99-401: Nuclear may refer to: Relating to the nucleus of the atom : Relating to the nucleus of the cell : Atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom , discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment . After the discovery of the neutron in 1932, models for
198-587: A farmers' market in Freiburg . Some authors, however, defend a possible German origin of Joyce's word quark . Gell-Mann went into further detail regarding the name of the quark in his 1994 book The Quark and the Jaguar : In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of Finnegans Wake , by James Joyce, I came across
297-469: A gold atom. For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin, until he found the word quark in James Joyce 's 1939 book Finnegans Wake : – Three quarks for Muster Mark! Sure he hasn't got much of a bark And sure any he has it's all beside the mark. The word quark is an outdated English word meaning to croak and the above-quoted lines are about
396-415: A vector whose length is measured in units of the reduced Planck constant ħ (pronounced "h bar"). For quarks, a measurement of the spin vector component along any axis can only yield the values + ħ / 2 or − ħ / 2 ; for this reason quarks are classified as spin- 1 / 2 particles. The component of spin along a given axis – by convention
495-443: A bar over the symbol for the corresponding quark, such as u for an up antiquark. As with antimatter in general, antiquarks have the same mass, mean lifetime , and spin as their respective quarks, but the electric charge and other charges have the opposite sign. Quarks are spin- 1 / 2 particles, which means they are fermions according to the spin–statistics theorem . They are subject to
594-486: A better description of the weak interaction (the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known leptons , and implied a mass formula that correctly reproduced the masses of the known mesons . Deep inelastic scattering experiments conducted in 1968 at the Stanford Linear Accelerator Center (SLAC) and published on October 20, 1969, showed that
693-480: A bird choir mocking king Mark of Cornwall in the legend of Tristan and Iseult . Especially in the German-speaking parts of the world there is a widespread legend, however, that Joyce had taken it from the word Quark , a German word of Slavic origin which denotes a curd cheese , but is also a colloquial term for "trivial nonsense". In the legend it is said that he had heard it on a journey to Germany at
792-414: A collective term for the constituents of hadrons (quarks, antiquarks, and gluons ). Richard Taylor , Henry Kendall and Jerome Friedman received the 1990 Nobel Prize in physics for their work at SLAC. The strange quark's existence was indirectly validated by SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it provided an explanation for
891-422: A color charge of 0 (or "white" color) and the formation of a meson . This is analogous to the additive color model in basic optics . Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with different anticolor charges, will result in the same "white" color charge and the formation of a baryon or antibaryon . In modern particle physics, gauge symmetries –
990-540: A great deal of speculation and experimentation. An estimate puts the needed temperature at (1.90 ± 0.02) × 10 kelvin . While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts by CERN in the 1980s and 1990s), recent experiments at the Relativistic Heavy Ion Collider have yielded evidence for liquid-like quark matter exhibiting "nearly perfect" fluid motion . The quark–gluon plasma would be characterized by
1089-508: A great increase in the number of heavier quark pairs in relation to the number of up and down quark pairs. It is believed that in the period prior to 10 seconds after the Big Bang (the quark epoch ), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable. Given sufficiently high baryon densities and relatively low temperatures – possibly comparable to those found in neutron stars – quark matter
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#17328487438501188-526: A half-life of 8.8 ms . Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons [NN, NNN] or protons [PP, PPP]. Nuclei which have a single neutron halo include Be and C. A two-neutron halo is exhibited by He, Li, B, B and C. Two-neutron halo nuclei break into three fragments, never two, and are called Borromean nuclei because of this behavior (referring to
1287-427: A kind of symmetry group – relate interactions between particles (see gauge theories ). Color SU(3) (commonly abbreviated to SU(3) c ) is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics. Just as the laws of physics are independent of which directions in space are designated x , y , and z , and remain unchanged if the coordinate axes are rotated to
1386-465: A mean square radius of about 0.8 fm. The shape of the atomic nucleus can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped. Nuclei are bound together by the residual strong force ( nuclear force ). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force
1485-410: A multitude of hadrons , among other particles. Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks, up , down , and strange , to which they ascribed properties such as spin and electric charge. The initial reaction of the physics community to the proposal was mixed. There
1584-408: A new orientation, the physics of quantum chromodynamics is independent of which directions in three-dimensional color space are identified as blue, red, and green. SU(3) c color transformations correspond to "rotations" in color space (which, mathematically speaking, is a complex space ). Every quark flavor f , each with subtypes f B , f G , f R corresponding to the quark colors, forms
1683-468: A nuclear atom with a dense center of positive charge and mass. The term nucleus is from the Latin word nucleus , a diminutive of nux ('nut'), meaning 'the kernel' (i.e., the 'small nut') inside a watery type of fruit (like a peach ). In 1844, Michael Faraday used the term to refer to the "central point of an atom". The modern atomic meaning was proposed by Ernest Rutherford in 1912. The adoption of
1782-452: A nucleus by the nuclear force . The diameter of the nucleus is in the range of 1.70 fm ( 1.70 × 10 m ) for hydrogen (the diameter of a single proton) to about 11.7 fm for uranium . These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 26,634 (uranium atomic radius is about 156 pm ( 156 × 10 m )) to about 60,250 ( hydrogen atomic radius
1881-444: A nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg . An atom is composed of a positively charged nucleus, with a cloud of negatively charged electrons surrounding it, bound together by electrostatic force . Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud . Protons and neutrons are bound together to form
1980-579: A particle classification system known as the Eightfold Way – or, in more technical terms, SU(3) flavor symmetry , streamlining its structure. Physicist Yuval Ne'eman had independently developed a scheme similar to the Eightfold Way in the same year. An early attempt at constituent organization was available in the Sakata model . At the time of the quark theory's inception, the " particle zoo " included
2079-440: A phenomenon known as color confinement , quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons , or in quark–gluon plasmas . For this reason, much of what is known about quarks has been drawn from observations of hadrons. Quarks have various intrinsic properties , including electric charge , mass , color charge , and spin . They are
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#17328487438502178-454: A positive charge as well. In his plum pudding model, Thomson suggested that an atom consisted of negative electrons randomly scattered within a sphere of positive charge. Ernest Rutherford later devised an experiment with his research partner Hans Geiger and with help of Ernest Marsden , that involved the deflection of alpha particles (helium nuclei) directed at a thin sheet of metal foil. He reasoned that if J. J. Thomson's model were correct,
2277-434: A property called color charge . There are three types of color charge, arbitrarily labeled blue , green , and red . Each of them is complemented by an anticolor – antiblue , antigreen , and antired . Every quark carries a color, while every antiquark carries an anticolor. The system of attraction and repulsion between quarks charged with different combinations of the three colors is called strong interaction , which
2376-530: A quark's mass: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark. These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically, quantum chromodynamics binding energy (QCBE) – and it
2475-434: A small atomic nucleus like that of helium-4 , in which the two protons and two neutrons separately occupy 1s orbitals analogous to the 1s orbital for the two electrons in the helium atom, and achieve unusual stability for the same reason. Nuclei with 5 nucleons are all extremely unstable and short-lived, yet, helium-3 , with 3 nucleons, is very stable even with lack of a closed 1s orbital shell. Another nucleus with 3 nucleons,
2574-428: A system of three interlocked rings in which breaking any ring frees both of the others). He and Be both exhibit a four-neutron halo. Nuclei which have a proton halo include B and P. A two-proton halo is exhibited by Ne and S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive electromagnetic forces of the halo proton(s). Although the standard model of physics
2673-427: A triplet: a three-component quantum field that transforms under the fundamental representation of SU(3) c . The requirement that SU(3) c should be local – that is, that its transformations be allowed to vary with space and time – determines the properties of the strong interaction. In particular, it implies the existence of eight gluon types to act as its force carriers. Two terms are used in referring to
2772-598: A valence quark and an antiquark. The most common baryons are the proton and the neutron, the building blocks of the atomic nucleus . A great number of hadrons are known (see list of baryons and list of mesons ), most of them differentiated by their quark content and the properties these constituent quarks confer. The existence of "exotic" hadrons with more valence quarks, such as tetraquarks ( q q q q ) and pentaquarks ( q q q q q ),
2871-447: A very short range, and essentially drops to zero just beyond the edge of the nucleus. The collective action of the positively charged nucleus is to hold the electrically negative charged electrons in their orbits about the nucleus. The collection of negatively charged electrons orbiting the nucleus display an affinity for certain configurations and numbers of electrons that make their orbits stable. Which chemical element an atom represents
