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87-585: V-A or V−A may refer to: "Vector minus axial", a theory of weak interaction Vetenskap & Allmänhet Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title V-A . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=V-A&oldid=933229076 " Category : Disambiguation pages Hidden categories: Short description

174-455: A ν μ (with T 3 = ⁠+ + 1 / 2 ⁠ ) and a μ (as a right-handed antiparticle, ⁠+ + 1 / 2 ⁠ ). For the development of the electroweak theory, another property, weak hypercharge , was invented, defined as where Y W is the weak hypercharge of a particle with electrical charge Q (in elementary charge units) and weak isospin T 3 . Weak hypercharge

261-504: A W  boson or by absorbing a W  boson. More precisely, the down-type quark becomes a quantum superposition of up-type quarks: that is to say, it has a possibility of becoming any one of the three up-type quarks, with the probabilities given in the CKM matrix tables. Conversely, an up-type quark can emit a W  boson, or absorb a W  boson, and thereby be converted into

348-425: A T 3 of ⁠− + 1 / 2 ⁠ and conversely. In any given strong, electromagnetic, or weak interaction, weak isospin is conserved : The sum of the weak isospin numbers of the particles entering the interaction equals the sum of the weak isospin numbers of the particles exiting that interaction. For example, a (left-handed) π , with a weak isospin of +1 normally decays into

435-535: A neutron is heavier than a proton (its partner nucleon ) and can decay into a proton by changing the flavour (type) of one of its two down quarks to an up quark. Neither the strong interaction nor electromagnetism permit flavour changing, so this can only proceed by weak decay ; without weak decay, quark properties such as strangeness and charm (associated with the strange quark and charm quark, respectively) would also be conserved across all interactions. All mesons are unstable because of weak decay. In

522-448: A quark or a lepton (e.g., an electron or a muon ) emits or absorbs a neutral Z boson . For example: Like the W  bosons, the Z  boson also decays rapidly, for example: Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge, and / or weak isospin, the neutral-current Z interaction can cause any two fermions in

609-566: A complex scalar Higgs field doublet. Likewise, there are four massless electroweak vector bosons, each similar to the photon . However, at low energies, this gauge symmetry is spontaneously broken down to the U(1) symmetry of electromagnetism, since one of the Higgs fields acquires a vacuum expectation value . Naïvely, the symmetry-breaking would be expected to produce three massless bosons , but instead those "extra" three Higgs bosons become incorporated into

696-503: A compound symmetry CP to be conserved. CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics . In 1973, Makoto Kobayashi and Toshihide Maskawa showed that CP violation in

783-412: A contact force with no range. In the mid-1950s, Chen-Ning Yang and Tsung-Dao Lee first suggested that the handedness of the spins of particles in weak interaction might violate the conservation law or symmetry. In 1957, Chien Shiung Wu and collaborators confirmed the symmetry violation . In the 1960s, Sheldon Glashow , Abdus Salam and Steven Weinberg unified the electromagnetic force and

870-433: A corresponding neutrino (with a charge of 0), where the type ("flavour") of neutrino (electron ν e , muon ν μ , or tau ν τ ) is the same as the type of lepton in the interaction, for example: Similarly, a down-type quark ( d , s , or b , with a charge of ⁠− + 1  / 3 ⁠ ) can be converted into an up-type quark ( u , c , or t , with a charge of ⁠+ + 2  / 3 ⁠ ), by emitting

957-408: A down-type quark, for example: The W boson is unstable so will rapidly decay, with a very short lifetime. For example: Decay of a W boson to other products can happen, with varying probabilities. In the so-called beta decay of a neutron (see picture, above), a down quark within the neutron emits a virtual W boson and is thereby converted into an up quark, converting

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1044-471: A fundamentally new type in 1903 and termed gamma rays . Alpha, beta, and gamma are the first three letters of the Greek alphabet . In 1900, Becquerel measured the mass-to-charge ratio ( m / e ) for beta particles by the method of J.J. Thomson used to study cathode rays and identify the electron. He found that m / e for a beta particle is the same as for Thomson's electron, and therefore suggested that

1131-572: A half-life of about 12.7 hours. This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. This particular nuclide (though not all nuclides in this situation) is almost equally likely to decay through proton decay by positron emission ( 18% ) or electron capture ( 43% ) to 28 Ni , as it is through neutron decay by electron emission ( 39% ) to 30 Zn . Most naturally occurring nuclides on earth are beta stable. Nuclides that are not beta stable have half-lives ranging from under

1218-455: A life of only about 10  seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about 10  seconds, or a hundred million times longer than a neutral pion. A particularly extreme example is the weak-force decay of a free neutron, which takes about 15 minutes. All particles have a property called weak isospin (symbol T 3 ), which serves as an additive quantum number that restricts how

1305-516: A lifetime of under 10  seconds. The weak interaction has a coupling constant (an indicator of how frequently interactions occur) between 10 and 10 , compared to the electromagnetic coupling constant of about 10 and the strong interaction coupling constant of about 1; consequently the weak interaction is "weak" in terms of intensity. The weak interaction has a very short effective range (around 10 to 10  m (0.01 to 0.1 fm)). At distances around 10  meters (0.001 fm),

1392-420: A neutrino: An example of electron capture is one of the decay modes of krypton-81 into bromine-81 : All emitted neutrinos are of the same energy. In proton-rich nuclei where the energy difference between the initial and final states is less than 2 m e c , β  decay is not energetically possible, and electron capture is the sole decay mode. If the captured electron comes from

1479-471: A neutron (an up quark is changed to a down quark), and an electron neutrino is emitted. Due to the large masses of the W ;bosons, particle transformations or decays (e.g., flavour change) that depend on the weak interaction typically occur much more slowly than transformations or decays that depend only on the strong or electromagnetic forces. For example, a neutral pion decays electromagnetically, and so has

1566-407: A neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks. Electron capture is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. In electron capture, an inner atomic electron is captured by a proton in the nucleus, transforming it into a neutron, and an electron neutrino

1653-589: A number of predictions, including a prediction of the masses of the Z and W  bosons before their discovery and detection in 1983. On 4 July 2012, the CMS and the ATLAS experimental teams at the Large Hadron Collider independently announced that they had confirmed the formal discovery of a previously unknown boson of mass between 125 and 127 GeV/ c , whose behaviour so far

1740-553: A positron identical to those found in cosmic rays (discovered by Carl David Anderson in 1932). This was the first example of β  decay ( positron emission ), which they termed artificial radioactivity since 15 P is a short-lived nuclide which does not exist in nature. In recognition of their discovery, the couple were awarded the Nobel Prize in Chemistry in 1935. The theory of electron capture

1827-503: A quantum number known as the lepton number , or the number of electrons and their associated neutrinos (other leptons are the muon and tau particles). These particles have lepton number +1, while their antiparticles have lepton number −1. Since a proton or neutron has lepton number zero, β decay (a positron, or antielectron) must be accompanied with an electron neutrino, while β decay (an electron) must be accompanied by an electron antineutrino. An example of electron emission (β decay)

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1914-585: A second to periods of time significantly greater than the age of the universe . One common example of a long-lived isotope is the odd-proton odd-neutron nuclide 19 K , which undergoes all three types of beta decay ( β , β and electron capture) with a half-life of 1.277 × 10  years . B = n q − n q ¯ 3 {\displaystyle B={\frac {n_{q}-n_{\bar {q}}}{3}}} where Beta decay just changes neutron to proton or, in

2001-506: A star. This is because it can convert a proton (hydrogen) into a neutron to form deuterium which is important for the continuation of nuclear fusion to form helium. The accumulation of neutrons facilitates the buildup of heavy nuclei in a star. Most fermions decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14 . It can also create radioluminescence , commonly used in tritium luminescence , and in

2088-562: A value of +1, antileptons −1, and non-leptonic particles 0. n → p + e − + ν ¯ e L : 0 = 0 + 1 − 1 {\displaystyle {\begin{matrix}&{\text{n}}&\rightarrow &{\text{p}}&+&{\text{e}}^{-}&+&{\bar {\nu }}_{\text{e}}\\L:&0&=&0&+&1&-&1\end{matrix}}} For allowed decays,

2175-470: A weak isospin value of either ⁠+ + 1 / 2 ⁠ or ⁠− + 1 / 2 ⁠ ; all right-handed fermions have 0 isospin. For example, the up quark has T 3 = ⁠+ + 1 / 2 ⁠ and the down quark has T 3 = ⁠− + 1 / 2 ⁠ . A quark never decays through the weak interaction into a quark of the same T 3 : Quarks with a T 3 of ⁠+ + 1 / 2 ⁠ only decay into quarks with

