Inphysics, asubatomic particle is aparticle smaller than anatom.[1] According to theStandard Model of particle physics, a subatomic particle can be either acomposite particle, which is composed of other particles (for example, abaryon, like aproton or aneutron, composed of threequarks; or ameson, composed of twoquarks), or anelementary particle, which is not composed of other particles (for example, quarks; orelectrons,muons, andtau particles, which are calledleptons).[2]Particle physics andnuclear physics study these particles and how they interact.[3] Most force-carrying particles likephotons orgluons are calledbosons 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 calledfermions. TheW and Z bosons, however, are an exception to this rule and have relatively large rest masses at approximately80 GeV/c2 and90 GeV/c2 respectively.
Experiments show that light could behave like astream of particles (calledphotons) as well as exhibiting wave-like properties. This led to the concept ofwave–particle duality to reflect that quantum-scaleparticles behave both like particles and likewaves; they are occasionally calledwavicles to reflect this.[4]
Even amongparticle physicists, the exact definition of a particle has diverse descriptions. These professional attempts at the definition of a particle include:[6]
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 word hadron comes from Greek and was introduced in 1962 byLev Okun.[8] Nearly all composite particles contain multiple quarks (and/or antiquarks) bound together by gluons (with a few exceptions with no quarks, such aspositronium andmuonium). Those containing few (≤ 5) quarks (including antiquarks) are calledhadrons. Due to a property known ascolor 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 thebaryons containing an odd number of quarks (almost always 3), of which theproton andneutron (the twonucleons) are by far the best known; and themesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which thepions andkaons 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 twoup quarks and onedown 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) formexotic nuclei.
Any subatomic particle, like any particle in thethree-dimensional space that obeys thelaws ofquantum mechanics, can be either a boson (with integerspin) 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 carrycolor charge and therefore feel the strong interaction, and theleptons 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 assupersymmetry 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.
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 (thetau particle) is heavier than the two lightest flavours of baryons (nucleons). It is also certain that any particle with anelectric 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.
Allmassless particles (particles whoseinvariant mass is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.
Most subatomic particles are not stable. All leptons, as well as baryonsdecay by either the strong force or weak force (except for the proton). Protons are not known todecay, 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 ofneutrino oscillations is thought to exist even in vacuums. The electron and its antiparticle, thepositron, are theoretically stable due tocharge conservation unless a lighter particle havingmagnitude of electric charge≤ e exists (which is unlikely). Its charge is not shown yet.
All observable subatomic particles have their electric charge aninteger multiple of theelementary charge. The Standard Model's quarks have "non-integer" electric charges, namely, multiple of1/3e, but quarks (and other combinations with non-integer electric charge) cannot be isolated due tocolor confinement. For baryons, mesons, and their antiparticles the constituent quarks' charges sum up to an integer multiple ofe.
Through the work ofAlbert Einstein,Satyendra Nath Bose,Louis de Broglie, and many others, current scientific theory holds thatall 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 ofconservation of energy andconservation 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 ofNewtonian mechanics, a series of statements and equations inPhilosophiae Naturalis Principia Mathematica, originally published in 1687.
The negatively charged electron has a mass of about1/1836 of that of ahydrogen atom. The remainder of the hydrogen atom's mass comes from the positively chargedproton. Theatomic 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. Differentisotopes of the same element contain the same number of protons but different numbers of neutrons. Themass number of an isotope is the total number ofnucleons (neutrons and protons collectively).
Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals andmolecules. The subatomic particles considered important in the understanding of chemistry are theelectron, theproton, and theneutron.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, requiresquantum mechanics. Analyzing processes that change the numbers and types of particles requiresquantum field theory. The study of subatomic particlesper se is calledparticle physics. The termhigh-energy physics is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result ofcosmic rays, or inparticle accelerators.Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.[12]
The term "subatomic particle" is largely aretronym of the 1960s, used to distinguish a large number ofbaryons andmesons (which comprisehadrons) from particles that are now thought to betruly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown.
^Hunter, Geoffrey; Wadlinger, Robert L. P. (August 23, 1987). Honig, William M.; Kraft, David W.; Panarella, Emilio (eds.).Quantum Uncertainties: Recent and Future Experiments and Interpretations. Springer US. pp. 331–343.doi:10.1007/978-1-4684-5386-7_18.The finite-field model of the photon is both a particle and a wave, and hence we refer to it by Eddington's name "wavicle".
^Okun, Lev (1962). "The theory of weak interaction".Proceedings of 1962 International Conference on High-Energy Physics at CERN. International Conference on High-Energy Physics (plenary talk). CERN, Geneva, CH. p. 845.Bibcode:1962hep..conf..845O.
^Eisberg, R. & Resnick, R. (1985).Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.).John Wiley & Sons. pp. 59–60.ISBN978-0-471-87373-0.For both large and small wavelengths, both matter and radiation have both particle and wave aspects. [...] But the wave aspects of their motion become more difficult to observe as their wavelengths become shorter. [...] For ordinary macroscopic particles the mass is so large that the momentum is always sufficiently large to make the de Broglie wavelength small enough to be beyond the range of experimental detection, and classical mechanics reigns supreme.
^Taiebyzadeh, Payam (2017).String Theory: A Unified Theory and Inner Dimension Of Elementary Particles (Baz Dahm). Iran: Shamloo Publications.ISBN978-6-00-116684-6.
^Zichichi, A. (1996)."Foundations of sequential heavy lepton searches"(PDF). In Newman, H.B.; Ypsilantis, T. (eds.).History of Original Ideas and Basic Discoveries in Particle Physics. NATO ASI Series (Series B: Physics). Vol. 352. Boston, MA: Springer. pp. 227–275.
Oerter, Robert (2006).The theory of almost everything: the Standard Model, the unsung triumph of modern physics. New York, New York:Pi Press.ISBN978-0-452-28786-0.
