Weak interaction
In nuclear physics and particle physics, the weak interaction, weak force or weak nuclear 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 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 ; however, the term QFD is rarely used, because the weak force is better understood by electroweak theory.
The effective range of the weak force is limited to subatomic distances and is less than the diameter of a proton.
Background
The Standard Model of particle physics provides a uniform framework for understanding electromagnetic, weak, and strong interactions. An interaction occurs when two particles exchange integer-spin, force-carrying bosons. The fermions involved in such exchanges can be either elementary or composite, although at the deepest levels, all weak interactions ultimately are between elementary particles.In the weak interaction, fermions can exchange three types of force carriers, namely,, and 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 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 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 a star. This is because it can convert a proton into a neutron that can fuse with another proton 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 the related field of betavoltaics.
The electroweak force is believed to have separated into the electromagnetic and weak forces during the quark epoch of the early universe.
History
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 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, the Wu experiment, carried out by Chien Shiung Wu and collaborators confirmed the symmetry violation.
In the 1960s, Sheldon Glashow, Abdus Salam and Steven 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.
The existence of the and bosons was not directly confirmed until 1983.
Properties
The electrically charged weak interaction is unique in a number of respects:- It is the only interaction that can change the flavour of quarks and leptons.
- It is the only interaction that violates P, or parity symmetry. It is also the only one that violates charge–parity symmetry.
- Both the electrically charged and the electrically neutral interactions are mediated by force carrier particles that have significant masses, an unusual feature which is explained in the Standard Model by the Higgs mechanism.
- Decay processes like beta decay governed by the weak interaction can only be observed when processes involving faster decays via electromagnetic or strong interaction are not competing.
The weak interaction affects all the fermions of the Standard Model, as well as the Higgs boson; neutrinos interact only through gravity and 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, a neutron is heavier than a proton and can decay into a proton by changing the flavour 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 would also be conserved across all interactions.
All mesons are unstable because of weak decay.
In the process known as beta decay, a down quark in the neutron can change into an up quark by emitting a virtual 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 a neutron, and an electron neutrino is emitted.
Due to the large masses of the W bosons, particle transformations or decays 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 a life of only about seconds. In contrast, a charged pion can only decay through the weak interaction, and so lives about 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.
Weak isospin and weak hypercharge
All particles have a property called weak isospin, which serves as an additive quantum number that restricts how the particle can interact with the of the weak force. Weak isospin plays the same role in the weak interaction with 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. All left-handed fermions have a weak isospin value of either or ; all right-handed fermions have 0 isospin. For example, the up quark has and the down quark has. A quark never decays through the weak interaction into a quark of the same : Quarks with a of only decay into quarks with a of 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 with a weak isospin of +1 normally decays into a and a .
For the development of the electroweak theory, another property, weak hypercharge, was invented, defined as
where is the weak hypercharge of a particle with electrical charge and weak isospin. Weak hypercharge is the generator of the U component of the electroweak gauge group; whereas some particles have a weak isospin of zero, all known spin- particles have a non-zero weak hypercharge.
Interaction types
There are two types of weak interaction. The first type is called the "charged-current interaction" because the weakly interacting fermions form a current with total electric charge that 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 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 and bosons, however the naming convention predates the concept of the mediator bosons, and clearly labels the charge of the current, not necessarily the bosons.Charged-current interaction
In one type of charged current interaction, a charged lepton can absorb a boson and be thereby converted into a corresponding neutrino, where the type of neutrino is the same as the type of lepton in the interaction, for example:Similarly, a down-type quark can be converted into an up-type quark, by emitting a boson or by absorbing a 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 boson, or absorb a 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.
In the so-called beta decay of a neutron, a down quark within the neutron emits a virtual boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the limited energy involved in the process, the virtual 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: