TheStandard Model ofparticle physics provides a uniform framework for understanding electromagnetic, weak, and strong interactions. An interaction occurs when two particles (typically, but not necessarily,half-integerspinfermions) exchange integer-spin, force-carryingbosons. The fermions involved in such exchanges can be either elementary (e.g.electrons orquarks) or composite (e.g.protons orneutrons), although at the deepest levels, all weak interactions ultimately are betweenelementary particles.
In the weak interaction, fermions can exchange three types of force carriers, namelyW+,W−, andZ bosons. Themasses 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.[3] In fact, the force is termedweak because itsfield strength over any set distance 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 breaksparity symmetry, and similarly, but far more rarely, the only interaction to breakcharge–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 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, duringbeta-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 thefusion of hydrogen into helium in 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.[3]
In 1933,Enrico Fermi proposed the first theory of the weak interaction, known asFermi's interaction. He suggested thatbeta decay could be explained by a four-fermion interaction, involving a contact force with no range.[5][6]
In the 1960s,Sheldon Glashow,Abdus Salam andSteven Weinberg unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force, now termed the electroweak force.[8][9]
A diagram depicting the decay routes for the sixquarks due to the charged weak interaction and some indication of their likelihood. The intensity of the lines is given by theCKM parameters.
The electrically charged weak interaction is unique in a number of respects:
It is the only interaction that can change theflavour of quarks and leptons (i.e., of changing one type of quark into another).[a]
Both the electrically charged and the electrically neutral interactions are mediated (propagated) byforce carrier particles that have significant masses, an unusual feature which is explained in theStandard Model by theHiggs mechanism.
Due to their large mass (approximately 90 GeV/c2[11]) these carrier particles, called theW andZ bosons, are short-lived with a lifetime of under 10−24 seconds.[12] The weak interaction has acoupling constant (an indicator of how frequently interactions occur) between 10−7 and 10−6, compared to theelectromagnetic coupling constant of about 10−2 and thestrong interaction coupling constant of about 1;[13] consequently the weak interaction is "weak" in terms of intensity.[14] The weak interaction has a very short effective range (around 10−17 to 10−16 m (0.01 to 0.1 fm)).[b][14][13] At distances around 10−18 meters (0.001 fm), the weak interaction has an intensity of a similar magnitude to the electromagnetic force, but this starts to decreaseexponentially with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3×10−17 m, the weak interaction becomes 10,000 times weaker.[15]
The weak interaction affects all thefermions of theStandard Model, as well as theHiggs boson;neutrinos interact only through gravity and the weak interaction. The weak interaction does not producebound states, nor does it involvebinding energy – something that gravity does on anastronomical scale, the electromagnetic force does at the molecular and atomic levels, and the strong nuclear force does only at the subatomic level, inside ofnuclei.[16]
Its most noticeable effect is due to its first unique feature: The charged weak interaction causesflavour change. For example, aneutron is heavier than aproton (its partnernucleon) and can decay into a proton by changing theflavour (type) of one of its twodown quarks to anup quark. Neither thestrong interaction norelectromagnetism permit flavour changing, so this can only proceed byweak 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.
Allmesons are unstable because of weak decay.[10](p29)[c]In the process known asbeta decay, adown quark in theneutron can change into anup quark by emitting avirtual W− boson, which then decays into anelectron and an electronantineutrino.[10](p28) Another example iselectron capture – a common variant ofradioactive decay – wherein a proton and an electron within an atom interact and are changed to 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.[d]For example, a neutralpion decays electromagnetically, and so has a life of only about 10−16 seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about 10−8 seconds, or a hundred million times longer than a neutral pion.[10](p30) A particularly extreme example is the weak-force decay of a free neutron, which takes about 15 minutes.[10](p28)
All of the above left-handed (regular) particles have corresponding right-handedanti-particles with equal and opposite weak isospin.
All right-handed (regular) particles and left-handed antiparticles have weak isospin of 0.
