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112-651: The Higgs boson , sometimes called the Higgs particle , is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field , one of the fields in particle physics theory. In the Standard Model, the Higgs particle is a massive scalar boson with zero spin , even (positive) parity , no electric charge , and no colour charge that couples to (interacts with) mass. It
224-424: A 40-year search , and the construction of one of the world's most expensive and complex experimental facilities to date, CERN 's Large Hadron Collider , in an attempt to create Higgs bosons and other particles for observation and study. On 4 July 2012, the discovery of a new particle with a mass between 125 and 127 GeV/ c was announced; physicists suspected that it was the Higgs boson. Since then,
336-728: A gauge theory interact with each other by the exchange of gauge bosons, usually as virtual particles . Photons , W and Z bosons , and gluons are gauge bosons. All known gauge bosons have a spin of 1; for comparison, the Higgs boson has spin zero and the hypothetical graviton has a spin of 2. Therefore, all known gauge bosons are vector bosons . Gauge bosons are different from the other kinds of bosons: first, fundamental scalar bosons (the Higgs boson); second, mesons , which are composite bosons, made of quarks ; third, larger composite, non-force-carrying bosons, such as certain atoms . The Standard Model of particle physics recognizes four kinds of gauge bosons: photons , which carry
448-438: A jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks. In the Standard Model, vector ( spin -1) bosons ( gluons , photons , and the W and Z bosons ) mediate forces, whereas the Higgs boson (spin-0) is responsible for the intrinsic mass of particles. Bosons differ from fermions in the fact that multiple bosons can occupy
560-599: A belief generally exists among physicists that there is likely to be "new" physics beyond the Standard Model , and the Standard Model will at some point be extended or superseded. The Higgs discovery, as well as the many measured collisions occurring at the LHC, provide physicists a sensitive tool to search their data for any evidence that the Standard Model seems to fail, and could provide considerable evidence guiding researchers into future theoretical developments. Below an extremely high temperature, electroweak symmetry breaking causes
672-636: A color-neutral baryon . Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral antibaryon . Quarks also carry fractional electric charges , but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Note that quarks have electric charges of either + + 2 / 3 e or − + 1 / 3 e , whereas antiquarks have corresponding electric charges of either − + 2 / 3 e or + + 1 / 3 e . Evidence for
784-425: A comprehensive theory for particle physics. In the late 1950s, Yoichiro Nambu recognised that spontaneous symmetry breaking , a process where a symmetric system becomes asymmetric, could occur under certain conditions. Symmetry breaking is when some variable that previously didn't affect the measured results ( it was originally a "symmetry" ) now does affect the measured results ( it's now "broken" and no longer
896-420: A fact explained by confinement . Every quark carries one of three color charges of the strong interaction ; antiquarks similarly carry anticolor. Color-charged particles interact via gluon exchange in the same way that charged particles interact via photon exchange. Gluons are themselves color-charged, however, resulting in an amplification of the strong force as color-charged particles are separated. Unlike
1008-622: A full relativistic model, independently and almost simultaneously, by three groups of physicists: by François Englert and Robert Brout in August 1964; by Peter Higgs in October 1964; and by Gerald Guralnik , Carl Hagen , and Tom Kibble (GHK) in November 1964. Higgs also wrote a short, but important, response published in September 1964 to an objection by Gilbert , which showed that if calculating within
1120-440: A loop (a one-dimensional sphere, that is, a circle). As a string moves through space it sweeps out something called a world sheet . String theory predicts 1- to 10-branes (a 1- brane being a string and a 10-brane being a 10-dimensional object) that prevent tears in the "fabric" of space using the uncertainty principle (e.g., the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in
1232-444: A major unanswered problem in physics. The six authors of the 1964 PRL papers , who received the 2010 J. J. Sakurai Prize for their work; from left to right: Kibble , Guralnik , Hagen , Englert , Brout ; right image: Higgs . Particle physicists study matter made from fundamental particles whose interactions are mediated by exchange particles – gauge bosons – acting as force carriers . At
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#17331191131591344-423: A neutron into a proton then decays into an electron and electron-antineutrino pair. The Z does not convert particle flavor or charges, but rather changes momentum; it is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction . These four gauge bosons form
1456-464: A new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs boson is not an elementary particle but a bound state of these objects. According to preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called preons, which is derived from "pre-quarks". In essence, preon theory tries to do for
1568-508: A non-zero value (or vacuum expectation ) everywhere . This non-zero value could in theory break electroweak symmetry. It was the first proposal capable of showing how the weak force gauge bosons could have mass despite their governing symmetry, within a gauge invariant theory. Although these ideas did not gain much initial support or attention, by 1972 they had been developed into a comprehensive theory and proved capable of giving "sensible" results that accurately described particles known at