2970-404: A very strong force must be present if it could deflect the massive and fast moving alpha particles. He realized that the plum pudding model could not be accurate and that the deflections of the alpha particles could only be explained if the positive and negative charges were separated from each other and that the mass of the atom was a concentrated point of positive charge. This justified the idea of
3069-414: Is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximum are unstable and tend to be increasingly short-lived with larger numbers of nucleons. However, bismuth-209 is also stable to beta decay and has the longest half-life to alpha decay of any known isotope, estimated at a billion times longer than the age of the universe. The residual strong force
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3168-487: Is a corresponding type of antiparticle , known as an antiquark , that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign . The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at
3267-500: Is about 52.92 pm ). The branch of physics involved with the study and understanding of the atomic nucleus, including its composition and the forces that bind it together, is called nuclear physics . The nucleus was discovered in 1911, as a result of Ernest Rutherford 's efforts to test Thomson's " plum pudding model " of the atom. The electron had already been discovered by J. J. Thomson . Knowing that atoms are electrically neutral, J. J. Thomson postulated that there must be
3366-469: Is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark. Spin is an intrinsic property of elementary particles, and its direction is an important degree of freedom . It is sometimes visualized as the rotation of an object around its own axis (hence the name " spin "), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be point-like . Spin can be represented by
3465-425: Is considered to be one of the basic quantities that any model must predict. For stable nuclei (not halo nuclei or other unstable distorted nuclei) the nuclear radius is roughly proportional to the cube root of the mass number ( A ) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations: The stable nucleus has approximately a constant density and therefore
3564-407: Is determined by the number of protons in the nucleus; the neutral atom will have an equal number of electrons orbiting that nucleus. Individual chemical elements can create more stable electron configurations by combining to share their electrons. It is that sharing of electrons to create stable electronic orbits about the nuclei that appears to us as the chemistry of our macro world. Protons define
3663-416: Is effective over a very short range (usually only a few femtometres (fm); roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons. For example, between a proton and a neutron to form a deuteron [NP], and also between protons and protons, and neutrons and neutrons. The effective absolute limit of the range of the nuclear force (also known as residual strong force )
3762-468: Is highly attractive at the distance of typical nucleon separation, and this overwhelms the repulsion between protons due to the electromagnetic force, thus allowing nuclei to exist. However, the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential ); thus only nuclei smaller than a certain size can be completely stable. The largest known completely stable nucleus (i.e. stable to alpha, beta , and gamma decay )
3861-532: Is highly stable without a closed second 1p shell orbital. For light nuclei with total nucleon numbers 1 to 6 only those with 5 do not show some evidence of stability. Observations of beta-stability of light nuclei outside closed shells indicate that nuclear stability is much more complex than simple closure of shell orbitals with magic numbers of protons and neutrons. For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict
3960-462: Is mediated by force carrying particles known as gluons ; this is discussed at length below. The theory that describes strong interactions is called quantum chromodynamics (QCD). A quark, which will have a single color value, can form a bound system with an antiquark carrying the corresponding anticolor. The result of two attracting quarks will be color neutrality: a quark with color charge ξ plus an antiquark with color charge − ξ will result in
4059-399: Is more stable than an odd number. A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory. These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells. In
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4158-443: Is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms together internally (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus). The nuclear force
4257-485: Is preserved. Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This causes asymptotic freedom : as quarks come closer to each other, the chromodynamic binding force between them weakens. Conversely, as the distance between quarks increases, the binding force strengthens. The color field becomes stressed, much as an elastic band is stressed when stretched, and more gluons of appropriate color are spontaneously created to strengthen