2262-417: Is allowed in proton-rich nuclides that do not have sufficient energy to emit a positron and neutrino. If the proton and neutron are part of an atomic nucleus , the above described decay processes transmute one chemical element into another. For example: Beta decay does not change the number ( A ) of nucleons in the nucleus, but changes only its charge   Z . Thus the set of all nuclides with

2349-467: Is defined as the total energy released in a given nuclear decay. In beta decay, Q is therefore also the sum of the kinetic energies of the emitted beta particle, neutrino, and recoiling nucleus. (Because of the large mass of the nucleus compared to that of the beta particle and neutrino, the kinetic energy of the recoiling nucleus can generally be neglected.) Beta particles can therefore be emitted with any kinetic energy ranging from 0 to Q . A typical Q

2436-439: Is different from Wikidata All article disambiguation pages All disambiguation pages Weak interaction In nuclear physics and particle physics , the weak interaction , also called the weak force , is one of the four known fundamental interactions , with the others being electromagnetism , the strong interaction , and gravitation . It is the mechanism of interaction between subatomic particles that

2523-424: Is known as the parent nuclide while the resulting element (in this case 7 N ) is known as the daughter nuclide . Another example is the decay of hydrogen-3 ( tritium ) into helium-3 with a half-life of about 12.3 years: An example of positron emission (β decay) is the decay of magnesium-23 into sodium-23 with a half-life of about 11.3 s: β decay also results in nuclear transmutation, with

2610-488: Is less than the diameter of a proton. The Standard Model of particle physics provides a uniform framework for understanding electromagnetic, weak, and strong interactions. An interaction occurs when two particles (typically, but not necessarily, half-integer spin fermions ) exchange integer-spin, force-carrying bosons . The fermions involved in such exchanges can be either elementary (e.g. electrons or quarks ) or composite (e.g. protons or neutrons ), although at

2697-503: Is much more matter than antimatter in the universe, and thus forms one of Andrei Sakharov 's three conditions for baryogenesis . Beta-minus decay Beta decay is a consequence of the weak force , which is characterized by relatively long decay times. Nucleons are composed of up quarks and down quarks , and the weak force allows a quark to change its flavour by means of a virtual W boson leading to creation of an electron/antineutrino or positron/neutrino pair. For example,

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2784-408: Is nonzero. The second type is called the " neutral-current interaction " because the weakly interacting fermions form a current with total electric charge of zero. It is responsible for the (rare) deflection of neutrinos . The two types of interaction follow different selection rules . This naming convention is often misunderstood to label the electric charge of the W and Z bosons , however

2871-415: Is only one known beta-stable isobar. For even  A , there are up to three different beta-stable isobars experimentally known; for example, 50 Sn , 52 Te , and 54 Xe are all beta-stable. There are about 350 known beta-decay stable nuclides . Usually unstable nuclides are clearly either "neutron rich" or "proton rich", with the former undergoing beta decay and

2958-413: Is released. The two types of beta decay are known as beta minus and beta plus . In beta minus (β ) decay, a neutron is converted to a proton, and the process creates an electron and an electron antineutrino ; while in beta plus (β ) decay, a proton is converted to a neutron and the process creates a positron and an electron neutrino. β decay is also known as positron emission . Beta decay conserves

3045-423: Is responsible for the radioactive decay of atoms: The weak interaction participates in nuclear fission and nuclear fusion . The theory describing its behaviour and effects is sometimes called quantum flavordynamics ( QFD ); however, the term QFD is rarely used, because the weak force is better understood by electroweak theory (EWT). The effective range of the weak force is limited to subatomic distances and

3132-399: Is the decay of carbon-14 into nitrogen-14 with a half-life of about 5,730 years: In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation . This new element has an unchanged mass number A , but an atomic number Z that is increased by one. As in all nuclear decays, the decaying element (in this case 6 C )

3219-482: Is the generator of the U(1) component of the electroweak gauge group ; whereas some particles have a weak isospin of zero, all known spin- ⁠ 1 / 2 ⁠ particles have a non-zero weak hypercharge. There are two types of weak interaction (called vertices ). The first type is called the " charged-current interaction " because the weakly interacting fermions form a current with total electric charge that

3306-494: Is then close to zero, so these mostly interact with the Z  boson through the axial coupling. The Standard Model of particle physics describes the electromagnetic interaction and the weak interaction as two different aspects of a single electroweak interaction. This theory was developed around 1968 by Sheldon Glashow , Abdus Salam , and Steven Weinberg , and they were awarded the 1979 Nobel Prize in Physics for their work. The Higgs mechanism provides an explanation for

