91-428: The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic , weak and strong interactions – excluding gravity ) in the universe and classifying all known elementary particles . It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in
182-498: A G a μ ν , {\displaystyle {\mathcal {L}}_{\text{QCD}}={\overline {\psi }}i\gamma ^{\mu }D_{\mu }\psi -{\frac {1}{4}}G_{\mu \nu }^{a}G_{a}^{\mu \nu },} where ψ {\displaystyle \psi } is a three component column vector of Dirac spinors , each element of which refers to a quark field with a specific color charge (i.e. red, blue, and green) and summation over flavor (i.e. up, down, strange, etc.)
273-614: A μ ν W μ ν a − 1 4 B μ ν B μ ν , {\displaystyle {\mathcal {L}}_{\text{EW}}={\overline {Q}}_{Lj}i\gamma ^{\mu }D_{\mu }Q_{Lj}+{\overline {u}}_{Rj}i\gamma ^{\mu }D_{\mu }u_{Rj}+{\overline {d}}_{Rj}i\gamma ^{\mu }D_{\mu }d_{Rj}+{\overline {\ell }}_{Lj}i\gamma ^{\mu }D_{\mu }\ell _{Lj}+{\overline {e}}_{Rj}i\gamma ^{\mu }D_{\mu }e_{Rj}-{\tfrac {1}{4}}W_{a}^{\mu \nu }W_{\mu \nu }^{a}-{\tfrac {1}{4}}B^{\mu \nu }B_{\mu \nu },} where
364-487: A Hilbert space , which is also treated in quantum field theory . Following the convention of particle physicists, 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. Ordinary matter is made from first- generation quarks ( up , down ) and leptons ( electron , electron neutrino ). Collectively, quarks and leptons are called fermions , because they have
455-558: A complete theory of fundamental interactions . For example, it does not fully explain why there is more matter than anti-matter , incorporate the full theory of gravitation as described by general relativity , or account for the universe's accelerating expansion as possibly described by dark energy . The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology . It also does not incorporate neutrino oscillations and their non-zero masses. The development of
546-402: A microsecond . They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays . Mesons are also produced in cyclotrons or other particle accelerators . Particles have corresponding antiparticles with the same mass but with opposite electric charges . For example, the antiparticle of the electron is the positron . The electron has
637-502: A quantum spin of half-integers (−1/2, 1/2, 3/2, etc.). This causes the fermions to obey the Pauli exclusion principle , where no two particles may occupy the same quantum state . Quarks have fractional elementary electric charge (−1/3 or 2/3) and leptons have whole-numbered electric charge (0 or 1). Quarks also have color charge , which is labeled arbitrarily with no correlation to actual light color as red, green and blue. Because
728-1058: A " Theory of Everything ", or "TOE". There are also other areas of work in theoretical particle physics ranging from particle cosmology to loop quantum gravity . 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 produce medical isotopes for research and treatment (for example, isotopes used in PET imaging ), or used directly in external beam radiotherapy . The development of superconductors has been pushed forward by their use in particle physics. The World Wide Web and touchscreen technology were initially developed at CERN . Additional applications are found in medicine, national security, industry, computing, science, and workforce development, illustrating
819-452: A fourth generation of fermions does not exist. Bosons are the mediators or carriers of fundamental interactions, such as electromagnetism , the weak interaction , and the strong interaction . Electromagnetism is mediated by the photon , the quanta of light . The weak interaction is mediated by the W and Z bosons . The strong interaction is mediated by the gluon , which can link quarks together to form composite particles. Due to
910-438: A graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze high-energy scattering experiments are in reasonable agreement with the data. However, perturbation theory (and with it the concept of a "force-mediating particle") fails in other situations. These include low-energy quantum chromodynamics, bound states , and solitons . The interactions between all
