [[Up quark|up]], [[Down quark|down]], [[Strange quark|strange]], [[Charm quark|charm]], [[Bottom quark|bottom]], and [[Top quark|top]];
* Six types of [[lepton]]s: [[electron]], [[electron neutrino]], [[muon]], [[muon neutrino]], [[tau (particle)|tau]], [[tau neutrino]];
* Twelve [[gauge boson]]s (force carriers): the photon of [[electromagnetism]], the three W and Z bosons of the [[weak interaction|weak force]], and the eight gluons of the [[strong force]];
* The [[Higgs boson]].
[[File:Standard Model of Elementary Particles.svg|thumb|upright=1.8|The [[Standard Model]] classification of elementary particles]]

All of these have now been discovered through experiments, with the latest being the top quark (1995), tau neutrino (2000), and Higgs boson (2012).

Various [[Physics beyond the Standard Model|extensions of the Standard Model]] predict the existence of an elementary [[graviton]] particle and [[List of elementary particles#Hypothetical particles|many other elementary particles]], but none have been discovered as of 2021.

==== Hadrons ====
The word hadron comes from Greek and was introduced in 1962 by [[Lev Okun]].<ref>{{cite conference |first=Lev |last=Okun |author-link=Lev Okun |year=1962 |title=The theory of weak interaction |conference=International Conference on High-Energy Physics |place=CERN, Geneva, CH |book-title=Proceedings of 1962 International Conference on High-Energy Physics at CERN |page=845 |type=plenary talk |bibcode=1962hep..conf..845O}}</ref> Nearly all composite particles contain multiple quarks (and/or antiquarks) bound together by gluons (with a few exceptions with no quarks, such as [[positronium]] and [[muonium]]). Those containing few (≤&nbsp;5) quarks (including antiquarks) are called [[hadron]]s. Due to a property known as [[color confinement]], quarks are never found singly but always occur in hadrons containing multiple quarks. 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 [[meson]]s containing an even number of quarks (almost always 2, one quark and one antiquark), of which the [[pion]]s and [[kaon]]s are the best known.

Except for the proton and neutron, all other hadrons are unstable and decay into other particles in microseconds or less. A proton is made of two [[up quark]]s and one [[down quark]], while the neutron is made of two down quarks and one up quark. These commonly bind together into an atomic nucleus, e.g. a helium-4 nucleus is composed of two protons and two neutrons. Most hadrons do not live long enough to bind into nucleus-like composites; those that do (other than the proton and neutron) form [[exotic nuclei]].