3393-596: Is typically several orders of magnitude less than that of the electromagnetic force, which itself is further orders of magnitude less than the strong nuclear force. The weak interaction is the only fundamental interaction that breaks parity symmetry , and similarly, but far more rarely, the only interaction to break charge–parity symmetry . Quarks , which make up composite particles like neutrons and protons, come in six "flavours" – up, down, charm, strange, top and bottom – which give those composite particles their properties. The weak interaction

3480-433: Is unique in that it allows quarks to swap their flavour for another. The swapping of those properties is mediated by the force carrier bosons. For example, during beta-minus decay , a down quark within a neutron is changed into an up quark, thus converting the neutron to a proton and resulting in the emission of an electron and an electron antineutrino. Weak interaction is important in the fusion of hydrogen into helium in

3567-493: The Cowan–Reines neutrino experiment . The properties of neutrinos were (with a few minor modifications) as predicted by Pauli and Fermi. In 1934, Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles to effect the nuclear reaction 2 He  +  13 Al  → 15 P  +  0 n , and observed that the product isotope 15 P emits

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3654-483: The Standard Model at lower energies, but dramatically different above symmetry breaking. The laws of nature were long thought to remain the same under mirror reflection . The results of an experiment viewed via a mirror were expected to be identical to the results of a separately constructed, mirror-reflected copy of the experimental apparatus watched through the mirror. This so-called law of parity conservation

3741-514: The mass number and atomic number of the decaying nucleus, and X and X′ are the initial and final elements, respectively. Another example is when the free neutron ( 0 n ) decays by β  decay into a proton ( p ): At the fundamental level (as depicted in the Feynman diagram on the right), this is caused by the conversion of the negatively charged ( − ⁠ 1 / 3 ⁠ e ) down quark to

3828-474: The proton-neutron model of the nucleus . Beta decay leaves the mass number unchanged, so the change of nuclear spin must be an integer. However, the electron spin is 1/2, hence angular momentum would not be conserved if beta decay were simply electron emission. From 1920 to 1927, Charles Drummond Ellis (along with Chadwick and colleagues) further established that the beta decay spectrum is continuous. In 1933, Ellis and Nevill Mott obtained strong evidence that

3915-449: The "neutrino" ('little neutral one' in Italian). In 1933, Fermi published his landmark theory for beta decay , where he applied the principles of quantum mechanics to matter particles, supposing that they can be created and annihilated, just as the light quanta in atomic transitions. Thus, according to Fermi, neutrinos are created in the beta-decay process, rather than contained in the nucleus;

4002-400: The absorption of a W . When a W boson is emitted, it decays into a positron and an electron neutrino : In all cases where β  decay (positron emission) of a nucleus is allowed energetically, so too is electron capture allowed. This is a process during which a nucleus captures one of its atomic electrons, resulting in the emission of

4089-417: The beta decay process. This spectrum was puzzling for many years. A second problem is related to the conservation of angular momentum . Molecular band spectra showed that the nuclear spin of nitrogen-14 is 1 (i.e., equal to the reduced Planck constant ) and more generally that the spin is integral for nuclei of even mass number and half-integral for nuclei of odd mass number. This was later explained by

4176-485: The beta particle is in fact an electron. In 1901, Rutherford and Frederick Soddy showed that alpha and beta radioactivity involves the transmutation of atoms into atoms of other chemical elements. In 1913, after the products of more radioactive decays were known, Soddy and Kazimierz Fajans independently proposed their radioactive displacement law , which states that beta (i.e., β ) emission from one element produces another element one place to

4263-507: The beta spectrum has an effective upper bound in energy. Niels Bohr had suggested that the beta spectrum could be explained if conservation of energy was true only in a statistical sense, thus this principle might be violated in any given decay. However, the upper bound in beta energies determined by Ellis and Mott ruled out that notion. Now, the problem of how to account for the variability of energy in known beta decay products, as well as for conservation of momentum and angular momentum in

