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Particle physics or high energy physics is the study of fundamental particles and interactions that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). There are three generations of fermions, which ordinary matter are made only from the first fermion generation. The first generation consists of up and down quarks composing protons and neutrons, electrons, and electron neutrinos. The three fundamental interactions known to be mediated by bosons are electromagnetism, the weak interaction, and the strong interaction. The conciliation of gravity to the current particle physics theory is not solved; many theories have proposed to do so, such as string theory and supersymmetry theory.

History

Early developments

The idea that all matter is fundamentally composed of elementary particles dates from at least the 6th century BC.^[1] In the 19th century, John Dalton, through his work on stoichiometry, concluded that each element of nature was composed of a single, unique type of particle.^[2] The word atom, after the Greek word atomos meaning "indivisible", has since then denoted the smallest particle of a chemical element.^{[citation needed]}

Physicists soon discovered that atoms are not, in fact, the fundamental particles of nature, but are conglomerates of even smaller particles, such as the electron. The early 20th century explorations of nuclear physics and quantum physics led to proofs of nuclear fission in 1939 by Lise Meitner (based on experiments by Otto Hahn), and nuclear fusion by Hans Bethe in that same year; both discoveries also led to the development of nuclear weapons.^{[citation needed]}

Quantum mechanics

Throughout the 1950s and 1960s, a bewildering variety of particles were found in collisions of particles from beams of increasingly high energy. It was referred to informally as the "particle zoo". Important discoveries such as the CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance.^{[citation needed]} 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 of quantum field theories. This reclassification marked the beginning of modern particle physics.^[3]^[4]

Standard Model

The current state of the classification of all elementary particles is explained by the Standard Model, which gained widespread acceptance in the mid-1970s after experimental confirmation of the existence of quarks. It describes the strong, weak, and electromagnetic fundamental interactions, using mediating gauge bosons. The species of gauge bosons are eight gluons,
W⁻
,
W⁺
and
Z
bosons, and the photon.^[5] The Standard Model also contains 24 fundamental fermions (12 particles and their associated anti-particles), which are the constituents of all matter.^[6] Finally, the Standard Model also predicted the existence of a type of boson known as the Higgs 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.^[7]

Subatomic particles

A subatomic particle is a particle that composes an atom.^{[citation needed]} Dynamics of these particles are governed by quantum mechanics; they exhibit wave–particle duality, displaying particle-like behaviour under certain experimental conditions and wave-like behaviour in others. In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. The term elementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.^[8] Most particles and their interactions observed to date can be described by the Standard Model.^[9]

Quarks and leptons

Ordinary matter is made from first-generation quarks (up, down) and leptons (electron, electron neutrino).^[10] Collectively, quarks and leptons are called fermions, because they have a quantum spin of half-integers (-1/2, 1/2, 3/2, etc.). This cause the fermions to obey the Pauli exclusion principle, where no two particles may occupy the same quantum state.^[11] Quarks have fractional elementary electric charge (-1/3 or 2/3)^[12] and leptons have whole-numbered electric charge (0 or 1).^{[citation needed]} Quarks also have color charge, which is labeled arbitrarily with no correlation to actual light color as red, green and blue.^[13] Because the interactions between the quarks stores energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is called color confinement.^[14]

There are three known generations of quarks (up and down, strange and charm, top and bottom) and leptons (electron and its neutrino, muon and its neutrino, tau and its neutrino), with strong indirect evidence that the fourth generation of fermions doesn't exist.^[15]

Bosons

Bosons are the mediators or carriers of fundamental interactions, such as electromagnetism, the weak interaction, and the strong interaction.^[16] Electromagnetism is mediated by the photon, the quanta of light.^[17]^: 29–30 The weak interaction is mediated by the W and Z bosons.^[18] The strong interaction is mediated by the gluon, which can link quarks together to form composite particles.^[19] Due to the aforementioned color confinement, gluons are never observed independently.^[20] The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism^[21] – the gluon and photon are expected to be massless.^[20] All bosons have an integer quantum spin (0 and 1) and can have the same quantum state.^[16]

Antiparticles and color charge

Most aforementioned particles have corresponding antiparticles, which compose antimatter. Normal particles have positive lepton or baryon number, and antiparticles have these numbers negative.^[22] 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 in superscript. For example, the electron and the positron are denoted
e⁻
and
e⁺
.^[23] When a particle and an antiparticle interacts with each other, they are annihilated and convert to other particles.^[24] Some particles have no antiparticles, such as the photon or gluon.^{[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.^[25] The gluon can have eight color charges, which are the result of quarks' interactions to form composite particles (gauge symmetry SU(3)).^[26]

Composite

The neutrons and protons in the atomic nuclei are baryons – 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.^[27] A baryon is composed of three quarks, and a meson is composed of two quarks (one normal, one anti). Baryons and mesons are collectively called hadrons. Quarks inside hadrons are governed by the strong interaction, thus are subjected to quantum chromodynamics (color charges). The bounded quarks must have their color charge to be neutral, or "white" for analogy with mixing the primary colors.^[28] More exotic hadrons can have other types, arrangement or number of quarks (tetraquark, pentaquark).^[29]

