physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, a subatomic particle is a particle smaller than an

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a

baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...

, like a

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

or a

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

, composed of three quarks; or a

meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...

, composed of two quarks), or an

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a c ...

, which is not composed of other particles (for example, quarks; or

electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

, muons, and

tau Tau (; uppercase Τ, lowercase τ or \boldsymbol\tau; ) is the nineteenth letter of the Greek alphabet, representing the voiceless alveolar plosive, voiceless dental or alveolar plosive . In the system of Greek numerals, it has a value of 300 ...

particles, which are called leptons).

Particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

and

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

study these particles and how they interact. Most force-carrying particles like photons or gluons are called bosons and, although they have quanta of energy, do not have rest mass or discrete diameters (other than pure energy wavelength) and are unlike the former particles that have rest mass and cannot overlap or combine which are called fermions. The W and Z bosons, however, are an exception to this rule and have relatively large rest masses at approximately and respectively. Experiments show that light could behave like a stream of particles (called

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

s) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum-scale behave both like particles and like

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

s; they are occasionally called ''wavicles'' to reflect this. Another concept, the

uncertainty principle The uncertainty principle, also known as Heisenberg's indeterminacy principle, is a fundamental concept in quantum mechanics. It states that there is a limit to the precision with which certain pairs of physical properties, such as position a ...

, states that some of their properties taken together, such as their simultaneous position and momentum, cannot be measured exactly. Interactions of particles in the framework of

quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines Field theory (physics), field theory and the principle of relativity with ideas behind quantum mechanics. QFT is used in particle physics to construct phy ...

are understood as creation and annihilation of '' quanta'' of corresponding fundamental interactions. This blends particle physics with field theory. Even among particle physicists, the exact definition of a particle has diverse descriptions. These professional attempts at the definition of a particle include: * A particle is a collapsed wave function * A particle is an excitation of a quantum field * A particle is an irreducible representation of the Poincaré group * A particle is an observed thing

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 The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

are: * Six "

flavors Flavour or flavor is either the sensory perception of taste or smell, or a flavoring in food that produces such perception. Flavour or flavor may also refer to: Science * Flavors (programming language), an early object-oriented extension to L ...

" of

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

s: up, down, strange, charm, bottom, and top; * Six types of leptons:

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

electron neutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name ''electron neutrino''. It was first hypothesized by Wolfga ...

, muon, muon neutrino,

, tau neutrino; * Twelve gauge bosons (force carriers): the photon of

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

, the three W and Z bosons of the weak force, and the eight gluons of the strong force; * The

Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the excited state, quantum excitation of the Higgs field, one of the field (physics), fields in particl ...

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 2021.

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

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

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

and

(the two

nucleons In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number. Until the 1960s, nucleons were thought to be ele ...

) are by far the best known; and the

s containing an even number of quarks (almost always 2, one quark and one antiquark), of which the

pion In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi (), is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the ...

s 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 quark The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (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

Any subatomic particle, like any particle in the

three-dimensional space In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values ('' coordinates'') are required to determine the position of a point. Most commonly, it is the three- ...

that obeys the

laws Law is a set of rules that are created and are law enforcement, enforceable by social or governmental institutions to regulate behavior, with its precise definition a matter of longstanding debate. It has been variously described as a Socia ...

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

, 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 Supersymmetry is a Theory, theoretical framework in physics that suggests the existence of a symmetry between Particle physics, particles with integer Spin (physics), spin (''bosons'') and particles with half-integer spin (''fermions''). It propo ...

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

special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity, "On the Ele ...

, the energy of a particle at rest equals its mass times the speed of light squared, ''E'' = ''mc''². That is,

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

can be expressed in terms of

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

and vice versa. If a particle has a

frame of reference In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system, whose origin (mathematics), origin, orientation (geometry), orientation, and scale (geometry), scale have been specified in physical space. It ...

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 Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

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 The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system. More precisely, ...

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 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 The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...

, 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

All observable subatomic particles have their electric charge an

integer An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...

multiple of the

elementary charge The elementary charge, usually denoted by , is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single electron, ...

