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A composite particle proton is made of two up quarks and one down quark, which are elementary particles.

In physics, a subatomic particle is a particle smaller than an atom.[1] 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, like a proton or a neutron, composed of three quarks; or a meson, composed of two quarks), or an elementary particle, which is not composed of other particles (for example, quarks; or electrons, muons, and tau particles, which are called leptons).[2] Particle physics and nuclear physics study these particles and how they interact.[3] 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 80 GeV/c2 and 90 GeV/c2 respectively.

Experiments show that light could behave like a stream of particles (called photons) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum-scale particles behave both like particles and like waves; they are occasionally called wavicles to reflect this.[4]

Another concept, the uncertainty principle, states that some of their properties taken together, such as their simultaneous position and momentum, cannot be measured exactly.[5] Interactions of particles in the framework of quantum field theory 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:[6]

Particles in the atom
Subatomic particle Symbol Type Location in atom Charge
[e]		 Mass
[Da]
proton p+ composite nucleus +1 ≈ 1
neutron n0 composite nucleus 0 ≈ 1
electron e− elementary shells −1 ≈ ⁠1/2000⁠

Classification

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By composition

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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:[7]

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 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

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The word hadron comes from Greek and was introduced in 1962 by Lev Okun.[8] 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

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Main article: Spin–statistics theorem
Overlap between bosons, 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

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In special relativity, the energy of a particle at rest equals its mass times the speed of light squared, E = mc 2. 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

[edit]

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, 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

[edit]

All observable subatomic particles have their electric charge an integer multiple of the elementary charge. The Standard Model's quarks have "non-integer" electric charges, namely, multiple of ⁠1/3⁠ 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, Satyendra Nath Bose, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature.[9] 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.[10]

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.[11] 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

[edit]

The negatively charged electron has a mass of about ⁠1/1836⁠ of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged proton. 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 electron, the proton, and the neutron. 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 quantum mechanics. Analyzing processes that change the numbers and types of particles requires quantum field theory. 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. Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.[12]

History

[edit]
Main articles: History of subatomic physics and Timeline of particle discoveries

The term "subatomic particle" is largely a retronym of the 1960s, used to distinguish a large number of baryons and mesons (which comprise hadrons) from particles that are now thought to be truly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown.

A list of important discoveries follows:

Particle Composition Theorized Discovered Comments
electron e−
 		elementary (lepton) G. Johnstone Stoney (1874)[13] J. J. Thomson (1897)[14] Minimum unit of electrical charge, for which Stoney suggested the name in 1891.[15] First subatomic particle to be identified.[16]
alpha particle α composite (atomic nucleus) never Ernest Rutherford (1899)[17] Proven by Rutherford and Thomas Royds in 1907 to be helium nuclei. Rutherford won the Nobel Prize for Chemistry in 1908 for this discovery.[18]
photon γ elementary (quantum) Max Planck (1900)[19] Albert Einstein (1905)[20] Necessary to solve the thermodynamic problem of black-body radiation.
proton p composite (baryon) William Prout (1815)[21] Ernest Rutherford (1919, named 1920)[22][23] The nucleus of 1
H.
neutron n composite (baryon) Ernest Rutherford (c.1920[24]) James Chadwick (1932) [25] The second nucleon.
antiparticles   Paul Dirac (1928)[26] Carl D. Anderson (e+
, 1932) 		Revised explanation uses CPT symmetry.
pions π composite (mesons) 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 μ−
 		elementary (lepton) never Carl D. Anderson (1936)[27] Called a "meson" at first; but today classed as a lepton.
tau τ−
 		elementary (lepton) Antonio Zichichi (1960) [28] Martin Lewis Perl (1975)
kaons K composite (mesons) never G. D. Rochester, C. C. Butler (1947)[29] Discovered in cosmic rays. The first strange particle.
lambda baryons Λ composite (baryons) never University of Melbourne (Λ0
, 1950)[30] 		The first hyperon discovered.
neutrino ν elementary (lepton) Wolfgang Pauli (1930), named by Enrico Fermi Clyde Cowan, Frederick Reines (ν
e, 1956) 		Solved the problem of energy spectrum of beta decay.
quarks
(u, d, s) 		elementary Murray Gell-Mann, George Zweig (1964) No particular confirmation event for the quark model.
charm quark c elementary (quark) Sheldon Glashow, John Iliopoulos, Luciano Maiani (1970) B. Richter, S. C. C. Ting (J/ψ, 1974)
bottom quark b elementary (quark) Makoto Kobayashi, Toshihide Maskawa (1973) Leon M. Lederman (ϒ, 1977)
gluons elementary (quantum) Harald Fritzsch, Murray Gell-Mann (1972)[31] DESY (1979)
weak gauge bosons W±
, Z0
 		elementary (quantum) Sheldon Glashow, Steven Weinberg, Abdus Salam (1968)[32][33][34] CERN (1983) Properties verified through the 1990s.
top quark t elementary (quark) Makoto Kobayashi, Toshihide Maskawa (1973)[35] Fermilab (1995)[36] Does not hadronize, but is necessary to complete the Standard Model.
Higgs boson elementary (quantum) Peter Higgs (1964)[37][38] CERN (2012)[39] Only known spin zero elementary particle.[40]
tetraquark composite ? Zc(3900), 2013, yet to be confirmed as a tetraquark A new class of hadrons.
pentaquark composite ? 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.[41]
magnetic monopole elementary (unclassified) Paul Dirac (1931)[42] hypothetical[43]: 25 