1680-449: A physical massive vector field [gauge bosons with mass]. This is what happens in superconductivity , a subject about which Anderson was (and is) one of the leading experts. [text condensed] The Higgs mechanism is a process by which vector bosons can acquire rest mass without explicitly breaking gauge invariance , as a byproduct of spontaneous symmetry breaking . Initially, the mathematical theory behind spontaneous symmetry breaking
1792-458: A result of these failures, gauge theories began to fall into disrepute. The problem was symmetry requirements for these two forces incorrectly predicted the weak force's gauge bosons ( W and Z ) would have "zero mass" (in the specialized terminology of particle physics, "mass" refers specifically to a particle's rest mass ). But experiments showed the W and Z gauge bosons had non-zero (rest) mass. Further, many promising solutions seemed to require
1904-477: A symmetry ). In 1962 physicist Philip Anderson , an expert in condensed matter physics , observed that symmetry breaking played a role in superconductivity , and suggested it could also be part of the answer to the problem of gauge invariance in particle physics. Specifically, Anderson suggested that the Goldstone bosons that would result from symmetry breaking might instead, in some circumstances, be "absorbed" by
2016-488: Is a consequence of the high masses of the W and Z bosons, which in turn are a consequence of the Higgs mechanism . Through the process of spontaneous symmetry breaking , the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence,
2128-439: Is a manifestation of potential energy transferred to fundamental particles when they interact ("couple") with the Higgs field, which had contained that mass in the form of energy . The Higgs field is the only scalar (spin-0) field to be detected; all the other fundamental fields in the Standard Model are spin- 1 / 2 fermions or spin-1 bosons. According to Rolf-Dieter Heuer , director general of CERN when
2240-455: Is also very unstable, decaying into other particles almost immediately upon generation. The Higgs field is a scalar field with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry. Its " Sombrero potential " leads it to take a nonzero value everywhere (including otherwise empty space), which breaks the weak isospin symmetry of
2352-453: Is also very unstable, decaying into other particles almost immediately via several possible pathways. The Higgs field is a scalar field , with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry. Unlike any other known quantum field, it has a Sombrero potential . This shape means that below extremely high energies of about 159.5 ± 1.5 GeV such as those seen during
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#17331191131592464-512: Is differentiated via the spin–statistics theorem : it is half-integer for fermions, and integer for bosons. Notes : [†] An anti-electron ( e ) is conventionally called a " positron ". [‡] The known force carrier bosons all have spin = 1. The hypothetical graviton has spin = 2; it is unknown whether it is a gauge boson as well. In the Standard Model , elementary particles are represented for predictive utility as point particles . Though extremely successful,
2576-442: Is massless, although some models containing massive Kaluza–Klein gravitons exist. Although experimental evidence overwhelmingly confirms the predictions derived from the Standard Model , some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance the hierarchy problem . Theories beyond the Standard Model attempt to resolve these shortcomings. One extension of
2688-415: Is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons . As a consequence of flavor and color combinations and antimatter , the fermions and bosons are known to have 48 and 13 variations, respectively. Among the 61 elementary particles embraced by the Standard Model number: electrons and other leptons , quarks , and
2800-414: Is strongly supported. The presence of the field, now confirmed by experimental investigation, explains why some fundamental particles have (a rest) mass , despite the symmetries controlling their interactions, implying that they should be "massless". It also resolves several other long-standing puzzles, such as the reason for the extremely short distance travelled by the weak force bosons, and, therefore,
2912-520: Is the existence of X and Y bosons , which cause proton decay . The non-observation of proton decay at the Super-Kamiokande neutrino observatory rules out the simplest GUTs, however, including SU(5) and SO(10). Supersymmetry extends the Standard Model by adding another class of symmetries to the Lagrangian . These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts
3024-437: Is the level of significance required to officially label experimental observations as a discovery . Research into the properties of the newly discovered particle continues. The graviton is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due to the difficulty inherent in its detection , it is sometimes included in tables of elementary particles. The conventional graviton
3136-412: The 1964 PRL symmetry breaking papers . All three groups reached similar conclusions and for all cases, not just some limited cases. They showed that the conditions for electroweak symmetry would be "broken" if an unusual type of field existed throughout the universe, and indeed, there would be no Goldstone bosons and some existing bosons would acquire mass . The field required for this to happen (which
3248-401: The 19th century , beginning with the electron , followed by the proton in 1919, the photon in the 1920s, and the neutron in 1932. By that time the advent of quantum mechanics had radically altered the definition of a "particle" by putting forward an understanding in which they carried out a simultaneous existence as matter waves . Many theoretical elaborations upon, and beyond ,