. The Standard Model's quarks have "non-integer" electric charges, namely, multiple of ''e'', but quarks (and other combinations with non-integer electric charge) cannot be isolated due to color confinement. For baryons, mesons, and their antiparticles the constituent quarks' charges sum up to an integer multiple of ''e''. Through the work of

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

, Satyendra Nath Bose, Louis de Broglie, and many others, current scientific theory holds that ''all'' particles also have a wave nature. This has been verified not only for elementary particles but also for compound particles like atoms and even molecules. In fact, according to traditional formulations of non-relativistic quantum mechanics, wave–particle duality applies to all objects, even macroscopic ones; although the wave properties of macroscopic objects cannot be detected due to their small wavelengths. Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles behave in collisions and interactions. The most fundamental of these are the laws of conservation of energy and conservation of momentum, which let us make calculations of particle interactions on scales of magnitude that range from stars to quarks. These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in ''Philosophiae Naturalis Principia Mathematica'', originally published in 1687.

Dividing an atom

The negatively charged electron has a mass of about of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged

. The atomic number of an element is the number of protons in its nucleus. Neutrons are neutral particles having a mass slightly greater than that of the proton. Different isotopes of the same element contain the same number of protons but different numbers of neutrons. The mass number of an isotope is the total number of nucleons (neutrons and protons collectively). Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules. The subatomic particles considered important in the understanding of chemistry are the

, the

, and the

. Nuclear physics deals with how protons and neutrons arrange themselves in nuclei. The study of subatomic particles, atoms and molecules, and their structure and interactions, requires

. Analyzing processes that change the numbers and types of particles requires

. The study of subatomic particles ''per se'' is called particle physics. The term ''high-energy physics'' is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result of cosmic rays, or in particle accelerators. phenomenology (particle physics), Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.

History

The term "''subatomic'' particle" is largely a retronym of the 1960s, used to distinguish a large number of

s and

s (which comprise

s) from particles that are now thought to be elementary particle, truly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown. A list of important discoveries follows: {, class="wikitable" !Particle !Composition !Theorized !Discovered !Comments , - ,

, elementary ( lepton) , G. Johnstone Stoney (1874) , J. J. Thomson (1897) , Minimum unit of electrical charge, for which Stoney suggested the name in 1891. First subatomic particle to be identified. , - , alpha particle , composite (atomic nucleus) , , Ernest Rutherford (1899) , Proven by Rutherford and Thomas Royds in 1907 to be helium nuclei. Rutherford won the Nobel Prize for Chemistry in 1908 for this discovery. , - ,

, elementary (quantum) , Max Planck (1900) ,

(1905) , Necessary to solve the thermodynamics, thermodynamic problem of black-body radiation. , - ,

, composite (

) , William Prout (1815) , Ernest Rutherford (1919, named 1920) , The nucleus of . , - ,

, composite (baryon) , Ernest Rutherford (1920) , James Chadwick (1932) , The second nucleon. , - , antiparticles , , Paul Dirac (1928) , Carl D. Anderson (, 1932) , Revised explanation uses CPT symmetry. , - ,

s , composite (

s) , Hideki Yukawa (1935) , César Lattes, Giuseppe Occhialini, Cecil Powell (1947) , Explains the nuclear force between nucleons. The first meson (by modern definition) to be discovered. , - , muon , elementary (lepton) , , Carl D. Anderson (1936) , Called a "meson" at first; but today classed as a lepton. , - ,

, elementary ( lepton) , Antonio Zichichi (1960) , Martin Lewis Perl (1975) , - , kaons {{subatomic particle, kaon , composite (mesons) , {{no, ''never'' , George Rochester, G. D. Rochester, Clifford Charles Butler, C. C. Butler (1947) , Discovered in cosmic rays. The first strange particle. , - , lambda baryons {{subatomic particle, Lambda , composite (baryons) , {{no, ''never'' , University of Melbourne ({{subatomic particle, Lambda0, 1950) , The first hyperon discovered. , - , neutrino {{math, {{subatomic particle, neutrino , elementary (lepton) , Wolfgang Pauli (1930), named by Enrico Fermi , Clyde Cowan, Frederick Reines ({{subatomic particle, electron neutrino, link=yes, 1956) , Solved the problem of energy spectrum of beta decay. , - ,

