Weak interaction
The
weak interaction (sometimes called the
weak nuclear force) is one of the four
fundamental interactions of nature. In the
Standard Model of
particle physics, it is due to the exchange of the heavy
W and Z bosons. Its most familiar effect is
beta decay (of
neutrons in
atomic nuclei) and the associated
radioactivity. The predicate "weak" derives from the fact that the field strength is some 10
13 times less than that of the
strong nuclear force.
The weak interaction affects all
left-handed leptons and
quarks. It is the only force affecting
neutrinos (except for
gravitation, which is negligible on laboratory scales). The weak interaction is unique in a number of respects:
# It is the only interaction capable of changing
flavour.# It is the only interaction which violates
parity symmetry
P (because it only acts on left-handed particles). It is also the only one which violates
CP.# It is mediated by heavy
gauge bosons. This unusual feature is explained in the
Standard Model by the
Higgs mechanism.
Due to the large mass of the weak interaction's carrier particles (about 90 GeV/c
2), their
mean life is limited to about 3×10
âˆ'25 seconds by the
uncertainty principle. Even at the
speed of light this effectively limits the range of the weak interaction to 10
âˆ'18 meters, about 1000 times smaller than the diameter of an
atomic nucleus.
Since the weak interaction is both very weak and very short range, its most noticeable effect is due to its other unique feature:
flavour changing. Consider a
neutron (
quark content
udd; one
up quark, two
down quarks). Although the neutron is heavier than its sister
nucleon, the
proton (quark content
uud), it cannot decay into a proton without changing the
flavour of one of its down quarks. Neither the
strong interaction nor
electromagnetism allow flavour changing, so this must proceed by
weak decay. In this process, a down quark in the neutron changes into an up quark by emitting a
W boson, which then breaks up into a high-energy
electron and an electron anti
neutrino. Since high-energy electrons are
beta radiation, this is called a
beta decay.
Due to the weakness of the weak interaction, weak decays are much slower than strong or electromagnetic decays. For example, an electromagnetically decaying neutral
pion has a life of about 10
âˆ'16 seconds; a weakly decaying charged pion lives about 10
âˆ'8 seconds, a hundred million times longer. A free neutron lives for over 11 minutes, making it the unstable
subatomic particle with the longest known
mean life.
Violation of symmetry
The
laws of nature were long thought to remain the same under mirror
reflection, the reversal of all
spatial axes. The results of an experiment viewed via a mirror were expected to be identical to the results of a mirror-reflected copy of the experimental apparatus. This so-called law of
parity conservation was known to be respected by classical
gravitation and
electromagnetism; it was assumed to be a universal law. However, in the mid-1950's
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 in fact maximally violates parity, earning Yang and Lee the
1957 Nobel Prize in Physics.
Although the weak interaction used to be described by
Fermi's theory of a contact four-
fermion interaction, the discovery of parity violation and
renormalization theory suggested a new approach was needed. In 1957, Robert Marshak and
George Sudarshan and, somewhat later,
Richard Feynman and
Murray Gell-Mann proposed a
Vâˆ'A (
vector minus
axial 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.
However, this theory allowed 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. Unlike parity violation, CP violation is a very small effect.
The
Standard Model of particle physics describes the
electromagnetic interaction and the weak interaction as two different aspects of a single
electroweak interaction, the theory of which was developed around 1968 by
Sheldon Glashow,
Abdus Salam and
Steven Weinberg (see
W and Z bosons). They were awarded the
1979 Nobel Prize in Physics for their work.
According to the electroweak theory, at very high energies, the universe has four identical massless
gauge bosons similar to the
photon and a scalar
Higgs field. However, at low energies the symmetry of the Higgs field is
spontaneously broken by the
Higgs mechanism. This symmetry breaking produces three massless
Goldstone bosons which are "eaten" by three of the photon-like fields, giving them mass. These three fields become the
W and Z bosons of the weak interaction, while the fourth field remains massless and is the photon of electromagnetism.
Although this theory has made a number of impressive predictions, including a prediction of the mass of the Z boson before its discovery, the
Higgs boson itself has never been observed. Producing Higgs bosons will be a major goal of the
Large Hadron Collider being built at
CERN.
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Formulation of the standard model*
Citation for 1957 Nobel Prize
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Citation for 1979 Nobel Prize
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Citation for 1980 Nobel Prize
