Particle physicsorhigh-energy physicsis the study offundamental particlesandforcesthat constitutematterandradiation.The field also studies combinations of elementary particles up to the scale ofprotonsandneutrons,while the study of combination of protons and neutrons is callednuclear physics.
Quarkscannot exist on their own but formhadrons.Hadrons that contain an odd number of quarks are calledbaryonsand those that contain an even number are calledmesons.Two baryons, theprotonand 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 incyclotronsor otherparticle accelerators.
Particles have correspondingantiparticleswith the samemassbut with oppositeelectric charges.For example, the antiparticle of theelectronis 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.
Practical particle physics is the study of these particles inradioactiveprocesses and in particle accelerators such as theLarge Hadron Collider.Theoretical particle physics is the study of these particles in the context ofcosmologyandquantum theory.The two are closely interrelated: theHiggs bosonwas postulated by theoretical particle physicists and its presence confirmed by practical experiments.
TheGeiger–Marsden experimentsobserved that a small fraction of the alpha particles experienced strong deflection when being struck by the gold foil.
The idea that allmatteris fundamentally composed ofelementary particlesdates 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 wordatomosmeaning "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 physicsandquantum physicsled to proofs ofnuclear fissionin 1939 byLise Meitner(based on experiments byOtto Hahn), andnuclear fusionbyHans Bethein that same year; both discoveries also led to the development ofnuclear weapons.
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 violationbyJames CroninandVal Fitchbrought new questions tomatter-antimatter imbalance.[3]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.[4][5]
The current state of the classification of all elementary particles is explained by theStandard Model,which gained widespread acceptance in the mid-1970s afterexperimental confirmationof the existence ofquarks.It describes thestrong,weak,andelectromagneticfundamental interactions,using mediatinggauge bosons.The species of gauge bosons are eightgluons, W− , W+ and Z bosons,and thephoton.[6]The Standard Model also contains 24fundamentalfermions(12 particles and their associated anti-particles), which are the constituents of allmatter.[7]Finally, the Standard Model also predicted the existence of a type ofbosonknown 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.[8]
The Standard Model, as currently formulated, has 61 elementary particles.[9]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 theexperimentaltests 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 ofneutrinomasshave provided the first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model.[10]
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 statevectors in aHilbert space,which is also treated inquantum field theory.Following the convention of particle physicists, the termelementary particlesis applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.[9]
Ordinarymatteris made from first-generationquarks (up,down) and leptons (electron,electron neutrino).[12]Collectively, quarks and leptons are calledfermions,because they have aquantum spinofhalf-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.[13]Quarks have fractionalelementary electric charge(−1/3 or 2/3)[14]and leptons have whole-numbered electric charge (0 or 1).[15]Quarks also havecolor charge,which is labeled arbitrarily with no correlation to actual lightcoloras red, green and blue.[16]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.[16]
There are three known generations of quarks (up and down,strangeandcharm,topandbottom) and leptons (electron and its neutrino,muonandits neutrino,tauandits neutrino), with strong indirect evidence that a fourth generation of fermions does not exist.[17]
Most aforementioned particles have correspondingantiparticles,which composeantimatter.Normal particles have positiveleptonorbaryon number,and antiparticles have these numbers negative.[24]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 denoted e− and e+ .[25]When a particle and an antiparticle interact with each other, they areannihilatedand convert to other particles.[26]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.[16]The gluon can haveeight color charges,which are the result of quarks' interactions to form composite particles (gauge symmetrySU(3)).[27]
Aprotonconsists of two up quarks and one down quark, linked together bygluons.The quarks' color charge are also visible.
Theneutronsandprotonsin theatomic nucleiarebaryons– 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.[28]A baryon is composed of three quarks, and amesonis 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). Theboundedquarks must have their color charge to be neutral, or "white" for analogy withmixing the primary colors.[29]Moreexotic hadronscan have other types, arrangement or number of quarks (tetraquark,pentaquark).[30]
An atom is made from protons, neutrons and electrons.[31]By modifying the particles inside a normal atom,exotic atomscan be formed.[32]A simple example would be thehydrogen-4.1,which has one of its electrons replaced with a muon.[33]
Thegravitonis a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories.[34]Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably,supersymmetricparticles aim to solve thehierarchy problem,axionsaddress thestrong CP problem,and various other particles are proposed to explain the origins ofdark matteranddark energy.