3360-420: The Higgs boson was announced to have been observed at CERN's Large Hadron Collider. Peter Higgs who first posited the existence of the Higgs boson was present at the announcement. The Higgs boson is believed to have a mass of approximately 125 GeV/ c . The statistical significance of this discovery was reported as 5 sigma, which implies a certainty of roughly 99.99994%. In particle physics, this
3472-434: The Higgs mechanism , a way for some particles to acquire mass . All fundamental particles known at the time should be massless at very high energies, but fully explaining how some particles gain mass at lower energies had been extremely difficult. If these ideas were correct, a particle known as a scalar boson should also exist (with certain properties). This particle was called the Higgs boson and could be used to test whether
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3584-519: The Large Hadron Collider ( ATLAS and CMS ). The Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, however, since it is not known if it is compatible with Einstein 's general relativity . There may be hypothetical elementary particles not described by the Standard Model, such as the graviton , the particle that would carry the gravitational force , and sparticles , supersymmetric partners of
3696-516: The Large Hadron Collider at CERN . String theory is a model of physics whereby all "particles" that make up matter are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according to M-theory , the leading version) or 12-dimensional (according to F-theory ) universe. These strings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A "string" can be open (a line) or closed in
3808-576: The Nobel Prize in Physics in 2013 for their theoretical predictions. Although Higgs's name has come to be associated with this theory, several researchers between about 1960 and 1972 independently developed different parts of it. In the media, the Higgs boson has often been called the " God particle " after the 1993 book The God Particle by Nobel Laureate Leon Lederman . The name has been criticised by physicists, including Peter Higgs . Physicists explain
3920-443: The Standard Model through the mechanism of mass generation . As more precise measurements of its properties are made, more advanced extensions may be suggested or excluded. As experimental means to measure the field's behaviours and interactions are developed, this fundamental field may be better understood. If the Higgs field had not been discovered, the Standard Model would have needed to be modified or superseded. Related to this,
4032-448: The Sun . The Higgs field is responsible for this symmetry breaking. The Higgs field is pivotal in generating the masses of quarks and charged leptons (through Yukawa coupling) and the W and Z gauge bosons (through the Higgs mechanism). The Higgs field does not "create" mass out of nothing (which would violate the law of conservation of energy ), nor is the Higgs field responsible for
4144-399: The Super-Kamiokande neutrino detector has yielded no evidence of X and Y bosons. The fourth fundamental interaction, gravity , may also be carried by a boson, called the graviton. In the absence of experimental evidence and a mathematically coherent theory of quantum gravity , it is unknown whether this would be a gauge boson or not. The role of gauge invariance in general relativity
4256-403: The atomic nucleus . Like quarks, gluons exhibit color and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons. There are three weak gauge bosons : W , W , and Z ; these mediate the weak interaction . The W bosons are known for their mediation in nuclear decay: The W converts
4368-779: The electromagnetic force and the weak nuclear force – and then to unify these interactions , were still unsuccessful. One known problem was that gauge invariant approaches, including non-abelian models such as Yang–Mills theory (1954), which held great promise for unified theories, also seemed to predict known massive particles as massless. Goldstone's theorem , relating to continuous symmetries within some theories, also appeared to rule out many obvious solutions, since it appeared to show that zero-mass particles known as Goldstone bosons would also have to exist that simply were "not seen". According to Guralnik , physicists had "no understanding" how these problems could be overcome. Particle physicist and mathematician Peter Woit summarised
4480-562: The electromagnetic force , which diminishes as charged particles separate, color-charged particles feel increasing force. Nonetheless, color-charged particles may combine to form color neutral composite particles called hadrons . A quark may pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral meson . Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form
4592-444: The electromagnetic interaction ; W and Z bosons, which carry the weak interaction ; and gluons , which carry the strong interaction . Isolated gluons do not occur because they are colour-charged and subject to colour confinement . In a quantized gauge theory , gauge bosons are quanta of the gauge fields . Consequently, there are as many gauge bosons as there are generators of the gauge field. In quantum electrodynamics ,
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4704-509: The electroweak interaction and, via the Higgs mechanism , gives a rest mass to all massive elementary particles of the Standard Model, including the Higgs boson itself. The existence of the Higgs field became the last unverified part of the Standard Model of particle physics, and for several decades was considered "the central problem in particle physics". Both the field and the boson are named after physicist Peter Higgs , who in 1964, along with five other scientists in three teams, proposed
4816-527: The electroweak interaction to manifest in part as the short-ranged weak force , which is carried by massive gauge bosons . In the history of the universe , electroweak symmetry breaking is believed to have happened at about 1 picosecond (10 s) after the Big Bang , when the universe was at a temperature 159.5 ± 1.5 GeV/ k B . This symmetry breaking is required for atoms and other structures to form, as well as for nuclear reactions in stars, such as
4928-486: The fundamental particles and forces of our universe in terms of the Standard Model – a widely accepted framework based on quantum field theory that predicts almost all known particles and forces aside from gravity with great accuracy. (A separate theory, general relativity , is used for gravity.) In the Standard Model, the particles and forces in nature (aside from gravity) arise from properties of quantum fields known as gauge invariance and symmetries . Forces in
5040-441: The inflaton responsible for this exponential expansion of the universe during the Big Bang . Such theories are highly tentative and face significant problems related to unitarity , but may be viable if combined with additional features such as large non-minimal coupling, a Brans–Dicke scalar, or other "new" physics, and they have received treatments suggesting that Higgs inflation models are still of interest theoretically. In
5152-407: The inflaton – a hypothetical field suggested as the explanation for the expansion of space during the first fraction of a second of the universe (known as the " inflationary epoch "). Some theories suggest that a fundamental scalar field might be responsible for this phenomenon; the Higgs field is such a field, and its existence has led to papers analysing whether it could also be
5264-409: The on-shell scheme . Estimates of the values of quark masses depend on the version of quantum chromodynamics used to describe quark interactions. Quarks are always confined in an envelope of gluons that confer vastly greater mass to the mesons and baryons where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of
5376-408: The speed of light in vacuum seems to give the identical result, whatever the location in time and space, and whatever the local gravitational field . In these kinds of theories, the gauge is an item whose value we can change. The fact that some changes leave the results we measure unchanged means it is a gauge invariant theory, and symmetries are the specific kinds of changes to the gauge which have
5488-428: The " multiverse " outside our known universe). Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like the graviton . Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. This means one adds
5600-465: The "Higgs Field", was hypothesized to exist throughout space, and to break some symmetry laws of the electroweak interaction , triggering the Higgs mechanism. It, therefore, would cause the W and Z gauge bosons of the weak force to be massive at all temperatures below an extremely high value. When the weak force bosons acquire mass, this affects the distance they can freely travel, which becomes very small, also matching experimental findings. Furthermore, it
5712-460: The Higgs boson suggest that our universe lies within a false vacuum of this kind, then it would imply – more than likely in many billions of years – that the universe's forces, particles, and structures could cease to exist as we know them (and be replaced by different ones), if a true vacuum happened to nucleate . It also suggests that the Higgs self-coupling λ and its β λ function could be very close to zero at
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#17331191131595824-400: The Higgs boson was discovered, this existence proof of a scalar field is almost as important as the Higgs's role in determining the mass of other particles. It suggests that other hypothetical scalar fields suggested by other theories, from the inflaton to quintessence , could perhaps exist as well. There has been considerable scientific research on possible links between the Higgs field and
5936-401: The Higgs field and its properties has been extremely significant for many reasons. The importance of the Higgs boson largely is that it is able to be examined using existing knowledge and experimental technology, as a way to confirm and study the entire Higgs field theory. Conversely, proof that the Higgs field and boson did not exist would have also been significant. The Higgs boson validates
6048-432: The Higgs field and the presently observed vacuum energy density of the universe has also come under scientific study. As observed, the present vacuum energy density is extremely close to zero, but the energy densities predicted from the Higgs field, supersymmetry, and other current theories are typically many orders of magnitude larger. It is unclear how these should be reconciled. This cosmological constant problem remains
6160-554: The Higgs field does not actually resist particles, and the effect of mass is not caused by resistance. In the Standard Model, the Higgs boson is a massive scalar boson whose mass must be found experimentally. Its mass has been determined to be 125.35 ± 0.15 GeV/ c by CMS (2022) and 125.11 ± 0.11 GeV/ c by ATLAS (2023). It is the only particle that remains massive even at very high energies. It has zero spin , even (positive) parity , no electric charge , and no colour charge , and it couples to (interacts with) mass. It
6272-489: The Higgs field was the correct explanation. After a 40-year search , a subatomic particle with the expected properties was discovered in 2012 by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN near Geneva , Switzerland. The new particle was subsequently confirmed to match the expected properties of a Higgs boson. Physicists from two of the three teams, Peter Higgs and François Englert , were awarded