4356-492: Is represented by halo nuclei such as lithium-11 or boron-14 , in which dineutrons , or other collections of neutrons, orbit at distances of about 10 fm (roughly similar to the 8 fm radius of the nucleus of uranium-238 ). These nuclei are not maximally dense. Halo nuclei form at the extreme edges of the chart of the nuclides —the neutron drip line and proton drip line—and are all unstable with short half-lives, measured in milliseconds ; for example, lithium-11 has
4455-490: Is strong indirect evidence that no more than three generations exist. Particles in higher generations generally have greater mass and less stability, causing them to decay into lower-generation particles by means of weak interactions . Only first-generation (up and down) quarks occur commonly in nature. Heavier quarks can only be created in high-energy collisions (such as in those involving cosmic rays ), and decay quickly; however, they are thought to have been present during
4554-401: Is the top quark, which may decay before it hadronizes. Hadrons contain, along with the valence quarks ( q v ) that contribute to their quantum numbers , virtual quark–antiquark ( q q ) pairs known as sea quarks ( q s ). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that
4653-458: Is this that contributes so greatly to the overall mass of the hadron (see mass in special relativity ). For example, a proton has a mass of approximately 938 MeV/ c , of which the rest mass of its three valence quarks only contributes about 9 MeV/ c ; much of the remainder can be attributed to the field energy of the gluons (see chiral symmetry breaking ). The Standard Model posits that elementary particles derive their masses from
4752-605: Is too weak to be relevant to individual particle interactions except at extremes of energy ( Planck energy ) and distance scales ( Planck distance ). However, since no successful quantum theory of gravity exists, gravitation is not described by the Standard Model. See the table of properties below for a more complete overview of the six quark flavors' properties. The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. The proposal came shortly after Gell-Mann's 1961 formulation of
4851-442: Is widely believed to completely describe the composition and behavior of the nucleus, generating predictions from theory is much more difficult than for most other areas of particle physics . This is due to two reasons: Historically, experiments have been compared to relatively crude models that are necessarily imperfect. None of these models can completely explain experimental data on nuclear structure. The nuclear radius ( R )
4950-431: The 2D Ising Model of MacGregor. Quarks A quark ( / k w ɔːr k , k w ɑːr k / ) is a type of elementary particle and a fundamental constituent of matter . Quarks combine to form composite particles called hadrons , the most stable of which are protons and neutrons , the components of atomic nuclei . All commonly observable matter is composed of up quarks, down quarks and electrons . Owing to
5049-495: The Higgs mechanism , which is associated to the Higgs boson . It is hoped that further research into the reasons for the top quark's large mass of ~ 173 GeV/ c , almost the mass of a gold atom, might reveal more about the origin of the mass of quarks and other elementary particles. In QCD, quarks are considered to be point-like entities, with zero size. As of 2014, experimental evidence indicates they are no bigger than 10 times
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#17328487438505148-448: The Pauli exclusion principle , which states that no two identical fermions can simultaneously occupy the same quantum state . This is in contrast to bosons (particles with integer spin), of which any number can be in the same state. Unlike leptons , quarks possess color charge , which causes them to engage in the strong interaction . The resulting attraction between different quarks causes
5247-637: The Stanford Linear Accelerator Center in 1968. Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered. The Standard Model is the theoretical framework describing all the known elementary particles . This model contains six flavors of quarks ( q ), named up ( u ), down ( d ), strange ( s ), charm ( c ), bottom ( b ), and top ( t ). Antiparticles of quarks are called antiquarks , and are denoted by
5346-536: The annihilation of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as "the sea". Sea quarks are much less stable than their valence counterparts, and they typically annihilate each other within the interior of the hadron. Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances. Under sufficiently extreme conditions, quarks may become "deconfined" out of bound states and propagate as thermalized "free" excitations in
5445-510: The elementary charge (e), depending on flavor. Up, charm, and top quarks (collectively referred to as up-type quarks ) have a charge of + 2 / 3 e; down, strange, and bottom quarks ( down-type quarks ) have a charge of − 1 / 3 e. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of − 2 / 3 e and down-type antiquarks have charges of + 1 / 3 e. Since
5544-500: The kaon ( K ) and pion ( π ) hadrons discovered in cosmic rays in 1947. In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani presented the GIM mechanism (named from their initials) to explain the experimental non-observation of flavor-changing neutral currents . This theoretical model required the existence of the as-yet undiscovered charm quark . The number of supposed quark flavors grew to
5643-927: The strange particles discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes. Glashow, who co-proposed the charm quark with Bjorken, is quoted as saying, "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world." The names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for up and down quarks". Alternative names for bottom and top quarks are "beauty" and "truth" respectively, but these names have somewhat fallen out of use. While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called " beauty factories ". Quarks have fractional electric charge values – either (− 1 / 3 ) or (+ 2 / 3 ) times
5742-448: The strong isospin quantum number , so two protons and two neutrons can share the same space wave function since they are not identical quantum entities. They are sometimes viewed as two different quantum states of the same particle, the nucleon . Two fermions, such as two protons, or two neutrons, or a proton + neutron (the deuteron) can exhibit bosonic behavior when they become loosely bound in pairs, which have integer spin. In
5841-409: The z axis – is often denoted by an up arrow ↑ for the value + 1 / 2 and down arrow ↓ for the value − 1 / 2 , placed after the symbol for flavor. For example, an up quark with a spin of + 1 / 2 along the z axis is denoted by u↑. A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of
5940-536: The " portmanteau " words in Through the Looking-Glass . From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark", in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly
6039-487: The Coulomb energy, the most stable form of nuclear matter would have the same number of neutrons as protons, since unequal numbers of neutrons and protons imply filling higher energy levels for one type of particle, while leaving lower energy levels vacant for the other type. Pairing energy . An energy which is a correction term that arises from the tendency of proton pairs and neutron pairs to occur. An even number of particles
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#17328487438506138-521: The above models, the nucleons may occupy orbitals in pairs, due to being fermions, which allows explanation of even/odd Z and N effects well known from experiments. The exact nature and capacity of nuclear shells differs from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nuclei) is quite different from the central electromagnetic potential well which binds electrons in atoms. Some resemblance to atomic orbital models may be seen in
6237-473: The additional quarks. In 1977, the bottom quark was observed by a team at Fermilab led by Leon Lederman . This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. It was not until 1995 that the top quark was finally observed, also by the CDF and DØ teams at Fermilab. It had a mass much larger than expected, almost as large as that of
6336-553: The current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks. Charm quarks were produced almost simultaneously by two teams in November 1974 (see November Revolution ) – one at SLAC under Burton Richter , and one at Brookhaven National Laboratory under Samuel Ting . The charm quarks were observed bound with charm antiquarks in mesons. The two parties had assigned
6435-461: The discovered meson two different symbols, J and ψ ; thus, it became formally known as the J/ψ; meson . The discovery finally convinced the physics community of the quark model's validity. In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper by Haim Harari was the first to coin the terms top and bottom for
6534-562: The down quarks in the neutron ( u d d ) decays into an up quark by emitting a virtual W boson, transforming the neutron into a proton ( u u d ). The W boson then decays into an electron and an electron antineutrino. Both beta decay and the inverse process of inverse beta decay are routinely used in medical applications such as positron emission tomography (PET) and in experiments involving neutrino detection . While
6633-439: The electric charge ( Q ) and all flavor quantum numbers ( B , I 3 , C , S , T , and B ′) are of opposite sign. Mass and total angular momentum ( J ; equal to spin for point particles) do not change sign for the antiquarks. As described by quantum chromodynamics , the strong interaction between quarks is mediated by gluons, massless vector gauge bosons . Each gluon carries one color charge and one anticolor charge. In
6732-410: The electric charge of a hadron is the sum of the charges of the constituent quarks, all hadrons have integer charges: the combination of three quarks (baryons), three antiquarks (antibaryons), or a quark and an antiquark (mesons) always results in integer charges. For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 e and +1 e respectively; the neutron
6831-429: The entire charge of a nucleus, and hence its chemical identity . Neutrons are electrically neutral, but contribute to the mass of a nucleus to nearly the same extent as the protons. Neutrons can explain the phenomenon of isotopes (same atomic number with different atomic mass). The main role of neutrons is to reduce electrostatic repulsion inside the nucleus. Protons and neutrons are fermions , with different values of
6930-401: The field. Above a certain energy threshold, pairs of quarks and antiquarks are created . These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known as color confinement : quarks never appear in isolation. This process of hadronization occurs before quarks formed in a high energy collision are able to interact in any other way. The only exception