1001-849: A long and growing list of beneficial practical applications with contributions from particle physics. Major efforts to look for physics beyond the Standard Model include the Future Circular Collider proposed for CERN and the Particle Physics Project Prioritization Panel (P5) in the US that will update the 2014 P5 study that recommended the Deep Underground Neutrino Experiment , among other experiments. GIM mechanism In particle physics ,
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#17327576899251092-1043: A member of the group SU(3), and ϕ a ( x ) {\displaystyle \phi ^{a}(x)} is an arbitrary function of spacetime. The electroweak sector is a Yang–Mills gauge theory with the symmetry group U(1) × SU(2) L , L EW = Q ¯ L j i γ μ D μ Q L j + u ¯ R j i γ μ D μ u R j + d ¯ R j i γ μ D μ d R j + ℓ ¯ L j i γ μ D μ ℓ L j + e ¯ R j i γ μ D μ e R j − 1 4 W
1183-430: A negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called antimatter . Some particles, such as the photon , are their own antiparticle. These elementary particles are excitations of the quantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics,
1274-476: A new particle with a mass of about 125 GeV/ c (about 133 proton masses, on the order of 10 kg ), which is "consistent with the Higgs boson". On 13 March 2013, it was confirmed to be the searched-for Higgs boson. Technically, quantum field theory provides the mathematical framework for the Standard Model, in which a Lagrangian controls the dynamics and kinematics of the theory. Each kind of particle
1365-465: A non-zero Vacuum expectation value , which generates masses for the Electroweak gauge fields (the Higgs' mechanism), and λ > 0 {\displaystyle \lambda >0} , so that the potential is bounded from below. The quartic term describes self-interactions of the scalar field φ {\displaystyle \varphi } . The minimum of the potential
1456-435: A wide range of exotic particles . All particles and their interactions observed to date can be described almost entirely by the Standard Model. Dynamics of particles are also 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
1547-425: Is a particle physics theory suggesting that systems with higher energy have a smaller number of dimensions. 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
1638-554: Is called the Standard Model . The reconciliation of gravity to the current particle physics theory is not solved; many theories have addressed this problem, such as loop quantum gravity , string theory and supersymmetry theory . Practical particle physics is the study of these particles in radioactive processes and in particle accelerators such as the Large Hadron Collider . Theoretical particle physics
1729-415: Is conventionally called a " positron ". The Standard Model includes 12 elementary particles of spin 1 ⁄ 2 , known as fermions . Fermions respect the Pauli exclusion principle , meaning that two identical fermions cannot simultaneously occupy the same quantum state in the same atom. Each fermion has a corresponding antiparticle , which are particles that have corresponding properties with
1820-431: Is degenerate with an infinite number of equivalent ground state solutions, which occurs when φ † φ = μ 2 2 λ {\displaystyle \varphi ^{\dagger }\varphi ={\tfrac {\mu ^{2}}{2\lambda }}} . It is possible to perform a gauge transformation on φ {\displaystyle \varphi } such that
1911-411: Is described in terms of a dynamical field that pervades space-time . The construction of the Standard Model proceeds following the modern method of constructing most field theories: by first postulating a set of symmetries of the system, and then by writing down the most general renormalizable Lagrangian from its particle (field) content that observes these symmetries. The global Poincaré symmetry
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#17327576899252002-532: 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 . The Standard Model also contains 24 fundamental fermions (12 particles and their associated anti-particles), which are
2093-477: Is implied. The gauge covariant derivative of QCD is defined by D μ ≡ ∂ μ − i g s 1 2 λ a G μ a {\displaystyle D_{\mu }\equiv \partial _{\mu }-ig_{s}{\frac {1}{2}}\lambda ^{a}G_{\mu }^{a}} , where The QCD Lagrangian is invariant under local SU(3) gauge transformations; i.e., transformations of
2184-595: Is in model building where model builders develop ideas for what physics may lie beyond the Standard Model (at higher energies or smaller distances). 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. Vanishing-dimensions theory
2275-405: Is mediated by mesons, such as the pion . The color charges inside the nucleon cancel out, meaning most of the gluon and quark fields cancel out outside of the nucleon. However, some residue is "leaked", which appears as the exchange of virtual mesons, that causes the attractive force between nucleons. The (fundamental) strong interaction is described by quantum chromodynamics, which is a component of
2366-404: Is postulated for all relativistic quantum field theories. It consists of the familiar translational symmetry , rotational symmetry and the inertial reference frame invariance central to the theory of special relativity . The local SU(3)×SU(2)×U(1) gauge symmetry is an internal symmetry that essentially defines the Standard Model. Roughly, the three factors of the gauge symmetry give rise to
2457-432: Is set by the mass-squared difference of the different virtual quarks exchanged in the box diagram, originally the u-c quarks, on the scale of the W mass. The smallness of this quantity accounts for the suppressed induced FCNC, dictating a rare decay , K L → μ + μ − {\displaystyle K_{L}\to \mu ^{+}\mu ^{-}} , illustrated in
2548-561: Is the Higgs doublet and φ ~ = i τ 2 φ ∗ {\displaystyle {\tilde {\varphi }}=i\tau _{2}\varphi ^{*}} is its charge conjugate state. The Yukawa terms are invariant under the SU ( 2 ) L × U ( 1 ) Y {\displaystyle \operatorname {SU} (2)_{\text{L}}\times \operatorname {U} (1)_{\text{Y}}} gauge symmetry of
2639-467: Is the electroweak gauge covariant derivative defined above and V ( φ ) {\displaystyle V(\varphi )} is the potential of the Higgs field. The square of the covariant derivative leads to three and four point interactions between the electroweak gauge fields W μ a {\displaystyle W_{\mu }^{a}} and B μ {\displaystyle B_{\mu }} and
2730-468: Is the only long-range force in the Standard Model. It is mediated by photons and couples to electric charge. Electromagnetism is responsible for a wide range of phenomena including atomic electron shell structure , chemical bonds , electric circuits and electronics . Electromagnetic interactions in the Standard Model are described by quantum electrodynamics. The weak interaction is responsible for various forms of particle decay , such as beta decay . It
2821-471: Is the study of these particles in the context of cosmology and quantum theory . The two are closely interrelated: the Higgs boson was postulated by theoretical particle physicists and its presence confirmed by practical experiments. The idea that all matter is fundamentally composed of elementary particles dates from at least the 6th century BC. In the 19th century, John Dalton , through his work on stoichiometry , concluded that each element of nature
Standard Model - Misplaced Pages Continue
2912-600: 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 . Others make use of lattice field theory and call themselves lattice theorists . Another major effort
3003-408: Is usually credited to Glashow , Iliopoulos , & Maiani (initials "G I M"). The mechanism relies on the unitarity of the charged weak current flavor mixing matrix , which enters in the two vertices of a one-loop box diagram involving W boson exchanges. Even though Z boson exchanges are flavor-neutral (i.e. prohibit FCNC), the box diagram induces FCNC, but at a very small level. The smallness
3094-424: Is weak and short-range, due to the fact that the weak mediating particles, W and Z bosons, have mass. W bosons have electric charge and mediate interactions that change the particle type (referred to as flavour) and charge. Interactions mediated by W bosons are charged current interactions . Z bosons are neutral and mediate neutral current interactions, which do not change particle flavour. Thus Z bosons are similar to
3185-477: The GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism ) is allegedly the mechanism through which flavour-changing neutral currents (FCNCs) are suppressed in loop diagrams . It also explains why weak interactions that change strangeness by 2 (Δ S = 2 transitions) are suppressed, while those that change strangeness by 1 (Δ S = 1 transitions) are allowed, but only in charged current interactions. The mechanism