6384-450: The LHC. Various analogies have been used to describe the Higgs field and boson, including analogies with well-known symmetry-breaking effects such as the rainbow and prism , electric fields , and ripples on the surface of water. Other analogies based on the resistance of macro objects moving through media (such as people moving through crowds, or some objects moving through syrup or molasses ) are commonly used but misleading, since
6496-407: The Planck scale, with "intriguing" implications, including theories of gravity and Higgs-based inflation. A future electron–positron collider would be able to provide the precise measurements of the top quark needed for such calculations. More speculatively, the Higgs field has also been proposed as the energy of the vacuum , which at the extreme energies of the first moments of the Big Bang caused
6608-433: The Standard Model are transmitted by particles known as gauge bosons . Gauge invariant theories are theories which have a useful feature, i.e.: some kinds of changes to the value of certain items do not make any difference to the outcomes or the measurements we make. For example: changing voltages in an electromagnet by +100 volts does not cause any change to the magnetic field it produces. Similarly, measuring
6720-403: The Standard Model attempts to combine the electroweak interaction with the strong interaction into a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism . This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification
6832-606: The Standard Model have been made since its codification in the 1970s. These include notions of supersymmetry , which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive. However, like an additional elementary boson mediating gravitation, such superpartners remain undiscovered as of 2024. All elementary particles are either bosons or fermions . These classes are distinguished by their quantum statistics : fermions obey Fermi–Dirac statistics and bosons obey Bose–Einstein statistics . Their spin
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#17331191131596944-416: The Standard Model is limited by its omission of gravitation and has some parameters arbitrarily added but unexplained. According to the current models of Big Bang nucleosynthesis , the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made up of one up and two down quarks, while protons are made of two up and one down quark. Since
7056-506: The Standard Model provides an accurate description of particle physics up to extreme energies of the Planck scale , then it is possible to calculate whether the vacuum is stable or merely long-lived. A Higgs mass of 125–127 GeV/ c seems to be extremely close to the boundary for stability, but a definitive answer requires much more precise measurements of the pole mass of the top quark. New physics can change this picture. If measurements of
7168-447: The Standard Model what the Standard Model did for the particle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to six more fundamental particles and the rules that govern their interactions. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s. Accelerons are the hypothetical subatomic particles that integrally link
7280-436: The Standard Model, the W and Z bosons gain mass via the Higgs mechanism . In the Higgs mechanism, the four gauge bosons (of SU(2)×U(1) symmetry) of the unified electroweak interaction couple to a Higgs field . This field undergoes spontaneous symmetry breaking due to the shape of its interaction potential. As a result, the universe is permeated by a non-zero Higgs vacuum expectation value (VEV). This VEV couples to three of
7392-437: The Standard Model, there exists the possibility that the underlying state of our universe – known as the "vacuum" – is long-lived, but not completely stable . In this scenario, the universe as we know it could effectively be destroyed by collapsing into a more stable vacuum state . This was sometimes misreported as the Higgs boson "ending" the universe. If the masses of the Higgs boson and top quark are known more precisely, and
7504-596: The Yang–Mills theory, that "considering the superconducting analog ... [t]hese two types of bosons seem capable of canceling each other out ... leaving finite mass bosons"), and in March 1964, Abraham Klein and Benjamin Lee showed that Goldstone's theorem could be avoided this way in at least some non-relativistic cases, and speculated it might be possible in truly relativistic cases. These approaches were quickly developed into
7616-436: The accuracy of its predictions led scientists to believe the theory might be true. By the 1980s, the question of whether the Higgs field existed, and therefore whether the entire Standard Model was correct, had come to be regarded as one of the most important unanswered questions in particle physics . The existence of the Higgs field became the last unverified part of the Standard Model of particle physics, and for several decades
7728-417: The beginning of the 1960s a number of these particles had been discovered or proposed, along with theories suggesting how they relate to each other, some of which had already been reformulated as field theories in which the objects of study are not particles and forces, but quantum fields and their symmetries . However, attempts to produce quantum field models for two of the four known fundamental forces –
7840-557: The current experimental and theoretical knowledge about elementary particle physics is the Particle Data Group , where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding. other pages are: Gauge boson In particle physics , a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions . Elementary particles whose interactions are described by