7029-512: The first fractions of a second after the Big Bang , when the universe was in an extremely hot and dense phase (the quark epoch ). Studies of heavier quarks are conducted in artificially created conditions, such as in particle accelerators . Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction. Gravitation
7128-461: The formation of composite particles known as hadrons (see § Strong interaction and color charge below). The quarks that determine the quantum numbers of hadrons are called valence quarks ; apart from these, any hadron may contain an indefinite number of virtual " sea " quarks, antiquarks, and gluons , which do not influence its quantum numbers. There are two families of hadrons: baryons , with three valence quarks, and mesons , with
7227-533: The four fundamental interactions in particle physics. By absorbing or emitting a W boson , any up-type quark (up, charm, and top quarks) can change into any down-type quark (down, strange, and bottom quarks) and vice versa. This flavor transformation mechanism causes the radioactive process of beta decay , in which a neutron ( n ) "splits" into a proton ( p ), an electron ( e ) and an electron antineutrino ( ν e ) (see picture). This occurs when one of
7326-408: The larger medium. In the course of asymptotic freedom , the strong interaction becomes weaker at increasing temperatures. Eventually, color confinement would be effectively lost in an extremely hot plasma of freely moving quarks and gluons. This theoretical phase of matter is called quark–gluon plasma . The exact conditions needed to give rise to this state are unknown and have been the subject of
7425-491: The lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay : the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe , whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators ). For every quark flavor there
7524-404: The magic numbers of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly-inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element. Similarly, the distance from shell-closure explains
7623-403: The manifestation of more elementary particles, called quarks , that are held in association by the nuclear strong force in certain stable combinations of hadrons , called baryons . The nuclear strong force extends far enough from each baryon so as to bind the neutrons and protons together against the repulsive electrical force between the positively charged protons. The nuclear strong force has
7722-445: The nuclear radius R can be approximated by the following formula, where A = Atomic mass number (the number of protons Z , plus the number of neutrons N ) and r 0 = 1.25 fm = 1.25 × 10 m. In this equation, the "constant" r 0 varies by 0.2 fm, depending on the nucleus in question, but this is less than 20% change from a constant. In other words, packing protons and neutrons in
7821-429: The nucleus and hence its binding energy is less. This surface energy term takes that into account and is therefore negative and is proportional to the surface area. Coulomb energy . The electric repulsion between each pair of protons in a nucleus contributes toward decreasing its binding energy. Asymmetry energy (also called Pauli Energy). An energy associated with the Pauli exclusion principle . Were it not for
7920-493: The nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight spherical or almost spherical bag (some stable nuclei are not quite spherical, but are known to be prolate ). Models of nuclear structure include: The cluster model describes the nucleus as a molecule-like collection of proton-neutron groups (e.g., alpha particles ) with one or more valence neutrons occupying molecular orbitals. Early models of
8019-492: The nucleus viewed the nucleus as a rotating liquid drop. In this model, the trade-off of long-range electromagnetic forces and relatively short-range nuclear forces, together cause behavior which resembled surface tension forces in liquid drops of different sizes. This formula is successful at explaining many important phenomena of nuclei, such as their changing amounts of binding energy as their size and composition changes (see semi-empirical mass formula ), but it does not explain
8118-401: The nucleus: [REDACTED] Volume energy . When an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. So, this nuclear energy is proportional to the volume. Surface energy . A nucleon at the surface of a nucleus interacts with fewer other nucleons than one in the interior of
8217-551: The only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions , also known as fundamental forces ( electromagnetism , gravitation , strong interaction , and weak interaction ), as well as the only known particles whose electric charges are not integer multiples of the elementary charge . There are six types, known as flavors , of quarks: up , down , charm , strange , top , and bottom . Up and down quarks have
8316-433: The positively charged alpha particles would easily pass through the foil with very little deviation in their paths, as the foil should act as electrically neutral if the negative and positive charges are so intimately mixed as to make it appear neutral. To his surprise, many of the particles were deflected at very large angles. Because the mass of an alpha particle is about 8000 times that of an electron, it became apparent that