3276-511: The GIM mechanism , predicting the charm quark . In 1973 Gross and Wilczek and Politzer independently discovered that non-Abelian gauge theories, like the color theory of the strong force, have asymptotic freedom . In 1976, Martin Perl discovered the tau lepton at the SLAC . In 1977, a team led by Leon Lederman at Fermilab discovered the bottom quark. The Higgs mechanism is believed to give rise to
3367-544: 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. 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
3458-469: The atomic nucleus , ultimately constituted of up and down quarks. On the other hand, second- and third-generation charged particles decay with very short half-lives and can only be observed in high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter. There are six quarks: up , down , charm , strange , top , and bottom . Quarks carry color charge , and hence interact via
3549-627: The electron , electron neutrino , muon , muon neutrino , tau , and tau neutrino . The leptons do not carry color charge, and do not respond to strong interaction. The main leptons carry an electric charge of -1 e , while the three neutrinos carry a neutral electric charge. Thus, the neutrinos' motion are only influenced by weak interaction and gravity , making them difficult to observe. The Standard Model includes 4 kinds of gauge bosons of spin 1, with bosons being quantum particles containing an integer spin. The gauge bosons are defined as force carriers , as they are responsible for mediating
3640-565: The fundamental interactions . The Standard Model explains the four fundamental forces as arising from the interactions, with fermions exchanging virtual force carrier particles, thus mediating the forces. At a macroscopic scale, this manifests as a force . As a result, they do not follow the Pauli exclusion principle that constrains fermions; bosons do not have a theoretical limit on their spatial density . The types of gauge bosons are described below. The Feynman diagram calculations, which are
3731-548: The masses of all the elementary particles in the Standard Model. This includes the masses of the W and Z bosons , and the masses of the fermions , i.e. the quarks and leptons . After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973, the electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics for discovering it. The W and Z bosons were discovered experimentally in 1983; and
Standard Model - Misplaced Pages Continue
3822-651: The mn term giving the coupling of the generations m and n , and h.c. means Hermitian conjugate of preceding terms. The fields Q L {\displaystyle Q_{\text{L}}} and ℓ L {\displaystyle \ell _{\text{L}}} are left-handed quark and lepton doublets. Likewise, u R , d R {\displaystyle u_{\text{R}},d_{\text{R}}} and e R {\displaystyle e_{\text{R}}} are right-handed up-type quark, down-type quark, and lepton singlets. Finally φ {\displaystyle \varphi }
3913-584: The strong interaction . The color confinement phenomenon results in quarks being strongly bound together such that they form color-neutral composite particles called hadrons ; quarks cannot individually exist and must always bind with other quarks. Hadrons can contain either a quark-antiquark pair ( mesons ) or three quarks ( baryons ). The lightest baryons are the nucleons : the proton and neutron . Quarks also carry electric charge and weak isospin , and thus interact with other fermions through electromagnetism and weak interaction . The six leptons consist of
4004-468: The universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). There are three generations of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists of up and down quarks which form protons and neutrons , and electrons and electron neutrinos . The three fundamental interactions known to be mediated by bosons are electromagnetism ,
4095-401: The weak interaction , and the strong interaction . Quarks cannot exist on their own but form hadrons . Hadrons that contain an odd number of quarks are called baryons and those that contain an even number are called mesons . Two baryons, the proton and the neutron , make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of
4186-476: The Higgs boson is massive, it must interact with itself. Because the Higgs boson is a very massive particle and also decays almost immediately when created, only a very high-energy particle accelerator can observe and record it. Experiments to confirm and determine the nature of the Higgs boson using the Large Hadron Collider (LHC) at CERN began in early 2010 and were performed at Fermilab 's Tevatron until its closure in late 2011. Mathematical consistency of