7952-547: The effect of leaving measurements unchanged. Symmetries of this kind are powerful tools for a deep understanding of the fundamental forces and particles of our physical world. Gauge invariance is therefore an important property within particle physics theory. They are closely connected to conservation laws and are described mathematically using group theory . Quantum field theory and the Standard Model are both gauge invariant theories – meaning they focus on properties of our universe, demonstrating this property of gauge invariance and
8064-663: The electroweak gauge bosons (W , W and Z), giving them mass; the remaining gauge boson remains massless (the photon). This theory also predicts the existence of a scalar Higgs boson , which has been observed in experiments at the LHC . The Georgi–Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons mediate interactions between quarks and leptons , hence violating conservation of baryon number and causing proton decay . Such bosons would be even more massive than W and Z bosons due to symmetry breaking . Analysis of data collected from such sources as
8176-468: The electroweak interaction among elementary particles. Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single electroweak force at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY . The differences at low energies
8288-531: The eventual theory published there was still almost no wider interest. For example, Coleman found in a study that "essentially no-one paid any attention" to Weinberg's paper prior to 1971 and discussed by David Politzer in his 2004 Nobel speech. – now the most cited in particle physics – and even in 1970 according to Politzer, Glashow's teaching of the weak interaction contained no mention of Weinberg's, Salam's, or Glashow's own work. In practice, Politzer states, almost everyone learned of
8400-543: The existence of supersymmetric particles , abbreviated as sparticles , which include the sleptons , squarks , neutralinos , and charginos . Each particle in the Standard Model would have a superpartner whose spin differs by 1 ⁄ 2 from the ordinary particle. Due to the breaking of supersymmetry , the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. Some physicists believe that sparticles will be detected by
8512-423: The existence of extra particles known as Goldstone bosons . But evidence suggested these did not exist either. This meant either gauge invariance was an incorrect approach, or something unknown was giving the weak force's W and Z bosons their mass, and doing it in a way that did not create Goldstone bosons. By the late 1950s and early 1960s, physicists were at a loss as to how to resolve these issues, or how to create
8624-483: The existence of quarks comes from deep inelastic scattering : firing electrons at nuclei to determine the distribution of charge within nucleons (which are baryons). If the charge is uniform, the electric field around the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but, above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and
8736-403: The factual existence of atoms remained controversial until 1905. In that year Albert Einstein published his paper on Brownian motion , putting to rest theories that had regarded molecules as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations of energy . Subatomic constituents of the atom were first identified toward the end of
8848-459: The first picosecond (10 s) of the Big Bang , the Higgs field in its ground state takes less energy to have a nonzero vacuum expectation (value) than a zero value. Therefore in today's universe the Higgs field has a nonzero value everywhere (including otherwise empty space). This nonzero value in turn breaks the weak isospin SU(2) symmetry of the electroweak interaction everywhere. (Technically
8960-517: The fundamental bosons . Subatomic particles such as protons or neutrons , which contain two or more elementary particles, are known as composite particles . Ordinary matter is composed of atoms , themselves once thought to be indivisible elementary particles. The name atom comes from the Ancient Greek word ἄτομος ( atomos ) which means indivisible or uncuttable . Despite the theories about atoms that had existed for thousands of years
9072-450: The gauge group is U(1) ; in this simple case, there is only one gauge boson, the photon. In quantum chromodynamics , the more complicated group SU(3) has eight generators, corresponding to the eight gluons. The three W and Z bosons correspond (roughly) to the three generators of SU(2) in electroweak theory . Gauge invariance requires that gauge bosons are described mathematically by field equations for massless particles. Otherwise,
9184-400: The mass of all particles. For example, approximately 99% of the mass of baryons ( composite particles such as the proton and neutron ), is due instead to quantum chromodynamic binding energy , which is the sum of the kinetic energies of quarks and the energies of the massless gluons mediating the strong interaction inside the baryons. In Higgs-based theories, the property of "mass"
9296-495: The mass terms add non-zero additional terms to the Lagrangian under gauge transformations, violating gauge symmetry. Therefore, at a naïve theoretical level, all gauge bosons are required to be massless, and the forces that they describe are required to be long-ranged. The conflict between this idea and experimental evidence that the weak and strong interactions have a very short range requires further theoretical insight. According to
9408-579: The massless W and Z bosons . If so, perhaps the Goldstone bosons would not exist, and the W and Z bosons could gain mass , solving both problems at once. Similar behaviour was already theorised in superconductivity. In 1964, this was shown to be theoretically possible by physicists Abraham Klein and Benjamin Lee , at least for some limited ( non-relativistic ) cases. Following the 1963 and early 1964 papers, three groups of researchers independently developed these theories more completely, in what became known as
