Weak neutral current interactions are one of the ways in which
subatomic particles can interact by means of the
weak force
Weak may refer to:
Songs
* Weak (AJR song), "Weak" (AJR song), 2016
* Weak (Melanie C song), "Weak" (Melanie C song), 2011
* Weak (SWV song), "Weak" (SWV song), 1993
* Weak (Skunk Anansie song), "Weak" (Skunk Anansie song), 1995
* "Weak", a song ...
. These interactions are mediated by the
Z boson. The discovery of weak neutral currents was a significant step toward the unification of
electromagnetism
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
and the weak force into the
electroweak force
In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
, and led to the discovery of the
W and Z bosons
In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , an ...
.
In simple terms
The weak force is best known for its role in nuclear decay. It has very short range but (apart from gravity) is the only force to interact with
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s. Like other subatomic forces, the weak force is mediated via exchange particles. Perhaps the most well known of the exchange particles for the weak force is the
W particle which is involved in
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
. W particles have
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
– there are both positive and negative W particles – however the Z boson is also an exchange particle for the weak force but does ''not'' have any electrical charge.
Exchange of a Z boson transfers
momentum,
spin, and
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
, but leaves the interacting particles' quantum numbers unaffected – charge,
flavor,
baryon number
In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as
::B = \frac\left(n_\text - n_\bar\right),
where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Baryo ...
,
lepton number
In particle physics, lepton number (historically also called lepton charge)
is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction.
Lepton number ...
, etc. Because there is no transfer of electrical charge involved, exchange of Z particles is referred to as "neutral" in the phrase "neutral current". However the word "current" here has nothing to do with electricity – it simply refers to the exchange of the Z particle.
The Z boson's neutral current interaction is determined by a derived quantum number called ''
weak charge'', which acts similarly to
weak isospin
In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol or , with the third component written as or . It c ...
for interactions with the W bosons.
Definition
The neutral current that gives the interaction its name is that of the interacting particles.
For example, the neutral current contribution to the → elastic
scattering amplitude
In quantum physics, the scattering amplitude is the probability amplitude of the outgoing spherical wave relative to the incoming plane wave in a stationary-state scattering process.neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
and of the electron are given by:
:
where:
[
:
and are the vector and axial vector couplings for fermion . denotes the ]weak isospin
In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol or , with the third component written as or . It c ...
of the fermions, their electric charge and their weak charge. These couplings amount to essentially left chiral for neutrinos and axial for charged leptons.
The Z boson can couple to any Standard Model particle, except gluons and photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s. However, any interaction between two charged particles that can occur via the exchange of a virtual Z boson can also occur via the exchange of a virtual photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
. Unless the interacting particles have energies on the order of the Z boson mass (91 GeV) or higher, the virtual Z boson exchange has an effect of a tiny correction () to the amplitude of the electromagnetic process.
Particle accelerators with energies necessary to observe neutral current interactions and to measure the mass of Z boson weren't available until 1983.
On the other hand, Z boson interactions involving neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s have distinctive signatures: They provide the only known mechanism for elastic scattering
Elastic scattering is a form of particle scattering in scattering theory, nuclear physics and particle physics. In this process, the kinetic energy of a particle is conserved in the center-of-mass frame, but its direction of propagation is modif ...
of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via Z boson exchange) as inelastically (via W boson exchange), of major experimental significance, in, e.g. , the Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory (SNO) was a neutrino observatory located 2100 m underground in Vale's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large ...
experiment.
Weak neutral currents were predicted by electroweak theory
In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
developed mainly by Abdus Salam
Mohammad Abdus Salam Salam adopted the forename "Mohammad" in 1974 in response to the anti-Ahmadiyya decrees in Pakistan, similarly he grew his beard. (; ; 29 January 192621 November 1996) was a Punjabi Pakistani theoretical physicist and a ...
, John Clive Ward
John Clive Ward, (1 August 1924 – 6 May 2000) was a British-Australian physicist. He introduced the Ward–Takahashi identity, also known as "Ward Identity" (or "Ward's Identities"). Andrei Sakharov said Ward was one of the titans of q ...
, Sheldon Glashow
Sheldon Lee Glashow (, ; born December 5, 1932) is a Nobel Prize-winning American theoretical physicist. He is the Metcalf Professor of Mathematics and Physics at Boston University and Eugene Higgins Professor of Physics, Emeritus, at Harvard U ...
and Steven Weinberg
Steven Weinberg (; May 3, 1933 – July 23, 2021) was an American theoretical physicist and Nobel laureate in physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interac ...
, and confirmed shortly thereafter in 1973, in a neutrino experiment in the Gargamelle
Gargamelle was a heavy liquid bubble chamber detector in operation at CERN between 1970 and 1979. It was designed to detect neutrinos and antineutrinos, which were produced with a beam from the Proton Synchrotron (PS) between 1970 and 1976, ...
bubble chamber
A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the 1 ...
at CERN.
See also
* Charged current
Charged current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the and bosons.
In simple terms
Charged current interactions are the most easily det ...
* Electric current
* Flavor changing neutral current
In particle physics, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical interactions that change the flavor of a fermion without altering its electric charge.
Details
If they occur in nature (as refl ...
* Quantum chromodynamics
* Sudbury Neutrino Observatory#Neutral current interaction
* Weak charge
References
External links
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{{DEFAULTSORT:Neutral Current
Electroweak theory