Particle physics orhigh-energy physics is the study offundamental particles andforces that constitutematter andradiation. The field also studies combinations of elementary particles up to the scale ofprotons andneutrons, while the study of combinations of protons and neutrons is callednuclear physics.

The fundamental particles in theuniverse are classified in theStandard Model asfermions (matter particles) andbosons (force-carrying particles). There are threegenerations of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists ofup anddown quarks which formprotons andneutrons, andelectrons andelectron neutrinos. The three fundamental interactions known to be mediated by bosons areelectromagnetism, theweak interaction, and thestrong interaction.

Quarks cannot exist on their own but formhadrons. Hadrons that contain an odd number of quarks are calledbaryons and those that contain an even number are calledmesons. Two baryons, theproton and theneutron, make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of amicrosecond. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons incosmic rays. Mesons are also produced incyclotrons or otherparticle accelerators.

Particles have correspondingantiparticles with the samemass but with oppositeelectric charges. For example, the antiparticle of theelectron is thepositron. The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter calledantimatter. Some particles, such as thephoton, are their own antiparticle.

Theseelementary particles are excitations of thequantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called theStandard Model. Thereconciliation of gravity to the current particle physics theory is not solved; many theories have addressed this problem, such asloop quantum gravity,string theory andsupersymmetry theory.

Experimental particle physics is the study of these particles inradioactive processes and in particle accelerators such as theLarge Hadron Collider. Theoretical particle physics is the study of these particles in the context ofcosmology andquantum theory. The two are closely interrelated: theHiggs boson was postulated theoretically before being confirmed by experiments.

The idea that allmatter is fundamentally composed ofelementary particles dates from at least the 6th century BC.^[1] In the 19th century,John Dalton, through his work onstoichiometry, concluded that each element of nature was composed of a single, unique type of particle.^[2] The wordatom, after the Greek wordatomos meaning "indivisible", has since then denoted the smallest particle of achemical element, but physicists later discovered that atoms are not, in fact, the fundamental particles of nature, but are conglomerates of even smaller particles, such as theelectron. The early 20th century explorations ofnuclear physics andquantum physics led to proofs ofnuclear fission in 1939 byLise Meitner (based on experiments byOtto Hahn), andnuclear fusion byHans Bethe in that same year; both discoveries also led to the development ofnuclear weapons. Bethe's 1947 calculation of theLamb shift is credited with having "opened the way to the modern era of particle physics".^[3]

Throughout the 1950s and 1960s, a bewildering variety of particles was found in collisions of particles from beams of increasingly high energy. It was referred to informally as the "particle zoo". Important discoveries such as theCP violation byJames Cronin andVal Fitch brought new questions tomatter-antimatter imbalance.^[4] After the formulation of the Standard Model during the 1970s, physicists clarified the origin of the particle zoo. The large number of particles was explained as combinations of a (relatively) small number of more fundamental particles and framed in the context ofquantum field theories. This reclassification marked the beginning of modern particle physics.^[5][6]

The current state of the classification of all elementary particles is explained by theStandard Model, which gained widespread acceptance in the mid-1970s afterexperimental confirmation of the existence ofquarks. It describes thestrong,weak, andelectromagneticfundamental interactions, using mediatinggauge bosons. The species of gauge bosons are eightgluons,W⁻
,W⁺
andZ bosons, and thephoton.^[7] The Standard Model also contains 24fundamentalfermions (12 particles and their associated anti-particles), which are the constituents of allmatter.^[8] Finally, the Standard Model also predicted the existence of a type ofboson known as theHiggs boson. On 4 July 2012, physicists with the Large Hadron Collider at CERN announced they had found a new particle that behaves similarly to what is expected from the Higgs boson.^[9]

The Standard Model, as currently formulated, has 61 elementary particles.^[10] Those elementary particles can combine to form composite particles, accounting for the hundreds of other species of particles that have been discovered since the 1960s. The Standard Model has been found to agree with almost all theexperimental tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature and that a more fundamental theory awaits discovery (SeeTheory of Everything). In recent years, measurements ofneutrinomass have provided the first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model.^[11]

Modern particle physics research is focused onsubatomic particles, including atomic constituents, such aselectrons,protons, andneutrons (protons and neutrons are composite particles calledbaryons, made ofquarks), that are produced byradioactive andscattering processes; such particles arephotons,neutrinos, andmuons, as well as a wide range ofexotic particles.^[12] All particles and their interactions observed to date can be described almost entirely by the Standard Model.^[7]

Dynamics of particles are also governed byquantum mechanics; they exhibitwave–particle duality, displaying particle-like behaviour under certain experimental conditions andwave-like behaviour in others. In more technical terms, they are described byquantum state vectors in aHilbert space, which is also treated inquantum field theory. Following the convention of particle physicists, the termelementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.^[10]