Budker Institute of Nuclear Physics(Novosibirsk,Russia). Its main projects are now the electron-positroncollidersVEPP-2000,[37]operated since 2006, and VEPP-4,[38]started experiments in 1994. Earlier facilities include the first electron–electron beam–beamcolliderVEP-1, which conducted experiments from 1964 to 1968; the electron-positroncollidersVEPP-2, operated from 1965 to 1974; and, its successor VEPP-2M,[39]performed experiments from 1974 to 2000.[40]
CMSdetector for LHCCERN(European Organization for Nuclear Research) (Franco-Swissborder, nearGeneva). 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.[41]
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 Modeland its tests. Theorists make quantitative predictions of observables atcolliderandastronomicalexperiments, 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 theoryandeffective field theory,referring to themselves asphenomenologists.[citation needed]Others make use oflattice field theoryand 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 problemand is constrained by existing experimental data.[47][48]It may involve work onsupersymmetry,alternatives to theHiggs mechanism,extra spatial dimensions (such as theRandall–Sundrum models),Preontheory, combinations of these, or other ideas.Vanishing-dimensions theoryis a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions.[49]
A third major effort in theoretical particle physics isstring theory.String theoristsattempt to construct a unified description ofquantum mechanicsandgeneral relativityby building a theory based on small strings, andbranesrather than particles. If the theory is successful, it may be considered a "Theory of Everything",or" TOE ".[50]
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 isotopesfor research and treatment (for example, isotopes used inPET imaging), or used directly inexternal beam radiotherapy.The development ofsuperconductorshas been pushed forward by their use in particle physics. TheWorld Wide Webandtouchscreentechnology 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.[51]
^Terranova, Francesco (2021).A Modern Primer in Particle and Nuclear Physics.Oxford Univ. Press.ISBN978-0-19-284524-5.
^B. Povh; K. Rith; C. Scholz; F. Zetsche; M. Lavelle (2004)."Part I: Analysis: The building blocks of matter".Particles and Nuclei: An Introduction to the Physical Concepts(4th ed.). Springer.ISBN978-3-540-20168-7.Archivedfrom the original on 22 April 2022.Retrieved28 July2022.Ordinary matter is composed entirely of first-generation particles, namely the u and d quarks, plus the electron and its neutrino.
^abCarroll, Sean(2007).Guidebook.Dark Matter, Dark Energy: The dark side of the universe. The Teaching Company. Part 2, p. 43.ISBN978-1598033502.... boson: A force-carrying particle, as opposed to a matter particle (fermion). Bosons can be piled on top of each other without limit. Examples are photons, gluons, gravitons, weak bosons, and the Higgs boson. The spin of a boson is always an integer: 0, 1, 2, and so on...
^Bernardi, G.; Carena, M.; Junk, T. (2007)."Higgs bosons: Theory and searches"(PDF).Review: Hypothetical particles and Concepts. Particle Data Group.Archived(PDF)from the original on 3 October 2018.Retrieved28 July2022.
^Tsan, Ung Chan (2013). "Mass, Matter, Materialization, Mattergenesis and Conservation of Charge".International Journal of Modern Physics E.22(5): 1350027.Bibcode:2013IJMPE..2250027T.doi:10.1142/S0218301313500274.Matter conservation means conservation of baryonic numberAand leptonic numberL,AandLbeing algebraic numbers. PositiveAandLare associated to matter particles, negativeAandLare associated to antimatter particles. All known interactions do conserve matter.
^Raith, W.; Mulvey, T. (2001).Constituents of Matter: Atoms, Molecules, Nuclei and Particles.CRC Press.pp. 777–781.ISBN978-0-8493-1202-1.
^§1.8,Constituents of Matter: Atoms, Molecules, Nuclei and Particles,Ludwig Bergmann, Clemens Schaefer, and Wilhelm Raith, Berlin: Walter de Gruyter, 1997,ISBN3-11-013990-1.