All particles have a property calledweak isospin (symbolT3), which serves as anadditive quantum number that restricts how the particle can interact with the W± of the weak force. Weak isospin plays the same role in the weak interaction with W± aselectric charge does inelectromagnetism, andcolor charge in thestrong interaction; a different number with a similar name,weak charge,discussed below, is used for interactions with the Z0 . All left-handedfermions have a weak isospin value of either++1/2 or−+1/2; all right-handed fermions have 0 isospin. For example, the up quark hasT3 =++1/2 and the down quark hasT3 =−+1/2. A quark never decays through the weak interaction into a quark of the sameT3: Quarks with aT3 of++1/2 only decay into quarks with aT3 of−+1/2 and conversely.
π+ decay through the weak interaction
In any given strong, electromagnetic, or weak interaction, weak isospin isconserved:[e] 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 a ν μ (withT3 =++1/2) and a μ+ (as a right-handed antiparticle,++1/2).[10](p30)
For the development of the electroweak theory, another property,weak hypercharge, was invented, defined as
whereYW is the weak hypercharge of a particle with electrical chargeQ (inelementary charge units) and weak isospinT3.Weak hypercharge is the generator of the U(1) component of the electroweakgauge group; whereas some particles have aweak isospin of zero, all knownspin-1/2 particles have a non-zero weak hypercharge.[f]
There are two types of weak interaction (calledvertices). The first type is called the "charged-current interaction" because theweakly interacting fermions form acurrent with totalelectric charge that is nonzero. The second type is called the "neutral-current interaction" because theweakly interacting fermions form acurrent with totalelectric charge of zero. It is responsible for the (rare) deflection ofneutrinos. The two types of interaction follow differentselection rules. This naming convention is often misunderstood to label the electric charge of theW andZ bosons, however 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.[g]
In one type of charged current interaction, a chargedlepton (such as anelectron or amuon, having a charge of −1) can absorb a W+ boson (a particle with a charge of +1) and be thereby converted into a correspondingneutrino (with a charge of 0), where the type ("flavour") of neutrino (electronνe, muonνμ, or tauντ) is the same as the type of lepton in the interaction, for example:
Similarly, a down-typequark (d,s, orb, with a charge of−+ 1 /3) can be converted into an up-type quark (u,c, ort, with a charge of++ 2 /3), by emitting a W− boson or by absorbing a W+ boson. More precisely, the down-type quark becomes aquantum 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 theCKM matrix tables. Conversely, an up-type quark can emit a W+ boson, or absorb a W− boson, and thereby be converted into 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.[18]
In the so-calledbeta decay of a neutron (see picture, above), a down quark within the neutron emits avirtual W− boson and is thereby converted into an up quark, converting 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.[19]At the quark level, the process can be represented as:
Like the W± bosons, the Z0 boson also decays rapidly,[18] for example:
Unlike the charged-current interaction, whose selection rules are strictly limited by chirality, electric charge,and / or weak isospin, the neutral-current Z0 interaction can cause any two fermions in 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.[h]
The quantum numberweak charge (QW) serves the same role in the neutral current interaction with the Z0 that electric charge (Q, with no subscript) does in theelectromagnetic interaction: It quantifies the vector part of the interaction. Its value is given by:[21]
since by convention, and for all fermions involved in the weak interaction. The weak charge of charged leptons is then close to zero, so these mostly interact with theZ boson through the axial coupling.