s
({{subatomic particle, up quark, {{subatomic particle, down quark, {{subatomic particle, strange quark) , elementary , Murray Gell-Mann, George Zweig (1964) , colspan=2 {{No particular confirmation event for the quark model. , - , charm quark {{subatomic particle, charm quark , elementary (quark) , Sheldon Glashow, John Iliopoulos, Luciano Maiani (1970) , Burton Richter, B. Richter, Samuel C. C. Ting, S. C. C. Ting ({{SubatomicParticle, J/psi, link=yes, 1974) , , - , bottom quark {{subatomic particle, bottom quark , elementary (quark) , Makoto Kobayashi (physicist), Makoto Kobayashi, Toshihide Maskawa (1973) , Leon M. Lederman ({{SubatomicParticle, Upsilon, link=yes, 1977) , , - , gluons , elementary (quantum) , Harald Fritzsch, Murray Gell-Mann (1972) , DESY (1979) , , - , W and Z bosons, weak gauge bosons {{SubatomicParticle, W boson+-, {{SubatomicParticle, Z boson0 , elementary (quantum) , Sheldon Glashow, Steven Weinberg, Abdus Salam (1968) , CERN (1983) , Properties verified through the 1990s. , - , top quark {{subatomic particle, top quark , elementary (quark) , Makoto Kobayashi (physicist), Makoto Kobayashi, Toshihide Maskawa (1973) , Fermilab (1995) , Does not hadronization, hadronize, but is necessary to complete the Standard Model. , - ,

, elementary (quantum) , Peter Higgs (1964) , CERN (2012) , Only known spin zero elementary particle. , - , tetraquark , composite , {{dunno , Zc(3900), Z_c(3900), 2013, yet to be confirmed as a tetraquark , A new class of hadrons. , - , pentaquark , composite , {{dunno , colspan=2 , Yet another class of hadrons. {{As of, 2019 several are thought to exist. , - , graviton , elementary (quantum) , Albert Einstein (1916) , , Interpretation of a gravitational wave as particles is controversial. , - , magnetic monopole , elementary (unclassified) , Paul Dirac (1931){{cite journal , last1=Dirac , first1=Paul A. M. , date=1931 , title=Quantised singularities in the electromagnetic field , url=https://royalsocietypublishing.org/doi/10.1098/rspa.1931.0130 , journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character , language=en , volume=133 , issue=821 , pages=60–72 , bibcode=1931RSPSA.133...60D , doi=10.1098/rspa.1931.0130 , issn=0950-1207, url-access=subscription , {{not yet, ''hypothetical''{{cite journal , last1=Navas , first1=S. , last2=Amsler , first2=C. , last3=Gutsche , first3=T. , last4=Hanhart , first4=C. , last5=Hernández-Rey , first5=J. J. , last6=Lourenço , first6=C. , last7=Masoni , first7=A. , last8=Mikhasenko , first8=M. , last9=Mitchell , first9=R. E. , last10=Patrignani , first10=C. , last11=Schwanda , first11=C. , last12=Spanier , first12=S. , last13=Venanzoni , first13=G. , last14=Yuan , first14=C. Z. , last15=Agashe , first15=K. , date=2024-08-01 , title=Review of Particle Physics , url=https://link.aps.org/doi/10.1103/PhysRevD.110.030001 , journal=Physical Review D , language=en , volume=110 , issue=3 , page=030001 , doi=10.1103/PhysRevD.110.030001 , issn=2470-0010, hdl=20.500.11850/695340 , hdl-access=free {{rp, loc=25 , {{portal, Physics

References

{{Reflist, refs= There was early debate on what to name the proton as seen in the follow commentary articles b
Soddy 1920
an
Lodge 1920

External links

{{Commons category, Subatomic particles
University of California: Particle Data Group.
{{Particles Subatomic particles Quantum mechanics