4277-1333: The Higgs' mass could not be predicted beforehand and had to be determined experimentally. The Yukawa interaction terms are: L Yukawa = ( Y u ) m n ( Q ¯ L ) m φ ~ ( u R ) n + ( Y d ) m n ( Q ¯ L ) m φ ( d R ) n + ( Y e ) m n ( ℓ ¯ L ) m φ ( e R ) n + h . c . {\displaystyle {\mathcal {L}}_{\text{Yukawa}}=(Y_{\text{u}})_{mn}({\bar {Q}}_{\text{L}})_{m}{\tilde {\varphi }}(u_{\text{R}})_{n}+(Y_{\text{d}})_{mn}({\bar {Q}}_{\text{L}})_{m}\varphi (d_{\text{R}})_{n}+(Y_{\text{e}})_{mn}({\bar {\ell }}_{\text{L}})_{m}{\varphi }(e_{\text{R}})_{n}+\mathrm {h.c.} } where Y u {\displaystyle Y_{\text{u}}} , Y d {\displaystyle Y_{\text{d}}} , and Y e {\displaystyle Y_{\text{e}}} are 3 × 3 matrices of Yukawa couplings, with
4368-509: The Standard Model and generate masses for all fermions after spontaneous symmetry breaking. The Standard Model describes three of the four fundamental interactions in nature; only gravity remains unexplained. In the Standard Model, such an interaction is described as an exchange of bosons between the objects affected, such as a photon for the electromagnetic force and a gluon for the strong interaction. Those particles are called force carriers or messenger particles . Despite being perhaps
4459-408: 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. The current state of the classification of all elementary particles
4550-476: The Standard Model requires that any mechanism capable of generating the masses of elementary particles must become visible at energies above 1.4 TeV ; therefore, the LHC (designed to collide two 7 TeV proton beams) was built to answer the question of whether the Higgs boson actually exists. On 4 July 2012, two of the experiments at the LHC ( ATLAS and CMS ) both reported independently that they had found
4641-510: The Standard Model was driven by theoretical and experimental particle physicists alike. The Standard Model is a paradigm of a quantum field theory for theorists, exhibiting a wide range of phenomena, including spontaneous symmetry breaking , anomalies , and non-perturbative behavior. It is used as a basis for building more exotic models that incorporate hypothetical particles , extra dimensions , and elaborate symmetries (such as supersymmetry ) to explain experimental results at variance with
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#17327576899254732-499: The Standard Model, such as the existence of dark matter and neutrino oscillations. In 1928, Paul Dirac introduced the Dirac equation which implied the existence of antimatter . In 1954, Yang Chen-Ning and Robert Mills extended the concept of gauge theory for abelian groups , e.g. quantum electrodynamics , to nonabelian groups to provide an explanation for strong interactions . In 1957, Chien-Shiung Wu demonstrated parity
4823-407: The Standard Model. Particle physics Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation . The field also studies combinations of elementary particles up to the scale of protons and neutrons , while the study of combination of protons and neutrons is called nuclear physics . The fundamental particles in
4914-465: The addition of fermion mass terms into the electroweak Lagrangian is forbidden, since terms of the form m ψ ¯ ψ {\displaystyle m{\overline {\psi }}\psi } do not respect U(1) × SU(2) L gauge invariance. Neither is it possible to add explicit mass terms for the U(1) and SU(2) gauge fields. The Higgs mechanism is responsible for the generation of
5005-571: The aforementioned color confinement, gluons are never observed independently. The Higgs boson gives mass to the W and Z bosons via the Higgs mechanism – the gluon and photon are expected to be massless . All bosons have an integer quantum spin (0 and 1) and can have the same quantum state . Most aforementioned particles have corresponding antiparticles , which compose antimatter . Normal particles have positive lepton or baryon number , and antiparticles have these numbers negative. Most properties of corresponding antiparticles and particles are
5096-545: The constituents of all matter . 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. The Standard Model, as currently formulated, has 61 elementary particles. Those elementary particles can combine to form composite particles, accounting for