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Subatomic particle

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Subatomic particles are microscopic entities smaller than atoms that serve as the fundamental constituents of all and in the . These particles include both elementary particles, which are considered indivisible point-like objects, and composite particles formed by combinations of elementary ones. Elementary subatomic particles are categorized primarily into quarks and leptons (collectively fermions), and bosons (including gauge bosons and the ) within the framework of the of , which describes their interactions through the fundamental forces of nature. Quarks combine to form composite particles such as protons and neutrons, which reside in the , while leptons include electrons that the nucleus and neutrinos that rarely interact with . Protons carry a positive and determine an atom's identity as an element, neutrons are neutral and contribute to , and electrons bear a negative charge essential for chemical bonding and . Beyond these, other notable subatomic particles include muons, taus, and various bosons like the , which imparts mass to other particles, discovered through high-energy experiments. The study of subatomic particles not only explains atomic structure but also underpins phenomena from nuclear reactions to origins. The organizes 17 fundamental particles—six quarks, six leptons, and five bosons—along with their antiparticles, providing a highly successful but incomplete theory that does not yet account for or . Ongoing at particle accelerators probes these particles' properties, symmetries, and potential extensions to the model, revealing insights into the universe's earliest moments and evolution.

Fundamentals

Definition and Scale

Subatomic particles are microscopic constituents of smaller than atoms, encompassing both composite particles, which are bound states of more fundamental entities, and elementary particles, which are considered indivisible according to current theories. Examples of composite particles include protons and neutrons, while elementary particles comprise quarks, leptons such as electrons, and gauge bosons like photons. This distinction arises from experimental evidence showing that composite particles have internal structure, whereas elementary ones do not exhibit substructure at probed energies. In terms of scale, atoms typically measure approximately 10−1010^{-10} meters in diameter, providing a benchmark for comparison. Composite subatomic particles, such as hadrons (including baryons like protons and neutrons, and mesons), have characteristic sizes on the order of 10−1510^{-15} meters, or 1 femtometer, which is about 100,000 times smaller than an atom. Elementary particles, in contrast, exhibit point-like behavior in high-energy scattering experiments, with no detectable size down to resolution limits of roughly 10−1910^{-19} meters or smaller, as probed by current experiments like the LHC, suggesting they may be truly fundamental point particles. The concept of subatomic particles emerged from late 19th-century experiments revealing that atoms are not indivisible, as previously thought. J.J. Thomson's 1897 discovery of the through cathode ray tube studies marked the first identification of a subatomic entity. This breakthrough, supported by subsequent work on and nuclear structure, laid the foundation for by demonstrating atomic divisibility.