According to the electroweak theory, at very high energies, the universe has four components of theHiggs field whose interactions are carried by four massless scalarbosons forming a complex scalar Higgs field doublet. Likewise, there are four massless electroweak vector bosons, each similar to thephoton. However, at low energies, this gauge symmetry isspontaneously broken down to theU(1) symmetry of electromagnetism, since one of the Higgs fields acquires avacuum expectation value. Naïvely, the symmetry-breaking would be expected to produce three masslessbosons, but instead those "extra" three Higgs bosons become incorporated into the three weak bosons, which then acquire mass through theHiggs mechanism. These three composite bosons are the W+ , W− , and Z0 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.[23]
This theory has made 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 theLarge Hadron Collider independently announced that they had confirmed the formal discovery of a previously unknown boson of mass between 125 and127 GeV/c2, whose behaviour so far 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.[24]
In a speculative case where theelectroweak symmetry breakingscale were lowered, the unbrokenSU(2) interaction would eventually becomeconfining. Alternative models whereSU(2) becomes confining above that scale appear quantitatively similar to theStandard Model at lower energies, but dramatically different above symmetry breaking.[25]
Thelaws of nature were long thought to remain the same under mirrorreflection. 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 ofparityconservation was known to be respected by classicalgravitation,electromagnetism and thestrong interaction; it was assumed to be a universal law.[26] However, in the mid-1950sChen-Ning Yang andTsung-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 the1957 Nobel Prize in Physics.[27]
Although the weak interaction was once described byFermi's theory, the discovery of parity violation andrenormalization theory suggested that a new approach was needed. In 1957,Robert Marshak andGeorge Sudarshan and, somewhat later,Richard Feynman andMurray Gell-Mann proposed aV − A (vector minusaxial vector or left-handed)Lagrangian for weak interactions. In this theory, 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. TheV − 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 a compound symmetryCP to be conserved.CP combines parityP (switching left to right) with charge conjugationC (switching particles with antiparticles). Physicists were again surprised when in 1964,James Cronin andVal Fitch provided clear evidence inkaon decays thatCP symmetry could be broken too, winning them the 1980Nobel Prize in Physics.[28] In 1973,Makoto Kobayashi andToshihide Maskawa showed thatCP violation in the weak interaction required more than two generations of particles,[29] effectively predicting the existence of a then unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics.[30]
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 is much more matter thanantimatter in the universe, and thus forms one ofAndrei Sakharov's three conditions forbaryogenesis.[31]
^Because of its unique ability to change particleflavour, analysis of the weak interaction is occasionally calledquantum flavour dynamics, in analogy to the namequantum chromodynamics sometimes used for thestrong force.
^The neutral pion ( π0 ), however, decays electromagnetically, and several othermesons (when their quantum numbers permit) mostly decay via astrong interaction.
^The prominent and possibly unique exception to this rule is the decay of thetop quark, whose mass exceeds the combined masses of thebottom quark and W+ boson that it decays into, hence it has a no energy constraint slowing its transition. Its unique speed of decay by the weak force is much higher than the speed with which thestrong interaction (or "color force") can bind it to other quarks.
^Only interactions with theHiggs boson violate conservation of weak isospin, and appear to always do so maximally:
^Some hypothesised fermions, such as thesterile neutrinos, would have zero weak hypercharge – in fact, nogauge charges of any known kind. Whether any such particles actually exist is an active area of research.
^The exchange of a virtualW boson can be equally well thought of as (say) the emission of aW+ or the absorption of aW−; that is, for time on the vertical co‑ordinate axis, as aW+ from left to right, or equivalently as aW− from right to left.
^The only fermions which the Z0 doesnot interact with at all are the hypothetical"sterile" neutrinos: Left-chiral anti-neutrinos and right-chiral neutrinos. They are called "sterile" because they would not interact with any Standard Model particle, except perhaps theHiggs boson. So far they remain entirely a conjecture: As of October 2021, no such neutrinos are known to actually exist.
"MicroBooNE has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques", says co-spokespersonBonnie Fleming of Yale. "They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino."[20]
... "eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model", says theorist Mikhail Shaposhnikov of EPFL. "But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment."[20]
^Langacker, Paul (2001) [1989]. "CP violation and cosmology". In Jarlskog, Cecilia (ed.).CP Violation. London,River Edge: World Scientific Publishing Co. p. 552.ISBN9789971505615 – via Google Books.
Coughlan, G. D.; Dodd, J. E.; Gripaios, B. M. (2006).The Ideas of Particle Physics: An introduction for scientists (3rd ed.). Cambridge University Press.ISBN978-0-521-67775-2.
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