5187-450: The development of nuclear 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 the CP violation by James Cronin and Val Fitch brought new questions to matter-antimatter imbalance . After the formulation of
5278-624: The exception of opposite charges . Fermions are classified based on how they interact, which is determined by the charges they carry, into two groups: quarks and leptons . Within each group, pairs of particles that exhibit similar physical behaviors are then grouped into generations (see the table). Each member of a generation has a greater mass than the corresponding particle of generations prior. Thus, there are three generations of quarks and leptons. As first-generation particles do not decay, they comprise all of ordinary ( baryonic ) matter. Specifically, all atoms consist of electrons orbiting around
5369-478: The first experimental deviations from the Standard Model, since neutrinos do not have mass in the Standard Model. Modern particle physics research is focused on subatomic particles , including atomic constituents, such as electrons , protons , and neutrons (protons and neutrons are composite particles called baryons , made of quarks ), that are produced by radioactive and scattering processes; such particles are photons , neutrinos , and muons , as well as
5460-486: The form ψ → ψ ′ = U ψ {\displaystyle \psi \rightarrow \psi '=U\psi } , where U = e − i g s λ a ϕ a ( x ) {\displaystyle U=e^{-ig_{s}\lambda ^{a}\phi ^{a}(x)}} is 3 × 3 {\displaystyle 3\times 3} unitary matrix with determinant 1, making it
5551-862: The gauge boson masses, and the fermion masses result from Yukawa-type interactions with the Higgs field. In the Standard Model, the Higgs field is an SU ( 2 ) L {\displaystyle \operatorname {SU} (2)_{\text{L}}} doublet of complex scalar fields with four degrees of freedom: φ = ( φ + φ 0 ) = 1 2 ( φ 1 + i φ 2 φ 3 + i φ 4 ) , {\displaystyle \varphi ={\begin{pmatrix}\varphi ^{+}\\\varphi ^{0}\end{pmatrix}}={\frac {1}{\sqrt {2}}}{\begin{pmatrix}\varphi _{1}+i\varphi _{2}\\\varphi _{3}+i\varphi _{4}\end{pmatrix}},} where
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#17327576899255642-538: The gravitational interaction, but it has not been detected or completely reconciled with current theories. Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably, supersymmetric particles aim to solve the hierarchy problem , axions address the strong CP problem , and various other particles are proposed to explain the origins of dark matter and dark energy . The world's major particle physics laboratories are: Theoretical particle physics attempts to develop
5733-428: The ground state is transformed to a basis where φ 1 = φ 2 = φ 4 = 0 {\displaystyle \varphi _{1}=\varphi _{2}=\varphi _{4}=0} and φ 3 = μ λ ≡ v {\displaystyle \varphi _{3}={\tfrac {\mu }{\sqrt {\lambda }}}\equiv v} . This breaks
5824-424: 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 the experimental 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 (See Theory of Everything ). In recent years, measurements of neutrino mass have provided
5915-614: The interactions between quarks and gluons, which is a Yang–Mills gauge theory with SU(3) symmetry, generated by T a = λ a / 2 {\displaystyle T^{a}=\lambda ^{a}/2} . Since leptons do not interact with gluons, they are not affected by this sector. The Dirac Lagrangian of the quarks coupled to the gluon fields is given by L QCD = ψ ¯ i γ μ D μ ψ − 1 4 G μ ν
6006-433: 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 called color confinement . 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
6097-642: The left-handed doublet and right-handed singlet lepton fields. The electroweak gauge covariant derivative is defined as D μ ≡ ∂ μ − i g ′ 1 2 Y W B μ − i g 1 2 τ → L W → μ {\displaystyle D_{\mu }\equiv \partial _{\mu }-ig'{\tfrac {1}{2}}Y_{\text{W}}B_{\mu }-ig{\tfrac {1}{2}}{\vec {\tau }}_{\text{L}}{\vec {W}}_{\mu }} , where Notice that
6188-406: The massive spin-zero particle, was proposed as the Higgs boson , and is a key building block in the Standard Model. It has no intrinsic spin , and for that reason is classified as a boson with spin-0. The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon , are massive. In particular, the Higgs boson explains why