4350-1107: The case of positive beta decay ( electron capture ) proton to neutron so the number of individual quarks doesn't change. It is only the baryon flavor that changes, here labelled as the isospin . Up and down quarks have total isospin I = 1 2 {\textstyle I={\frac {1}{2}}} and isospin projections I z = { 1 2 up quark − 1 2 down quark {\displaystyle I_{\text{z}}={\begin{cases}{\frac {1}{2}}&{\text{up quark}}\\-{\frac {1}{2}}&{\text{down quark}}\end{cases}}} All other quarks have I = 0 . In general I z = 1 2 ( n u − n d ) {\displaystyle I_{\text{z}}={\frac {1}{2}}(n_{\text{u}}-n_{\text{d}})} L ≡ n ℓ − n ℓ ¯ {\displaystyle L\equiv n_{\ell }-n_{\bar {\ell }}} so all leptons have assigned

4437-403: The decay of a proton inside the nucleus to a neutron: However, β  decay cannot occur in an isolated proton because it requires energy, due to the mass of the neutron being greater than the mass of the proton. β  decay can only happen inside nuclei when the daughter nucleus has a greater binding energy (and therefore a lower total energy) than

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4524-441: The deepest levels, all weak interactions ultimately are between elementary particles . In the weak interaction, fermions can exchange three types of force carriers, namely W , W , and Z  bosons . The masses of these bosons are far greater than the mass of a proton or neutron, which is consistent with the short range of the weak force. In fact, the force is termed weak because its field strength over any set distance

4611-503: The innermost shell of the atom, the K-shell , which has the highest probability to interact with the nucleus, the process is called K-capture. If it comes from the L-shell, the process is called L-capture, etc. Electron capture is a competing (simultaneous) decay process for all nuclei that can undergo β decay. The converse, however, is not true: electron capture is the only type of decay that

4698-434: The kinetic energy distribution, or spectrum, of beta particles measured by Lise Meitner and Otto Hahn in 1911 and by Jean Danysz in 1913 showed multiple lines on a diffuse background. These measurements offered the first hint that beta particles have a continuous spectrum. In 1914, James Chadwick used a magnetic spectrometer with one of Hans Geiger's new counters to make more accurate measurements which showed that

4785-510: The latter undergoing electron capture (or more rarely, due to the higher energy requirements, positron decay). However, in a few cases of odd-proton, odd-neutron radionuclides, it may be energetically favorable for the radionuclide to decay to an even-proton, even-neutron isobar either by undergoing beta-positive or beta-negative decay. An often-cited example is the single isotope 29 Cu (29 protons, 35 neutrons), which illustrates three types of beta decay in competition. Copper-64 has

4872-521: The momentum difference (called " running ") between the particles involved. Hence since by convention ⁠ sgn ⁡ T 3 ≡ sgn ⁡ Q {\displaystyle \operatorname {sgn} T_{3}\equiv \operatorname {sgn} Q} ⁠ , and for all fermions involved in the weak interaction ⁠ T 3 = ± 1 2 {\displaystyle T_{3}=\pm {\tfrac {1}{2}}} ⁠ . The weak charge of charged leptons

4959-402: The mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron, and a neutrino and into the kinetic energy of these particles. This process is opposite to negative beta decay, in that the weak interaction converts a proton into a neutron by converting an up quark into a down quark resulting in the emission of a W or

5046-420: The naming convention predates the concept of the mediator bosons, and clearly (at least in name) labels the charge of the current (formed from the fermions), not necessarily the bosons. In one type of charged current interaction, a charged lepton (such as an electron or a muon , having a charge of −1) can absorb a W  boson (a particle with a charge of +1) and be thereby converted into

5133-442: The net orbital angular momentum is zero, hence only spin quantum numbers are considered. The electron and antineutrino are fermions , spin-1/2 objects, therefore they may couple to total S = 1 {\displaystyle S=1} (parallel) or S = 0 {\displaystyle S=0} (anti-parallel). For forbidden decays, orbital angular momentum must also be taken into consideration. The Q value

5220-436: The neutron into a proton. Because of the limited energy involved in the process (i.e., the mass difference between the down quark and the up quark), the virtual W boson can only carry sufficient energy to produce an electron and an electron-antineutrino – the two lowest-possible masses among its prospective decay products. At the quark level, the process can be represented as: In neutral current interactions,

5307-460: The new elements polonium and radium . In 1899, Ernest Rutherford separated radioactive emissions into two types: alpha and beta (now beta minus), based on penetration of objects and ability to cause ionization. Alpha rays could be stopped by thin sheets of paper or aluminium, whereas beta rays could penetrate several millimetres of aluminium. In 1900, Paul Villard identified a still more penetrating type of radiation, which Rutherford identified as

