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: • Six "
flavors" of
quarks:
up,
down,
strange,
charm,
bottom, and
top; • Six types of
leptons:
electron,
electron neutrino,
muon,
muon neutrino,
tau,
tau neutrino; • Twelve
gauge bosons (force carriers): the photon of
electromagnetism, the three W and Z bosons of the
weak force, and the eight gluons of the
strong force; • The
Higgs boson. 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
extensions of the Standard Model predict the existence of an elementary
graviton particle and
many other elementary particles, but none have been discovered as of 2026.
Hadrons The word hadron comes from Greek and was introduced in 1962 by
Lev Okun. 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 (≤ 5) quarks (including antiquarks) are called
hadrons. 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
mesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which the
pions and
kaons 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 quarks 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 s,
hadrons, and
fermions Any subatomic particle, like any particle in the
three-dimensional space that obeys the
laws of
quantum mechanics, can be either a boson (with integer
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
energy of a particle at rest equals its mass times the speed of light squared,
E =
mc2. 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
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 (
nucleons). 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 particles (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
decay by either the strong force or the weak force (except for the proton). Protons are not known to
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 oscillations 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
magnitude of electric charge
e exists (which is unlikely). Its charge is not shown yet. == Other properties ==