Editing Subatomic particle (section)

== Classification ==

=== 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]].

=== By statistics ===
{{main|Spin–statistics theorem}}
[[File:Bosons-Hadrons-Fermions-RGB.svg|thumb|upright=1.6|Overlap between [[boson]]s, [[hadron]]s, and [[fermion]]s]]
Any subatomic particle, like any particle in the [[three-dimensional space]] that obeys the [[Scientific law|laws]] of [[quantum mechanics]], can be either a boson (with integer [[Spin (physics)|spin]]) or a fermion (with odd half-integer spin).

In the Standard Model, all the elementary fermions have spin 1/2, and are divided into the quarks which carry [[color charge]] and therefore feel the strong interaction, and the [[leptons]] which do not. The elementary bosons comprise the gauge bosons (photon, W and Z, gluons) with spin 1, while the Higgs boson is the only elementary particle with spin zero.

The hypothetical graviton is required theoretically to have spin 2, but is not part of the Standard Model. Some extensions such as [[supersymmetry]] predict additional elementary particles with spin 3/2, but none have been discovered as of 2023.

Due to the laws for spin of composite particles, the baryons (3 quarks) have spin either 1/2 or 3/2 and are therefore fermions; the mesons (2 quarks) have integer spin of either 0 or 1 and are therefore bosons.

=== By mass ===
In [[special relativity]], the [[Mass–energy equivalence|energy of a particle at rest equals its mass times the speed of light squared]], {{nowrap begin}}''E'' = ''mc''<sup>2</sup>{{nowrap end}}. That is, [[mass]] can be expressed in terms of [[energy]] and vice versa. If a particle has a [[frame of reference]] in which it lies [[rest (physics)|at rest]], then it has a positive [[rest mass]] and is referred to as ''massive''.

All composite particles are massive. Baryons (meaning "heavy") tend to have greater mass than mesons (meaning "intermediate"), which in turn tend to be heavier than leptons (meaning "lightweight"), but the heaviest lepton (the [[tau particle]]) is heavier than the two lightest flavours of baryons ([[nucleon]]s). It is also certain that any particle with an [[electric charge]] is massive.

When originally defined in the 1950s, the terms baryons, mesons and leptons referred to masses; however, after the quark model became accepted in the 1970s, it was recognised that baryons are composites of three quarks, mesons are composites of one quark and one antiquark, while leptons are elementary and are defined as the elementary fermions with no color charge.

All [[massless particle]]s (particles whose [[invariant mass]] is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.

=== By decay ===
Most subatomic particles are not stable. All leptons, as well as baryons [[particle decay|decay]] by either the strong force or weak force (except for the proton). Protons are not known to [[Proton decay|decay]], although whether they are "truly" stable is unknown, as some very important Grand Unified Theories (GUTs) actually require it. The μ and τ muons, as well as their antiparticles, decay by the weak force. Neutrinos (and antineutrinos) do not decay, but a related phenomenon of [[neutrino oscillation]]s is thought to exist even in vacuums. The electron and its antiparticle, the [[positron]], are theoretically stable due to [[charge conservation]] unless a lighter particle having [[absolute value|magnitude]] of electric charge {{abbr|≤|less than or equal}}&nbsp;[[elementary charge|''e'']] exists (which is unlikely). Its charge is not shown yet.