9520-452: The newfound mass of the neutrino to the dark energy conjectured to be accelerating the expansion of the universe . In this theory, neutrinos are influenced by a new force resulting from their interactions with accelerons, leading to dark energy. Dark energy results as the universe tries to pull neutrinos apart. Accelerons are thought to interact with matter more infrequently than they do with neutrinos. The most important address about
9632-551: The non-zero expectation value converts the Lagrangian 's Yukawa coupling terms into mass terms.) When this happens, three components of the Higgs field are "absorbed" by the SU(2) and U(1) gauge bosons (the " Higgs mechanism ") to become the longitudinal components of the now-massive W and Z bosons of the weak force . The remaining electrically neutral component either manifests as a Higgs boson, or may couple separately to other particles known as fermions (via Yukawa couplings ), causing these to acquire mass as well. Evidence of
9744-477: The observable universe. The number of protons in the observable universe is called the Eddington number . In terms of number of particles, some estimates imply that nearly all the matter, excluding dark matter , occurs in neutrinos, which constitute the majority of the roughly 10 elementary particles of matter that exist in the visible universe. Other estimates imply that roughly 10 elementary particles exist in
9856-466: The only elementary fermions with neither electric nor color charge . The remaining six particles are quarks (discussed below). The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: millions of electron-volts relative to square of light speed (MeV/ c ). For example, the most accurately known quark mass is of the top quark ( t ) at 172.7 GeV/ c , estimated using
9968-475: The ordinary particles. The 12 fundamental fermions are divided into 3 generations of 4 particles each. Half of the fermions are leptons , three of which have an electric charge of −1 e , called the electron ( e ), the muon ( μ ), and the tau ( τ ); the other three leptons are neutrinos ( ν e , ν μ , ν τ ), which are
10080-501: The other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all baryons , in turn consist of up quarks and down quarks. Some estimates imply that there are roughly 10 baryons (almost entirely protons and neutrons) in
10192-445: The particle has been shown to behave, interact, and decay in many of the ways predicted for Higgs particles by the Standard Model, as well as having even parity and zero spin , two fundamental attributes of a Higgs boson. This also means it is the first elementary scalar particle discovered in nature. By March 2013, the existence of the Higgs boson was confirmed, and therefore, the concept of some type of Higgs field throughout space
10304-444: The radiation gauge, Goldstone's theorem and Gilbert's objection would become inapplicable. Higgs later described Gilbert's objection as prompting his own paper. Properties of the model were further considered by Guralnik in 1965, by Higgs in 1966, by Kibble in 1967, and further by GHK in 1967. The original three 1964 papers demonstrated that when a gauge theory is combined with an additional charged scalar field that spontaneously breaks
10416-430: The same quantum state ( Pauli exclusion principle ). Also, bosons can be either elementary, like photons, or a combination, like mesons . The spin of bosons are integers instead of half integers. Gluons mediate the strong interaction , which join quarks and thereby form hadrons , which are either baryons (three quarks) or mesons (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form
10528-459: The state of research at the time: Yang and Mills work on non-abelian gauge theory had one huge problem: in perturbation theory it has massless particles which don't correspond to anything we see. One way of getting rid of this problem is now fairly well understood, the phenomenon of confinement realized in QCD , where the strong interactions get rid of the massless "gluon" states at long distances. By
10640-399: The surrounding gluons, slight differences in the calculation make large differences in the masses. There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, the antielectron (positron) e is the electron's antiparticle and has an electric charge of +1 e . Isolated quarks and antiquarks have never been detected,
10752-399: The symmetries which are involved. Quantum field theories based on gauge invariance had been used with great success in understanding the electromagnetic and strong forces , but by around 1960, all attempts to create a gauge invariant theory for the weak force (and its combination with the electromagnetic force, known together as the electroweak interaction ) had consistently failed. As
10864-409: The symmetry, the gauge bosons may consistently acquire a finite mass. In 1967, Steven Weinberg and Abdus Salam independently showed how a Higgs mechanism could be used to break the electroweak symmetry of Sheldon Glashow 's unified model for the weak and electromagnetic interactions , (itself an extension of work by Schwinger ), forming what became the Standard Model of particle physics. Weinberg
10976-403: The theory due to physicist Benjamin Lee , who combined the work of Veltman and 't Hooft with insights by others, and popularised the completed theory. In this way, from 1971, interest and acceptance "exploded" and the ideas were quickly absorbed in the mainstream. Elementary particle In particle physics , an elementary particle or fundamental particle is a subatomic particle that
11088-403: The time, and which, with exceptional accuracy, predicted several other particles discovered during the following years . During the 1970s these theories rapidly became the Standard Model of particle physics. To allow symmetry breaking, the Standard Model includes a field of the kind needed to "break" electroweak symmetry and give particles their correct mass. This field, which became known as