8415-533: The process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by a mathematical table , called the Cabibbo–Kobayashi–Maskawa matrix (CKM matrix). Enforcing unitarity , the approximate magnitudes of the entries of the CKM matrix are: where V ij represents
8514-429: The proton contained much smaller, point-like objects and was therefore not an elementary particle. Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them " partons " – a term coined by Richard Feynman . The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered. Nevertheless, "parton" remains in use as
8613-526: The rare case of a hypernucleus , a third baryon called a hyperon , containing one or more strange quarks and/or other unusual quark(s), can also share the wave function. However, this type of nucleus is extremely unstable and not found on Earth except in high-energy physics experiments. The neutron has a positively charged core of radius ≈ 0.3 fm surrounded by a compensating negative charge of radius between 0.3 fm and 2 fm. The proton has an approximately exponentially decaying positive charge distribution with
8712-508: The shape of the potential well to fit experimental data, but the question remains whether these mathematical manipulations actually correspond to the spatial deformations in real nuclei. Problems with the shell model have led some to propose realistic two-body and three-body nuclear force effects involving nucleon clusters and then build the nucleus on this basis. Three such cluster models are the 1936 Resonating Group Structure model of John Wheeler, Close-Packed Spheron Model of Linus Pauling and
8811-500: The size of a proton, i.e. less than 10 metres. The following table summarizes the key properties of the six quarks. Flavor quantum numbers ( isospin ( I 3 ), charm ( C ), strangeness ( S , not to be confused with spin), topness ( T ), and bottomness ( B ′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The baryon number ( B ) is + 1 / 3 for all quarks, as baryons are made of three quarks. For antiquarks,
8910-410: The special stability which occurs when nuclei have special "magic numbers" of protons or neutrons. The terms in the semi-empirical mass formula, which can be used to approximate the binding energy of many nuclei, are considered as the sum of five types of energies (see below). Then the picture of a nucleus as a drop of incompressible liquid roughly accounts for the observed variation of binding energy of
9009-515: The standard framework of particle interactions (part of a more general formulation known as perturbation theory ), gluons are constantly exchanged between quarks through a virtual emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while each quark's color constantly changes, their strong interaction
9108-544: The tendency of a quark of flavor i to change into a quark of flavor j (or vice versa). There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix). Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear. According to quantum chromodynamics (QCD), quarks possess
9207-499: The term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule , that "the atom is composed of the kernel and an outer atom or shell. " Similarly, the term kern meaning kernel is used for nucleus in German and Dutch. The nucleus of an atom consists of neutrons and protons, which in turn are
9306-417: The triton hydrogen-3 is unstable and will decay into helium-3 when isolated. Weak nuclear stability with 2 nucleons {NP} in the 1s orbital is found in the deuteron hydrogen-2 , with only one nucleon in each of the proton and neutron potential wells. While each nucleon is a fermion, the {NP} deuteron is a boson and thus does not follow Pauli Exclusion for close packing within shells. Lithium-6 with 6 nucleons
9405-422: The unusual instability of isotopes which have far from stable numbers of these particles, such as the radioactive elements 43 ( technetium ) and 61 ( promethium ), each of which is preceded and followed by 17 or more stable elements. There are however problems with the shell model when an attempt is made to account for nuclear properties well away from closed shells. This has led to complex post hoc distortions of
9504-444: The way quarks occur in nature. Zweig preferred the name ace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted. The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components of isospin , which they carry. Strange quarks were given their name because they were discovered to be components of
9603-413: The word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark", as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork". But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like
9702-438: Was conjectured from the beginnings of the quark model but not discovered until the early 21st century. Elementary fermions are grouped into three generations , each comprising two leptons and two quarks. The first generation includes up and down quarks, the second strange and charm quarks, and the third bottom and top quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there
9801-466: Was particular contention about whether the quark was a physical entity or a mere abstraction used to explain concepts that were not fully understood at the time. In less than a year, extensions to the Gell-Mann–Zweig model were proposed. Sheldon Glashow and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm . The addition was proposed because it allowed for
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