Ordinarymatter is made from first-generation quarks (up,down) and leptons (electron,electron neutrino).^[13] Collectively, quarks and leptons are calledfermions, because they have aquantum spin ofhalf-integers (−1/2, 1/2, 3/2, etc.). This causes the fermions to obey thePauli exclusion principle, where no two particles may occupy the samequantum state.^[14] Quarks have fractionalelementary electric charge (−1/3 or 2/3)^[15] and leptons have whole-numbered electric charge (0 or -1).^[16] Quarks also havecolor charge, which is labeled arbitrarily with no correlation to actual lightcolor as red, green and blue.^[17] Because the interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is calledcolor confinement.^[17]

There are three known generations of quarks (up and down,strange andcharm,top andbottom) and leptons (electron and its neutrino,muon andits neutrino,tau andits neutrino), with strong indirect evidence that a fourth generation of fermions does not exist.^[18]

Bosons are themediators or carriers of fundamental interactions, such aselectromagnetism, theweak interaction, and thestrong interaction.^[19] Electromagnetism is mediated by thephoton, thequanta oflight.^{[20]: 29–30} The weak interaction is mediated by theW and Z bosons.^[21] The strong interaction is mediated by thegluon, which can link quarks together to form composite particles.^[22] Due to the aforementioned color confinement, gluons are never observed independently.^[23] TheHiggs boson gives mass to the W and Z bosons via theHiggs mechanism^[24] – the gluon and photon are expected to bemassless.^[23] All bosons have an integer quantum spin (0 and 1) and can have the samequantum state.^[19]

Most aforementioned particles have correspondingantiparticles, which composeantimatter. Normal particles have positivelepton orbaryon number, and antiparticles have these numbers negative.^[25] Most properties of corresponding antiparticles and particles are the same, with a few gets reversed; the electron's antiparticle, positron, has an opposite charge. To differentiate between antiparticles and particles, a plus or negative sign is added insuperscript. For example, the electron and the positron are denotede⁻
ande⁺
.^[26] However, in the case that the particle has a charge of 0 (equal to that of the antiparticle), the antiparticle is denoted with a line above the symbol. As such, an electron neutrino isν
_e, whereas its antineutrino isν
_e. When a particle and an antiparticle interact with each other, they areannihilated and convert to other particles.^[27] Some particles, such as the photon or gluon, have no antiparticles.^{[citation needed]}

Quarks and gluons additionally have color charges, which influences the strong interaction. Quark's color charges are called red, green and blue (though the particle itself have no physical color), and in antiquarks are called antired, antigreen and antiblue.^[17] The gluon can haveeight color charges, which are the result of quarks' interactions to form composite particles (gauge symmetrySU(3)).^[28]

Theneutrons andprotons in theatomic nuclei arebaryons – the neutron is composed of two down quarks and one up quark, and the proton is composed of two up quarks and one down quark.^[29] A baryon is composed of three quarks, and ameson is composed of two quarks (one normal, one anti). Baryons and mesons are collectively calledhadrons. Quarks inside hadrons are governed by the strong interaction, thus are subjected toquantum chromodynamics (color charges). Thebounded quarks must have their color charge to be neutral, or "white" for analogy withmixing the primary colors.^[30] Moreexotic hadrons can have other types, arrangement or number of quarks (tetraquark,pentaquark).^[31]

An atom is made from protons, neutrons and electrons.^[32] By modifying the particles inside a normal atom,exotic atoms can be formed.^[33] A simple example would be thehydrogen-4.1, which has one of its electrons replaced with a muon.^[34]

Thegraviton is a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories.^[35] Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably,supersymmetric particles aim to solve thehierarchy problem,axions address thestrong CP problem, and various other particles are proposed to explain the origins ofdark matter anddark energy.

The world's major particle physics laboratories are:

• Brookhaven National Laboratory (Long Island, New York,United States). Its main facility is theRelativistic Heavy Ion Collider (RHIC), which collidesheavy ions such as gold ions and polarized protons. It is the world's first heavy ion collider, and the world's only polarized proton collider.^[36][37]
• Budker Institute of Nuclear Physics (Novosibirsk,Russia). Its main projects are now the electron-positroncollidersVEPP-2000,^[38] operated since 2006, and VEPP-4,^[39] started experiments in 1994. Earlier facilities include the first electron–electron beam–beamcollider VEP-1, which conducted experiments from 1964 to 1968; the electron-positroncolliders VEPP-2, operated from 1965 to 1974; and, its successor VEPP-2M,^[40] performed experiments from 1974 to 2000.^[41]