11200-409: The universe at any given moment). String theory proposes that our universe is merely a 4-brane, inside which exist the three space dimensions and the one time dimension that we observe. The remaining 7 theoretical dimensions either are very tiny and curled up (and too small to be macroscopically accessible) or simply do not/cannot exist in our universe (because they exist in a grander scheme called
11312-444: The universe to be a kind of featureless symmetry of undifferentiated, extremely high energy. In this kind of speculation, the single unified field of a Grand Unified Theory is identified as (or modelled upon) the Higgs field, and it is through successive symmetry breakings of the Higgs field, or some similar field, at phase transitions that the presently known forces and fields of the universe arise. The relationship (if any) between
11424-475: The very early sixties, people had begun to understand another source of massless particles: spontaneous symmetry breaking of a continuous symmetry. What Philip Anderson realized and worked out in the summer of 1962 was that, when you have both gauge symmetry and spontaneous symmetry breaking, the massless Nambu–Goldstone mode [which gives rise to Goldstone bosons] can combine with the massless gauge field modes [which give rise to massless gauge bosons] to produce
11536-405: The visible universe (not including dark matter ), mostly photons and other massless force carriers. The Standard Model of particle physics contains 12 flavors of elementary fermions , plus their corresponding antiparticles , as well as elementary bosons that mediate the forces and the Higgs boson , which was reported on July 4, 2012, as having been likely detected by the two main experiments at
11648-533: The weak force's extremely short range. As of 2018, in-depth research shows the particle continuing to behave in line with predictions for the Standard Model Higgs boson. More studies are needed to verify with higher precision that the discovered particle has all of the properties predicted or whether, as described by some theories, multiple Higgs bosons exist. The nature and properties of this field are now being investigated further, using more data collected at
11760-419: Was conceived and published within particle physics by Yoichiro Nambu in 1960 (and somewhat anticipated by Ernst Stueckelberg in 1938), and the concept that such a mechanism could offer a possible solution for the "mass problem" was originally suggested in 1962 by Philip Anderson, who had previously written papers on broken symmetry and its outcomes in superconductivity. Anderson concluded in his 1963 paper on
11872-500: Was considered "the central problem in particle physics". For many decades, scientists had no way to determine whether the Higgs field existed because the technology needed for its detection did not exist at that time. If the Higgs field did exist, then it would be unlike any other known fundamental field, but it also was possible that these key ideas, or even the entire Standard Model, were somehow incorrect. The hypothesised Higgs theory made several key predictions. One crucial prediction
11984-611: Was far from easy. In principle, it can be proved to exist by detecting its excitations , which manifest as Higgs particles (the Higgs boson ), but these are extremely difficult to produce and detect due to the energy required to produce them and their very rare production even if the energy is sufficient. It was, therefore, several decades before the first evidence of the Higgs boson could be found. Particle colliders , detectors, and computers capable of looking for Higgs bosons took more than 30 years ( c. 1980–2010 ) to develop. The importance of this fundamental question led to
12096-425: Was later realised that the same field would also explain, in a different way, why other fundamental constituents of matter (including electrons and quarks ) have mass. Unlike all other known fields, such as the electromagnetic field , the Higgs field is a scalar field , and has a non-zero average value in vacuum . There was not yet any direct evidence that the Higgs field existed, but even without direct proof,
12208-400: Was possible in two papers covering massless, and then massive, fields. Their contribution, and the work of others on the renormalisation group – including "substantial" theoretical work by Russian physicists Ludvig Faddeev , Andrei Slavnov , Efim Fradkin , and Igor Tyutin – was eventually "enormously profound and influential", but even with all key elements of
12320-451: Was purely hypothetical at the time) became known as the Higgs field (after Peter Higgs , one of the researchers) and the mechanism by which it led to symmetry breaking became known as the Higgs mechanism . A key feature of the necessary field is that it would take less energy for the field to have a non-zero value than a zero value, unlike all other known fields, therefore, the Higgs field has
12432-431: Was that a matching particle , called the "Higgs boson", should also exist. Proving the existence of the Higgs boson would prove whether the Higgs field existed, and therefore finally prove whether the Standard Model's explanation was correct. Therefore, there was an extensive search for the Higgs boson , as a way to prove the Higgs field itself existed. Although the Higgs field would exist everywhere, proving its existence
12544-426: Was the first to observe that this would also provide mass terms for the fermions. At first, these seminal papers on spontaneous breaking of gauge symmetries were largely ignored, because it was widely believed that the (non-Abelian gauge) theories in question were a dead-end, and in particular that they could not be renormalised . In 1971–72, Martinus Veltman and Gerard 't Hooft proved renormalisation of Yang–Mills
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