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5394-415: The particle can interact with the W of the weak force. Weak isospin plays the same role in the weak interaction with W as electric charge does in electromagnetism , and color charge in the strong interaction ; a different number with a similar name, weak charge , discussed below , is used for interactions with the Z . All left-handed fermions have

5481-609: The positively charged ( + ⁠ 2 / 3 ⁠ e ) up quark promoteby by a virtual W boson ; the W boson subsequently decays into an electron and an electron antineutrino: In β  decay, or positron emission, the weak interaction converts an atomic nucleus into a nucleus with atomic number decreased by one, while emitting a positron ( e ) and an electron neutrino ( ν e ). β  decay generally occurs in proton-rich nuclei. The generic equation is: This may be considered as

5568-484: The presence of three massive gauge bosons ( W , W , Z , the three carriers of the weak interaction), and the photon ( γ , the massless gauge boson that carries the electromagnetic interaction). According to the electroweak theory, at very high energies, the universe has four components of the Higgs field whose interactions are carried by four massless scalar bosons forming

5655-413: The process known as beta decay , a down quark in the neutron can change into an up quark by emitting a virtual W  boson, which then decays into an electron and an electron antineutrino . Another example is electron capture  – a common variant of radioactive decay  – wherein a proton and an electron within an atom interact and are changed to

5742-531: The process, became acute. In a famous letter written in 1930, Wolfgang Pauli attempted to resolve the beta-particle energy conundrum by suggesting that, in addition to electrons and protons, atomic nuclei also contained an extremely light neutral particle, which he called the neutron. He suggested that this "neutron" was also emitted during beta decay (thus accounting for the known missing energy, momentum, and angular momentum), but it had simply not yet been observed. In 1931, Enrico Fermi renamed Pauli's "neutron"

5829-428: The related field of betavoltaics (but not similar radium luminescence ). The electroweak force is believed to have separated into the electromagnetic and weak forces during the quark epoch of the early universe . In 1933, Enrico Fermi proposed the first theory of the weak interaction, known as Fermi's interaction . He suggested that beta decay could be explained by a four- fermion interaction, involving

5916-409: The resulting element having an atomic number that is decreased by one. The beta spectrum, or distribution of energy values for the beta particles, is continuous. The total energy of the decay process is divided between the electron, the antineutrino, and the recoiling nuclide. In the figure to the right, an example of an electron with 0.40 MeV energy from the beta decay of Bi is shown. In this example,

6003-417: The right in the periodic table , while alpha emission produces an element two places to the left. The study of beta decay provided the first physical evidence for the existence of the neutrino . In both alpha and gamma decay, the resulting alpha or gamma particle has a narrow energy distribution , since the particle carries the energy from the difference between the initial and final nuclear states. However,

6090-468: The same happens to electrons. The neutrino interaction with matter was so weak that detecting it proved a severe experimental challenge. Further indirect evidence of the existence of the neutrino was obtained by observing the recoil of nuclei that emitted such a particle after absorbing an electron. Neutrinos were finally detected directly in 1956 by the American physicists Clyde Cowan and Frederick Reines in

6177-460: The same  A can be introduced; these isobaric nuclides may turn into each other via beta decay. For a given A there is one that is most stable. It is said to be beta stable, because it presents a local minimum of the mass excess : if such a nucleus has ( A , Z ) numbers, the neighbour nuclei ( A , Z −1) and ( A , Z +1) have higher mass excess and can beta decay into ( A , Z ) , but not vice versa. For all odd mass numbers A , there

6264-400: The spectrum was continuous. The distribution of beta particle energies was in apparent contradiction to the law of conservation of energy . If beta decay were simply electron emission as assumed at the time, then the energy of the emitted electron should have a particular, well-defined value. For beta decay, however, the observed broad distribution of energies suggested that energy is lost in

6351-419: The standard model to deflect: Either particles or anti-particles, with any electric charge, and both left- and right-chirality, although the strength of the interaction differs. The quantum number weak charge ( Q W ) serves the same role in the neutral current interaction with the Z that electric charge ( Q , with no subscript) does in the electromagnetic interaction : It quantifies

6438-409: The three weak bosons, which then acquire mass through the Higgs mechanism . These three composite bosons are the W , W , and Z  bosons actually observed in the weak interaction. The fourth electroweak gauge boson is the photon ( γ ) of electromagnetism, which does not couple to any of the Higgs fields and so remains massless. This theory has made