6279-595: The mid-1970s upon experimental confirmation of the existence of quarks . Since then, proof of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy. Although the Standard Model is believed to be theoretically self-consistent and has demonstrated some success in providing experimental predictions , it leaves some physical phenomena unexplained and so falls short of being
6370-497: The models, theoretical framework, and mathematical tools to understand current experiments and make predictions for future experiments (see also theoretical physics ). There are several major interrelated efforts being made in theoretical particle physics today. 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
6461-415: The most familiar fundamental interaction, gravity is not described by the Standard Model, due to contradictions that arise when combining general relativity, the modern theory of gravity, and quantum mechanics. However, gravity is so weak at microscopic scales, that it is essentially unmeasurable. The graviton is postulated to be the mediating particle, but has not yet been proved to exist. Electromagnetism
6552-427: The particles described by the Standard Model are summarized by the diagrams on the right of this section. The Higgs particle is a massive scalar elementary particle theorized by Peter Higgs ( and others ) in 1964, when he showed that Goldstone's 1962 theorem (generic continuous symmetry, which is spontaneously broken) provides a third polarisation of a massive vector field. Hence, Goldstone's original scalar doublet,
6643-431: The photon has no mass, while the W and Z bosons are very heavy. Elementary-particle masses and the differences between electromagnetism (mediated by the photon) and the weak force (mediated by the W and Z bosons) are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory , the Higgs boson generates the masses of the leptons (electron, muon, and tau) and quarks. As
6734-483: The photon or gluon, have no antiparticles. 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. The gluon can have eight color charges , which are the result of quarks' interactions to form composite particles (gauge symmetry SU(3) ). The neutrons and protons in
6825-423: The photon, aside from them being massive and interacting with the neutrino. The weak interaction is also the only interaction to violate parity and CP . Parity violation is maximal for charged current interactions, since the W boson interacts exclusively with left-handed fermions and right-handed antifermions. In the Standard Model, the weak force is understood in terms of the electroweak theory, which states that
6916-426: The primary colors . More exotic hadrons can have other types, arrangement or number of quarks ( tetraquark , pentaquark ). An atom is made from protons, neutrons and electrons. By modifying the particles inside a normal atom, exotic atoms can be formed. A simple example would be the hydrogen-4.1 , which has one of its electrons replaced with a muon. The graviton is a hypothetical particle that can mediate
7007-444: The ratio of their masses was found to be as the Standard Model predicted. The theory of the strong interaction (i.e. quantum chromodynamics , QCD), to which many contributed, acquired its modern form in 1973–74 when asymptotic freedom was proposed (a development which made QCD the main focus of theoretical research) and experiments confirmed that the hadrons were composed of fractionally charged quarks. The term "Standard Model"
7098-444: 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 . When a particle and an antiparticle interact with each other, they are annihilated and convert to other particles. Some particles, such as
7189-649: The scalar field φ {\displaystyle \varphi } . The scalar potential is given by V ( φ ) = − μ 2 φ † φ + λ ( φ † φ ) 2 , {\displaystyle V(\varphi )=-\mu ^{2}\varphi ^{\dagger }\varphi +\lambda \left(\varphi ^{\dagger }\varphi \right)^{2},} where μ 2 > 0 {\displaystyle \mu ^{2}>0} , so that φ {\displaystyle \varphi } acquires
7280-688: The scale of electroweak physics. This is the only dimensional parameter of the Standard Model and has a measured value of ~ 246 GeV/ c . After symmetry breaking, the masses of the W {\displaystyle {\text{W}}} and Z {\displaystyle {\text{Z}}} are given by m W = 1 2 g v {\displaystyle m_{\text{W}}={\frac {1}{2}}gv} and m Z = 1 2 g 2 + g ′ 2 v {\displaystyle m_{\text{Z}}={\frac {1}{2}}{\sqrt {g^{2}+g'^{2}}}v} , which can be viewed as predictions of