•  

  CERN (European Organization for Nuclear Research) (Franco-Swiss border, nearGeneva, Switzerland). Its main project is now theLarge Hadron Collider (LHC), which had its first beam circulation on 10 September 2008, and is now the world's most energetic collider of protons. It also became the most energetic collider of heavy ions after it began colliding lead ions. Earlier facilities include theLarge Electron–Positron Collider (LEP), which was stopped on 2 November 2000 and then dismantled to give way for LHC; and theSuper Proton Synchrotron, which is being reused as a pre-accelerator for the LHC and for fixed-target experiments.^[42]
• DESY (Deutsches Elektronen-Synchrotron) (Hamburg,Germany). Its main facility was theHadron Elektron Ring Anlage (HERA), which collided electrons and positrons with protons.^[43] The accelerator complex is now focused on the production ofsynchrotron radiation withPETRA III,FLASH and theEuropean XFEL.
• Fermi National Accelerator Laboratory (Fermilab) (Batavia, Illinois,United States). Its main facility until 2011 was theTevatron, which collided protons and antiprotons and was the highest-energy particle collider on earth until the Large Hadron Collider surpassed it on 29 November 2009.^[44]
• Institute of High Energy Physics (IHEP) (Beijing,China). IHEP manages a number of China's major particle physics facilities, including theBeijing Electron–Positron Collider II(BEPC II), the Beijing Spectrometer (BES), the Beijing Synchrotron Radiation Facility (BSRF), theInternational Cosmic-Ray Observatory at Yangbajing in Tibet, theDaya Bay Reactor Neutrino Experiment, theChina Spallation Neutron Source, theHard X-ray Modulation Telescope (HXMT), and the Accelerator-driven Sub-critical System (ADS) as well as theJiangmen Underground Neutrino Observatory (JUNO).^[45]
• KEK (Tsukuba,Japan). It is the home of a number of experiments such as theK2K experiment, aneutrino oscillation experiment andBelle II, an experiment measuring theCP violation ofB mesons.^[46]
• SLAC National Accelerator Laboratory (Menlo Park, California,United States). Its 2-mile-long linear particle accelerator began operating in 1962 and was the basis for numerouselectron andpositron collision experiments until 2008. Since then the linear accelerator is being used for theLinac Coherent Light SourceX-ray laser as well as advanced accelerator design research. SLAC staff continue to participate in developing and building manyparticle detectors around the world.^[47]

Theoretical particle physics attempts to develop the models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments (see alsotheoretical physics). There are several major interrelated efforts being made in theoretical particle physics today.

One important branch attempts to better understand theStandard Model and its tests. Theorists make quantitative predictions of observables atcollider andastronomical experiments, which along with experimental measurements is used to extract the parameters of the Standard Model with less uncertainty. This work probes the limits of the Standard Model and therefore expands scientific understanding of nature's building blocks. Those efforts are made challenging by the difficulty of calculating high precision quantities inquantum chromodynamics. Some theorists working in this area use the tools of perturbativequantum field theory andeffective field theory, referring to themselves asphenomenologists.^{[citation needed]} Others make use oflattice field theory and call themselveslattice theorists.

Another major effort is in model building where model builders develop ideas for what physics may liebeyond the Standard Model (at higher energies or smaller distances). This work is often motivated by thehierarchy problem and is constrained by existing experimental data.^[48][49] It may involve work onsupersymmetry, alternatives to theHiggs mechanism, extra spatial dimensions (such as theRandall–Sundrum models),Preon theory, combinations of these, or other ideas.Vanishing-dimensions theory is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions.^[50]

A third major effort in theoretical particle physics isstring theory.String theorists attempt to construct a unified description ofquantum mechanics andgeneral relativity by building a theory based on small strings, andbranes rather than particles. If the theory is successful, it may be considered a "Theory of Everything", or "TOE".^[51]

There are also other areas of work in theoretical particle physics ranging fromparticle cosmology toloop quantum gravity.^{[citation needed]}

In principle, all physics (and practical applications developed therefrom) can be derived from the study of fundamental particles. In practice, even if "particle physics" is taken to mean only "high-energy atom smashers", many technologies have been developed during these pioneering investigations that later find wide uses in society. Particle accelerators are used to producemedical isotopes for research and treatment (for example, isotopes used inPET imaging), or used directly inexternal beam radiotherapy. The development ofsuperconductors has been pushed forward by their use in particle physics. TheWorld Wide Web andtouchscreen technology were initially developed atCERN. Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating a long and growing list of beneficial practical applications with contributions from particle physics.^[52]

Major efforts to look forphysics beyond the Standard Model include theFuture Circular Collider proposed for CERN^[53] and theParticle Physics Project Prioritization Panel (P5) in the US that will update the 2014 P5 study that recommended theDeep Underground Neutrino Experiment, among other experiments.