Role in Matter Structure

Subatomic particles serve as the fundamental constituents of atoms, which in turn form the basis of all ordinary . The atom consists of a central nucleus surrounded by a of electrons. The nucleus is composed of protons and neutrons, collectively known as nucleons, while electrons, being leptons, occupy probabilistic orbitals around the nucleus due to their negative charge. Protons carry a positive equal in magnitude to that of the , and their number in the nucleus defines the , determining the element's identity. Neutrons, being electrically neutral, contribute to the nucleus's mass and stability without altering the charge. Together, protons and neutrons are bound within the nucleus by the residual strong nuclear force, which overcomes the electromagnetic repulsion between positively charged protons. At a deeper level, protons and neutrons are composite particles known as hadrons, each made up of three quarks held together by gluons through the strong force. A proton comprises two up quarks and one , while a neutron consists of one and two down quarks; these up and down quarks are the lightest and most prevalent flavors. Gluons, as force carriers, mediate the interactions that confine the quarks within these nucleons, preventing their isolation under normal conditions. This structure establishes a of : fundamental particles like quarks and leptons (including electrons) combine to form hadrons such as protons and neutrons, which assemble into atomic nuclei; nuclei then attract electrons via the electromagnetic force to create neutral atoms, and atoms bond through electron interactions to form molecules. The electromagnetic force governs atomic binding by attracting oppositely charged electrons to the nucleus and facilitating chemical bonds between atoms. In contrast, the strong force dominates at the nuclear and subnuclear scales, ensuring the cohesion of nucleons and quarks. Electrons play a pivotal role in enabling chemistry, as their arrangement in outer shells—particularly the valence electrons—dictates an atom's reactivity and ability to form chemical bonds, while the nuclear composition primarily influences physical properties like and stability.

Classification

By Composition

Subatomic particles are classified by composition into elementary and composite categories, reflecting whether they possess internal structure under the framework of the of . Elementary particles are fundamental constituents considered indivisible, with no substructure observed at current scales, and serve as the building blocks for all and forces. They are divided into fermions, which obey the and comprise , and bosons, which mediate interactions. Fermionic elementary particles include quarks and leptons, each grouped into three generations. Quarks—up, down, charm, strange, top, and bottom—carry fractional electric charges and participate in the strong through , existing in three color varieties (red, green, blue) that combine to form color-neutral states. Leptons consist of charged particles (, , ) with integer charges of -1 and neutral neutrinos (electron, muon, tau), which interact primarily via the weak force and (for charged ones). Bosonic elementary particles encompass gauge bosons—photons for , gluons (eight types) for the strong force, and W⁺, W⁻, Z for the weak force—and the , which imparts mass to other particles via the Higgs field. Composite particles, in contrast, are bound states of elementary particles, primarily formed through the strong binding into hadrons. Hadrons are categorized as baryons or mesons based on their quark content. Baryons, which are fermions with spin, consist of three (or quark-antiquark pairs plus additional quarks), exemplified by the proton (uuduud, where uu denotes and dd ) and neutron (uddudd). These nucleons form the nucleus of atoms, with the proton's positive charge arising from two (each +2/3+2/3) and one (−1/3-1/3). Mesons, bosons with integer spin, are composed of a quark-antiquark pair, such as the positively charged π+\pi^+ (udˉu\bar{d}, where dˉ\bar{d} is the anti-down quark). Other hadrons include heavier baryons like the delta resonances and mesons like the rho, but all share this quark-based structure. Every subatomic particle has a corresponding , a with identical mass but opposite quantum numbers, such as . For elementary particles, antiquarks and antileptons (e.g., as antielectron) follow this rule, while for composites, the (uˉuˉdˉ\bar{u}\bar{u}\bar{d}) exemplifies the charge-reversed structure of the proton. Antiparticles annihilate with their counterparts upon contact, releasing , and are integral to understanding - symmetry in the universe. Mass ranges for these particles vary widely, from near-zero for neutrinos and photons to hundreds of GeV for top quarks and Higgs, as detailed in specialized classifications.

