Subatomic particle

 ( Subatomic Particles ) 

In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model, 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 ), or an elementary particle, which is not composed of other particles (for example, quarks; or electrons, muons, and tau particles, which are called leptons).

Experiments show that light could behave like a stream of particles (called ) 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 ; they are occasionally called wavicles to reflect this.

Another concept, the uncertainty principle, 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 are understood as creation and annihilation of quantum of corresponding fundamental interactions. This blends particle physics with field theory.

Even among particle physics, 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

The elementary particles of the Standard Model are:

• Six "flavors" of : Up quark, Down quark, Strange quark, Charm quark, Bottom quark, and Top quark;
• Six types of : electron, electron neutrino, muon, muon neutrino, tau, tau neutrino;
• Twelve (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak interaction, and the eight gluons of the strong force;
• The Higgs boson.

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.

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 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.

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.

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 (). 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.

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.

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.

Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and . 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 , or in particle accelerators. Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.

A list of important discoveries follows:

!Particle
!Composition
!Theorized
!Discovered
!Comments

• (book)
• (book)
• CPT invariance
• Dark matter
• Hot spot effect in subatomic physics
• List of fictional elements, materials, isotopes and atomic particles
• List of particles
• Poincaré symmetry

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• Brian Greene (2025). 9780393058581, Norton. ISBN 9780393058581
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• Bruce A. Schumm (2025). 9780801879715, Johns Hopkins University Press. . ISBN 9780801879715
• Martinus Veltman (2025). 9789812381484, World Scientific. . ISBN 9789812381484

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• David J. Griffiths (2025). 9780471603863, Wiley. . ISBN 9780471603863
• Gordon L. Kane (2025). 9781107165083, Cambridge University Press. . ISBN 9781107165083

• University of California: Particle Data Group.