A normal atom is made from protons, neutrons and electrons.^{[citation needed]} By modifying the particles inside a normal atom, exotic atoms can be formed.^[30] A simple example would be the hydrogen-4.1, which has one of its electrons replaced with a muon.^[31]

Hypothetical

Graviton is a hypothetical particle that can mediate the gravitational interaction, but it has not being detected nor completely reconciled with current theories.^[32]

Fundamental interactions

Quantum fields

Conservation laws

Quantum gravity problem

Another major effort is in model building where model builders develop ideas for what physics may lie beyond the Standard Model (at higher energies or smaller distances). There are several powerful experimental reasons to expect new physics, including dark matter and neutrino mass. This work is often motivated by the hierarchy problem and is constrained by existing experimental data. It may involve work on supersymmetry, alternatives to the Higgs mechanism, extra spatial dimensions (such as the Randall–Sundrum models), Preon theory, combinations of these, or other ideas. A third major effort in theoretical particle physics is string theory. String theorists attempt to construct a unified description of quantum mechanics and general relativity by building a theory based on small strings, and branes rather than particles. If the theory is successful, it may be considered a "Theory of Everything".^{[citation needed]}

Experimental evidence

One important branch attempts to better understand the Standard Model and its tests. Theorists make quantitative predictions of observables at collider and astronomical 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 in quantum chromodynamics. Some theorists working in this area use the tools of perturbative quantum field theory and effective field theory, referring to themselves as phenomenologists.^{[citation needed]} Others make use of lattice field theory and call themselves lattice theorists.

Facilities

The world's major particle physics laboratories are:

Brookhaven National Laboratory (Long Island, United States). Its main facility is the Relativistic Heavy Ion Collider (RHIC), which collides heavy 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.^[33]^[34]
Budker Institute of Nuclear Physics (Novosibirsk, Russia). Its main projects are now the electron-positron colliders VEPP-2000,^[35] operated since 2006, and VEPP-4,^[36] started experiments in 1994. Earlier facilities include the first electron–electron beam–beam collider VEP-1, which conducted experiments from 1964 to 1968; the electron-positron colliders VEPP-2, operated from 1965 to 1974; and, its successor VEPP-2M,^[37] performed experiments from 1974 to 2000.^[38]
CERN (European Organization for Nuclear Research) (Franco-Swiss border, near Geneva). Its main project is now the Large 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 the Large Electron–Positron Collider (LEP), which was stopped on 2 November 2000 and then dismantled to give way for LHC; and the Super Proton Synchrotron, which is being reused as a pre-accelerator for the LHC and for fixed-target experiments.^[39]
DESY (Deutsches Elektronen-Synchrotron) (Hamburg, Germany). Its main facility was the Hadron Elektron Ring Anlage (HERA), which collided electrons and positrons with protons.^[40] The accelerator complex is now focused on the production of synchrotron radiation with PETRA III, FLASH and the European XFEL.
Fermi National Accelerator Laboratory (Fermilab) (Batavia, United States). Its main facility until 2011 was the Tevatron, 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.^[41]
Institute of High Energy Physics (IHEP) (Beijing, China). IHEP manages a number of China's major particle physics facilities, including the Beijing Electron–Positron Collider II(BEPC II), the Beijing Spectrometer (BES), the Beijing Synchrotron Radiation Facility (BSRF), the International Cosmic-Ray Observatory at Yangbajing in Tibet, the Daya Bay Reactor Neutrino Experiment, the China Spallation Neutron Source, the Hard X-ray Modulation Telescope (HXMT), and the Accelerator-driven Sub-critical System (ADS) as well as the Jiangmen Underground Neutrino Observatory (JUNO).^[42]
KEK (Tsukuba, Japan). It is the home of a number of experiments such as the K2K experiment, a neutrino oscillation experiment and Belle II, an experiment measuring the CP violation of B mesons.^[43]
SLAC National Accelerator Laboratory (Menlo Park, United States). Its 2-mile-long linear particle accelerator began operating in 1962 and was the basis for numerous electron and positron collision experiments until 2008. Since then the linear accelerator is being used for the Linac Coherent Light Source X-ray laser as well as advanced accelerator design research. SLAC staff continue to participate in developing and building many particle detectors around the world.^[44]

Many other particle accelerators also exist. The techniques required for modern experimental particle physics are quite varied and complex, constituting a sub-specialty nearly completely distinct from the theoretical side of the field.^{[citation needed]}

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v t e Major branches of physics
Divisions	Pure Applied Engineering
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Modern	Relativistic mechanics Special General Nuclear physics Quantum mechanics Particle physics Atomic, molecular, and optical physics Atomic Molecular Modern optics Condensed matter physics
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Related	History of physics Nobel Prize in Physics Philosophy of physics Physics education Timeline of physics discoveries

v t e The fundamental interactions of physics
Physical forces	Strong interaction fundamental residual Electroweak interaction weak interaction electromagnetism Gravitation
Radiations	Electromagnetic radiation Gravitational radiation
Hypothetical forces	Fifth force Quintessence Weak gravity conjecture
Glossary of physics Particle physics Philosophy of physics Universe Weakless universe