By Particle Statistics

Subatomic particles are classified by their , which dictate how they behave in multi-particle systems, into two primary categories: fermions and bosons. This classification arises from their intrinsic spin and adherence to either Fermi-Dirac or Bose-Einstein , fundamentally influencing phenomena from atomic structure to force interactions. Fermions are particles with spin, such as 1/2, that obey the , preventing two identical fermions from occupying the same simultaneously. This antisymmetric under particle exchange ensures fermions form the stable building blocks of , including quarks—which combine to form protons and neutrons—and leptons, such as electrons and neutrinos. In contrast, bosons possess spin values, like 0, 1, or 2, and follow symmetric statistics, allowing multiple identical bosons to occupy the same without restriction. These particles primarily act as mediators of fundamental forces, exemplified by photons carrying the electromagnetic force, gluons mediating the strong , W and Z bosons for the weak force, and the , which imparts mass to other particles. The connection between spin and statistics is formalized by the in relativistic , which mandates that particles with spin are fermions exhibiting anticommuting fields, while those with spin are bosons with commuting fields. This , proven for local fields, ensures consistency in quantum theories and has been experimentally verified through interference patterns in particle systems. The statistical behaviors have profound implications for physical systems: the for fermions enables the diverse electronic configurations in atoms, fostering stable chemical bonds and the solidity of . Bosons, by enabling quantum coherence, facilitate interactions like transmission and phenomena such as Bose-Einstein , where, for instance, photons in a achieve by collectively occupying identical states, producing coherent light.

By Mass and Stability

Subatomic particles are often classified by their rest masses, which are conventionally expressed in energy units via Einstein's mass-energy equivalence E=mc2E = mc^2, where mm is the rest mass and cc is the speed of light; this yields units such as electronvolts per speed of light squared (eV/c²), mega-electronvolts per speed of light squared (MeV/c²), or giga-electronvolts per speed of light squared (GeV/c²), as standardized in particle physics reviews. This convention facilitates comparisons across the vast range of particle masses, from nearly zero to hundreds of GeV/c². Stability, closely tied to mass, refers to a particle's mean lifetime before decay; stable particles have effectively infinite lifetimes under normal conditions, while unstable ones decay rapidly or slowly depending on their mass and interactions. Particles are grouped into broad mass categories: light, with rest masses below about 1 MeV/c²; medium, ranging from roughly 1 MeV/c² to 10 GeV/c²; and heavy, exceeding 10 GeV/c². Light particles include the , with a rest mass of 0.510 998 950 00(15) MeV/c², and neutrinos, whose masses are extremely small with an upper limit on the sum of the three flavors around 0.12 eV/c² from cosmological and oscillation data. Medium-mass examples encompass the , at 105.6583755(23) MeV/c², which is about 207 times heavier than the . Heavy particles, such as the top quark, have rest masses around 172.57 ± 0.29 GeV/c² (PDG 2024 average), making it the heaviest known and roughly 340,000 times more massive than the . Stability varies independently of mass category but correlates with it for certain particles; for instance, light particles like the , proton (though composite, with constituents), and (massless) are stable with infinite lifetimes in isolation. In contrast, the medium-mass , despite its low mass of 939.565 MeV/c², is unstable and decays via with a mean lifetime of 878.4 ± 0.5 s (PDG 2024, ultracold neutron average) into a proton, , and antineutrino. Neutrinos, though light and stable against decay, exhibit oscillatory flavor changes implying nonzero masses. Heavy particles like the top are highly unstable, decaying almost instantaneously due to their large mass enabling kinematically allowed weak decays. The origin of these masses, except for the massless photon and gluons, arises primarily from the in the , where particles acquire mass through interactions with the Higgs field via , as detailed in foundational reviews of electroweak theory. This mechanism explains why fermions and W/Z bosons have finite masses while preserving gauge invariance, though the precise values depend on Yukawa couplings whose origins remain an open question beyond the .