7371-430: The strong force becomes weaker, as the energy scale increases. The strong force overpowers the electrostatic repulsion of protons and quarks in nuclei and hadrons respectively, at their respective scales. While quarks are bound in hadrons by the fundamental strong interaction, which is mediated by gluons, nucleons are bound by an emergent phenomenon termed the residual strong force or nuclear force . This interaction
7462-522: The subscript j {\displaystyle j} sums over the three generations of fermions; Q L , u R {\displaystyle Q_{L},u_{R}} , and d R {\displaystyle d_{R}} are the left-handed doublet, right-handed singlet up type, and right handed singlet down type quark fields; and ℓ L {\displaystyle \ell _{L}} and e R {\displaystyle e_{R}} are
7553-782: The superscripts + and 0 indicate the electric charge Q {\displaystyle Q} of the components. The weak hypercharge Y W {\displaystyle Y_{\text{W}}} of both components is 1. Before symmetry breaking, the Higgs Lagrangian is L H = ( D μ φ ) † ( D μ φ ) − V ( φ ) , {\displaystyle {\mathcal {L}}_{\text{H}}=\left(D_{\mu }\varphi \right)^{\dagger }\left(D^{\mu }\varphi \right)-V(\varphi ),} where D μ {\displaystyle D_{\mu }}
7644-451: The symmetry of the ground state. The expectation value of φ {\displaystyle \varphi } now becomes ⟨ φ ⟩ = 1 2 ( 0 v ) , {\displaystyle \langle \varphi \rangle ={\frac {1}{\sqrt {2}}}{\begin{pmatrix}0\\v\end{pmatrix}},} where v {\displaystyle v} has units of mass and sets
7735-473: The theory. The photon remains massless. The mass of the Higgs Boson is m H = 2 μ 2 = 2 λ v {\displaystyle m_{\text{H}}={\sqrt {2\mu ^{2}}}={\sqrt {2\lambda }}v} . Since μ {\displaystyle \mu } and λ {\displaystyle \lambda } are free parameters,
7826-434: The three fundamental interactions. The fields fall into different representations of the various symmetry groups of the Standard Model (see table). Upon writing the most general Lagrangian, one finds that the dynamics depends on 19 parameters, whose numerical values are established by experiment. The parameters are summarized in the table (made visible by clicking "show") above. The quantum chromodynamics (QCD) sector defines
7917-546: The weak and electromagnetic interactions become united into a single electroweak interaction at high energies. The strong nuclear force is responsible for hadronic and nuclear binding . It is mediated by gluons, which couple to color charge. Since gluons themselves have color charge, the strong force exhibits confinement and asymptotic freedom . Confinement means that only color-neutral particles can exist in isolation, therefore quarks can only exist in hadrons and never in isolation, at low energies. Asymptotic freedom means that
8008-682: Was composed of a single, unique type of particle. The word atom , after the Greek word atomos meaning "indivisible", has since then denoted the smallest particle of a chemical 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 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
8099-701: Was introduced by Abraham Pais and Sam Treiman in 1975, with reference to the electroweak theory with four quarks. Steven Weinberg , has since claimed priority, explaining that he chose the term Standard Model out of a sense of modesty and used it in 1973 during a talk in Aix-en-Provence in France. The Standard Model includes members of several classes of elementary particles, which in turn can be distinguished by other characteristics, such as color charge . All particles can be summarized as follows: Notes : [†] An anti-electron ( e )
8190-500: Was not conserved in the weak interaction . In 1961, Sheldon Glashow combined the electromagnetic and weak interactions . In 1964, Murray Gell-Mann and George Zweig introduced quarks and that same year Oscar W. Greenberg implicitly introduced color charge of quarks. In 1967 Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into Glashow's electroweak interaction , giving it its modern form. In 1970, Sheldon Glashow, John Iliopoulos, and Luciano Maiani introduced
8281-399: Was put forth in a famous paper by Glashow, Iliopoulos & Maiani (1970) ; at that time, only three quarks ( up , down , and strange ) were thought to exist. Bjorken & Glashow (1964) had previously predicted a fourth quark , but there was little evidence for its existence. The GIM mechanism however, required the existence of a fourth quark, and the prediction of the charm quark
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