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55-648: In previous years, the PDG has published the Pocket Diary for Physicists , a calendar with the dates of key international conferences and contact information of major high energy physics institutions, which is now discontinued. PDG also further maintains the standard numbering scheme for particles in event generators , in association with the event generator authors. The Review of Particle Physics (formerly Review of Particle Properties , Data on Particles and Resonant States , and Data on Elementary Particles and Resonant States )

110-487: 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

165-402: A hadron ( / ˈ h æ d r ɒ n / ; from Ancient Greek ἁδρός (hadrós)  'stout, thick') is a composite subatomic particle made of two or more quarks held together by the strong interaction . They are analogous to molecules , which are held together by the electric force . Most of the mass of ordinary matter comes from two hadrons: the proton and

220-402: 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

275-487: A proton is composed of two up quarks (each with electric charge + + 2 ⁄ 3 , for a total of + 4 ⁄ 3 together) and one down quark (with electric charge − + 1 ⁄ 3 ). Adding these together yields the proton charge of +1. Although quarks also carry color charge , hadrons must have zero total color charge because of a phenomenon called color confinement . That is, hadrons must be "colorless" or "white". The simplest ways for this to occur are with

330-498: 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

385-1055: 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

440-452: 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

495-791: 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. Hadron In particle physics ,

550-430: 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,

605-527: A new category term: Notwithstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that " strongly interacting particles " is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called "non- leptonic ". This definition

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660-425: A quark of one color and an antiquark of the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type of meson , and those with the second arrangement are a type of baryon . Massless virtual gluons compose the overwhelming majority of particles inside hadrons, as well as the major constituents of its mass (with the exception of the heavy charm and bottom quarks ;

715-435: 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

770-694: A year later into the 1964 Data on Elementary Particles and Resonant States . The publication underwent three renamings thereafter: 1965 into Data on Particles and Resonant States , 1970 into Review of Particle Properties , and 1996 into the present form Review of Particle Physics . Starting with 1972, the Review no longer appear exclusively in Reviews of Modern Physics , but also in Physics Letters B , European Physical Journal C , Journal of Physics G , Physical Review D , and Chinese Physics C (depending on

825-425: 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

880-401: Is a voluminous, 1,200+ page reference work which summarizes particle properties and reviews the current status of elementary particle physics , general relativity and big-bang cosmology. Usually singled out for citation analysis , it is currently the most cited article in high energy physics , being cited more than 2,000 times annually in the scientific literature (as of 2009). The Review

935-538: 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

990-569: Is currently divided into 3 sections: A condensed version of the Review , with the Summary Tables , a significantly shortened Reviews, Tables and Plots , and without the Particle Listings , is available as a 300-page, pocket-sized Particle Physics Booklet . The history of Review of Particle Physics can be traced back to the 1957 article Hyperons and Heavy Mesons (Systematics and Decay) by Murray Gell-Mann and Arthur H. Rosenfeld , and

1045-532: 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

1100-591: 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

1155-835: Is known as asymptotic freedom , has been experimentally confirmed in the energy range between 1  GeV (gigaelectronvolt) and 1  TeV (teraelectronvolt). All free hadrons except ( possibly ) the proton and antiproton are unstable . Baryons are hadrons containing an odd number of valence quarks (at least 3). Most well-known baryons such as the proton and neutron have three valence quarks, but pentaquarks with five quarks—three quarks of different colors, and also one extra quark-antiquark pair—have also been proven to exist. Because baryons have an odd number of quarks, they are also all fermions , i.e. , they have half-integer spin . As quarks possess baryon number B  =  1 ⁄ 3 , baryons have baryon number B  = 1. Pentaquarks also have B  = 1, since

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1210-547: Is not exact because "non-leptonic" may also signify photonic. In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient. — L. B. Okun (1962) According to the quark model , the properties of hadrons are primarily determined by their so-called valence quarks . For example,

1265-402: Is not meaningful to ask which quark is real and which virtual; only the small excess is apparent from the outside in the form of a hadron. Therefore, when a hadron or anti-hadron is stated to consist of (typically) two or three quarks, this technically refers to the constant excess of quarks versus antiquarks. Like all subatomic particles , hadrons are assigned quantum numbers corresponding to

1320-471: 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

1375-600: 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

1430-515: The LHCb collaboration. Mesons are hadrons containing an even number of valence quarks (at least two). Most well known mesons are composed of a quark-antiquark pair, but possible tetraquarks (four quarks) and hexaquarks (six quarks, comprising either a dibaryon or three quark-antiquark pairs) may have been discovered and are being investigated to confirm their nature. Several other hypothetical types of exotic meson may exist which do not fall within

1485-519: The LHCb collaboration. There are several more exotic hadron candidates and other colour-singlet quark combinations that may also exist. Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an atomic nucleus ) are believed to be unstable and eventually decay into other particles. The only known possible exception is free protons, which appear to be stable , or at least, take immense amounts of time to decay (order of 10  years). By way of comparison, free neutrons are

1540-544: 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

1595-415: The longest-lived unstable particle , and decay with a half-life of about 611 seconds, and have a mean lifetime of 879 seconds, see free neutron decay . Hadron physics is studied by colliding hadrons, e.g. protons, with each other or the nuclei of dense, heavy elements , such as lead (Pb) or gold (Au), and detecting the debris in the produced particle showers . A similar process occurs in

1650-413: The neutron , while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force. Hadrons are categorized into two broad families: baryons , made of an odd number of quarks (usually three) and mesons , made of an even number of quarks (usually two: one quark and one antiquark ). Protons and neutrons (which make the majority of

1705-472: The representations of the Poincaré group : J ( m ), where J is the spin quantum number, P the intrinsic parity (or P-parity ), C the charge conjugation (or C-parity ), and m is the particle's mass . Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence , most of the mass comes from the large amount of energy associated with

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1760-718: The strong interaction . Hadrons may also carry flavor quantum numbers such as isospin ( G-parity ), and strangeness . All quarks carry an additive, conserved quantum number called a baryon number ( B ), which is + + 1 ⁄ 3 for quarks and − + 1 ⁄ 3 for antiquarks. This means that baryons (composite particles made of three, five or a larger odd number of quarks) have B  = 1 whereas mesons have B  = 0. Hadrons have excited states known as resonances . Each ground state hadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about 10   seconds ) via

1815-484: The top quark vanishes before it has time to bind into a hadron). The strength of the strong-force gluons which bind the quarks together has sufficient energy ( E ) to have resonances composed of massive ( m ) quarks ( E ≥ mc ). One outcome is that short-lived pairs of virtual quarks and antiquarks are continually forming and vanishing again inside a hadron. Because the virtual quarks are not stable wave packets (quanta), but an irregular and transient phenomenon, it

1870-468: 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 ,

1925-401: 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

1980-408: 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

2035-571: 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

2090-584: 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

2145-447: 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

2200-488: The extra quark's and antiquark's baryon numbers cancel. Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example, just as a proton is made of two up quarks and one down quark, its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark. As of August 2015, there are two known pentaquarks, P c (4380) and P c (4450) , both discovered in 2015 by

2255-478: 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

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2310-538: 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

2365-424: 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

2420-433: 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

2475-596: The mass of an atom ) are examples of baryons; pions are an example of a meson. "Exotic" hadrons , containing more than three valence quarks, have been discovered in recent years. A tetraquark state (an exotic meson ), named the Z(4430) , was discovered in 2007 by the Belle Collaboration and confirmed as a resonance in 2014 by the LHCb collaboration. Two pentaquark states ( exotic baryons ), named P c (4380) and P c (4450) , were discovered in 2015 by

2530-497: 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

2585-401: The natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions of cosmic rays with rarefied gas particles in the outer atmosphere. The term "hadron" is a new Greek word introduced by L. B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics at CERN . He opened his talk with the definition of

2640-483: 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

2695-426: 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

2750-447: The quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons ). Because mesons have an even number of quarks, they are also all bosons , with integer spin , i.e. , 0, +1, or −1. They have baryon number B = ⁠ 1 / 3 ⁠ − ⁠ 1 / 3 ⁠ = 0 . Examples of mesons commonly produced in particle physics experiments include pions and kaons . Pions also play

2805-444: 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

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2860-416: The strong nuclear force. In other phases of matter the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics (QCD) predicts that quarks and gluons will no longer be confined within hadrons, "because the strength of the strong interaction diminishes with energy ". This property, which

2915-415: The unpublished update tables for its data with the title Data for Elementary Particle Physics ( University of California Radiation Laboratory Technical Report UCRL-8030 ) that were circulated before the actual publication of the original article. In 1963, Matts Roos independently published a compilation Data on Elementary Particles and Resonant States . On his suggestion, the two publications were merged

2970-404: The year). High energy 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

3025-630: 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

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