By Interactions and Decay

Subatomic particles participate in four fundamental interactions: the electromagnetic, strong, weak, and gravitational forces, each governing specific behaviors and binding mechanisms at the quantum level. The electromagnetic interaction acts on all charged particles, such as electrons and quarks, enabling phenomena like atomic structure and light emission; it is infinitely ranged and mediated by the massless . The strong interaction, confined to short distances on the order of 10^{-15} meters, binds quarks into protons, neutrons, and other hadrons through the , exclusively involving quarks and gluons as mediators—eight massless gluons that carry themselves. The facilitates processes involving flavor changes, such as radioactive , and affects all fermions (quarks and leptons), including neutrinos, which interact solely through this force at the subatomic scale; it is mediated by the massive charged bosons (W⁺ and W⁻) and neutral boson, with masses around 80–91 GeV/c², limiting its range to about 10^{-18} meters. Gravitation influences all particles with non-zero mass or energy but is overwhelmingly weak compared to the other forces in contexts, playing no significant role in subatomic dynamics. These interactions determine not only how particles bind or scatter but also their decay pathways, classifying unstable particles by lifetime into (indefinite lifetime), long-lived (lifetimes exceeding 10^{-6} seconds), and short-lived or resonant (lifetimes below 10^{-20} seconds). particles, like the and proton, do not decay due to conservation laws prohibiting lighter final states, while neutrinos are effectively stable given their minuscule masses and lack of observed decays. Long-lived particles include the , with a mean lifetime of 2.197 μs, decaying primarily via the into an , electron antineutrino, and , and the free , with a lifetime of approximately 880 seconds, undergoing into a proton, , and electron antineutrino. Short-lived resonances, such as the Δ (a of the proton or composed of three quarks), exist fleetingly with lifetimes around 10^{-24} seconds before decaying strongly into a and , reflecting their role as intermediate states in high-energy collisions. Specific decay examples illustrate these classifications and the mediating interactions. The neutron's weak decay, n→p+e−+νˉen \to p + e^- + \bar{\nu}_e, releases about 0.782 MeV and proceeds via a virtual W⁻ boson exchange, changing a down quark to an up quark; this process is crucial for understanding stellar nucleosynthesis and has a measured branching ratio near 100%. Similarly, the negatively charged pion decays through the weak interaction as π−→μ−+νˉμ\pi^- \to \mu^- + \bar{\nu}_\mu, with a lifetime of 2.60 × 10^{-8} seconds and a dominant branching ratio of 99.99%, highlighting the suppression of charged-current weak processes compared to electromagnetic ones. These decays underscore how interaction strengths and conservation principles dictate particle stability, with strong decays being the fastest (e.g., resonances), followed by electromagnetic, and then weak processes for long-lived cases.

Physical Properties

Electric Charge and Color Charge

Subatomic particles possess as a fundamental property that determines their interactions via the electromagnetic force. This charge is quantized in integer multiples of the ee, defined exactly as 1.602176634×10−191.602176634 \times 10^{-19} coulombs since the 2019 redefinition of the SI units. Among leptons, the carries a charge of −e-e, while neutrinos have zero charge; protons, composed of quarks, have +e+e, and neutrons are electrically neutral. Quarks exhibit fractional charges: up-type quarks (u, c, t) have +23e+\frac{2}{3}e, and down-type quarks (d, s, b) have −13e-\frac{1}{3}e. Electric charge is strictly conserved in all known physical processes, including electromagnetic, weak, and strong interactions, ensuring that the total charge before and after any interaction remains unchanged. This conservation law underpins the stability of atoms and nuclei, as charge imbalances would lead to unstable configurations. The electromagnetic force between charged particles follows Coulomb's law, where the force FF is proportional to q1q2r2\frac{q_1 q_2}{r^2}, with q1q_1 and q2q_2 as the charges and rr the separation distance, mediating repulsion between like charges and attraction between opposites. In addition to electric charge, quarks and gluons carry color charge, a quantum number associated with the strong nuclear force described by quantum chromodynamics (QCD). Color charge transforms under the SU(3) gauge group, with quarks possessing one of three color states—arbitrarily labeled red, green, or blue—and antiquarks carrying the corresponding anticolors (antired, antigreen, antiblue). Gluons, the mediators of the strong force, are eight massless bosons that carry a color-anticolor combination, enabling them to couple to quarks and other gluons, unlike photons in electromagnetism. Color charge is conserved in strong interactions, but physical particles observed in nature, such as hadrons, must be color singlets—combinations where the net color is zero, like a white light formed by mixing red, green, and blue. A key consequence of QCD is : color-charged particles like quarks and are never observed in isolation due to the strong force increasing with distance, binding them into color-neutral hadrons such as protons and mesons. This phenomenon arises from the non-Abelian nature of SU(3), leading to gluon self-interactions that generate a linear potential at large separations, preventing free quarks despite at short distances. Leptons, lacking , do not participate in strong interactions.

Spin and Angular Momentum

Spin is an intrinsic form of possessed by subatomic particles, independent of their orbital motion, and quantified in units of the reduced Planck's constant ℏ\hbar. The magnitude of this spin angular momentum for a particle is given by s(s+1)ℏ2s(s+1)\hbar^2, where ss is the , while its projection along a chosen axis is msℏm_s \hbar with msm_s ranging from −s-s to +s+s in integer steps. Particles are classified based on their spin values: fermions, such as electrons and quarks, have half-integer spins (e.g., s=1/2s = 1/2 for the electron), whereas bosons, like photons and gluons, have integer spins (e.g., s=1s = 1 for the photon). The spin of subatomic particles is measured through experiments that exploit their interaction with magnetic fields or polarization effects. The Stern-Gerlach experiment, conducted in 1922, demonstrated the quantized nature of electron spin by passing a beam of silver atoms (whose magnetism arises from unpaired electron spins) through an inhomogeneous , resulting in discrete deflections corresponding to spin projections of ±ℏ/2\pm \hbar/2. For massless particles like photons and originally assumed massless neutrinos, spin is characterized by helicity, the projection of spin along the direction of motion, which is fixed at ±s\pm s due to the absence of a ; for instance, neutrinos in the are left-handed with helicity −1/2-1/2. The value of spin has profound implications for particle behavior, dictating their quantum statistics via the spin-statistics theorem, which connects half-integer spin to antisymmetric wave functions (Fermi-Dirac statistics) and integer spin to symmetric ones (Bose-Einstein statistics). This theorem underpins phenomena like the for electrons in atoms. Additionally, spin influences atomic spectra through the , where spin-orbit coupling—the interaction between an electron's spin and its orbital motion in the of the nucleus—splits energy levels, leading to closely spaced spectral lines observable in hydrogen's . Intrinsic spin must be distinguished from orbital , which arises from a particle's motion around a center. The total angular momentum J\mathbf{J} of a particle or system is the vector sum J=L+S\mathbf{J} = \mathbf{L} + \mathbf{S}, where L\mathbf{L} is the orbital contribution (with integer ll) and S\mathbf{S} is the spin (with ss). This determines the possible total angular momentum jj from ∣l−s∣|l - s| to l+sl + s, affecting selection rules in transitions and the overall structure of matter.

Magnetic Moment and Other Intrinsic Properties

The magnetic moment of a subatomic particle arises from its spin angular momentum and is described by the relation μ⃗=ge2mS⃗\vec{\mu} = g \frac{e}{2m} \vec{S}
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