6525-422: The total decay energy is 1.16 MeV, so the antineutrino has the remaining energy: 1.16 MeV − 0.40 MeV = 0.76 MeV . An electron at the far right of the curve would have the maximum possible kinetic energy, leaving the energy of the neutrino to be only its small rest mass. Radioactivity was discovered in 1896 by Henri Becquerel in uranium , and subsequently observed by Marie and Pierre Curie in thorium and in

6612-550: The vector part of the interaction. Its value is given by: Since the weak mixing angle ⁠ θ w ≈ 29 ∘ {\displaystyle \theta _{\mathsf {w}}\approx 29^{\circ }} ⁠ , the parenthetic expression ⁠ ( 1 − 4 sin 2 ⁡ θ w ) ≈ 0.060 {\displaystyle (1-4\,\sin ^{2}\theta _{\mathsf {w}})\approx 0.060} ⁠ , with its value varying slightly with

6699-617: The weak force. In recognition of their theoretical work, Lee and Yang were awarded the Nobel Prize for Physics in 1957. However Wu, who was female, was not awarded the Nobel prize. In β  decay, the weak interaction converts an atomic nucleus into a nucleus with atomic number increased by one, while emitting an electron ( e ) and an electron antineutrino ( ν e ). β  decay generally occurs in neutron-rich nuclei. The generic equation is: where A and Z are

6786-448: The weak force. They sketched the design for an experiment for testing conservation of parity in the laboratory. Later that year, Chien-Shiung Wu and coworkers conducted the Wu experiment showing an asymmetrical beta decay of Co at cold temperatures that proved that parity is not conserved in beta decay. This surprising result overturned long-held assumptions about parity and

6873-465: The weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity. The V − A theory was developed before the discovery of the Z ;boson, so it did not include the right-handed fields that enter in the neutral current interaction. However, this theory allowed

6960-420: The weak interaction by showing them to be two aspects of a single force, now termed the electroweak force. The existence of the W and Z  bosons was not directly confirmed until 1983. The electrically charged weak interaction is unique in a number of respects: Due to their large mass (approximately 90 GeV/ c ) these carrier particles, called the W and Z  bosons, are short-lived with

7047-460: The weak interaction has an intensity of a similar magnitude to the electromagnetic force, but this starts to decrease exponentially with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3 × 10  m, the weak interaction becomes 10,000 times weaker. The weak interaction affects all the fermions of the Standard Model , as well as the Higgs boson ; neutrinos interact only through gravity and

7134-401: The weak interaction required more than two generations of particles, effectively predicting the existence of a then unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics. Unlike parity violation, CP  violation occurs only in rare circumstances. Despite its limited occurrence under present conditions, it is widely believed to be the reason that there

7221-404: The weak interaction was once described by Fermi's theory , the discovery of parity violation and renormalization theory suggested that a new approach was needed. In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V − A ( vector minus axial vector or left-handed) Lagrangian for weak interactions. In this theory,

7308-469: The weak interaction. The weak interaction does not produce bound states , nor does it involve binding energy  – something that gravity does on an astronomical scale , the electromagnetic force does at the molecular and atomic levels, and the strong nuclear force does only at the subatomic level, inside of nuclei . Its most noticeable effect is due to its first unique feature: The charged weak interaction causes flavour change . For example,

7395-522: Was "consistent with" a Higgs boson, while adding a cautious note that further data and analysis were needed before positively identifying the new boson as being a Higgs boson of some type. By 14 March 2013, a Higgs boson was tentatively confirmed to exist. In a speculative case where the electroweak symmetry breaking scale were lowered, the unbroken SU(2) interaction would eventually become confining . Alternative models where SU(2) becomes confining above that scale appear quantitatively similar to

7482-509: Was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed in 1937 by Luis Alvarez , in the nuclide V. Alvarez went on to study electron capture in Ga and other nuclides. In 1956, Tsung-Dao Lee and Chen Ning Yang noticed that there was no evidence that parity was conserved in weak interactions, and so they postulated that this symmetry may not be preserved by

7569-430: Was known to be respected by classical gravitation , electromagnetism and the strong interaction ; it was assumed to be a universal law. However, in the mid-1950s Chen-Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law. Chien Shiung Wu and collaborators in 1957 discovered that the weak interaction violates parity, earning Yang and Lee the 1957 Nobel Prize in Physics . Although

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