In particle physics, the weak interaction
(the weak force or weak nuclear force) is
the mechanism of interaction between subatomic
particles that causes radioactive decay and
thus plays an essential role in nuclear fission.
The theory of the weak interaction is sometimes
called quantum flavordynamics (QFD), in analogy
with the terms quantum chromodynamics (QCD)
dealing with the strong interaction and quantum
electrodynamics (QED) dealing with the electromagnetic
force. However, the term QFD is rarely used
because the weak force is best understood
in terms of electroweak theory (EWT).The weak
interaction takes place only at very small,
sub-atomic distances, less than the diameter
of a proton. It is one of the four known fundamental
interactions of nature, alongside the strong
interaction, electromagnetism, and gravitation.
== Background ==
The Standard Model of particle physics provides
a uniform framework for understanding the
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 the deepest levels, all weak interactions
ultimately are between elementary particles.
In the case of the weak interaction, fermions
can exchange three distinct types of force
carriers known as the W+, W−, and Z bosons.
The mass of each of these bosons is far greater
than the mass of a proton or neutron, which
is consistent with the short range of the
weak force. The force is in fact termed weak
because its field strength over a given distance
is typically several orders of magnitude less
than that of the strong nuclear force or electromagnetic
force.
Quarks, which make up composite particles
like neutrons and protons, come in six "flavors"
– up, down, strange, charm, top and bottom
– which give those composite particles their
properties. The weak interaction is unique
in that it allows for quarks to swap their
flavor 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.
The weak interaction is the only fundamental
interaction that breaks parity-symmetry, and
similarly, the only one to break charge parity
symmetry.
Other important examples of phenomena involving
the weak interaction include beta decay, and
the fusion of hydrogen into helium that powers
the Sun's thermonuclear process. Most fermions
will 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 illumination, and
in the related field of betavoltaics.During
the quark epoch of the early universe, the
electroweak force separated into the electromagnetic
and weak forces.
== 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.However,
it is better described as a non-contact force
field having a finite range, albeit very short.
In 1968, 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 W and Z bosons was not directly confirmed
until 1983.
== Properties ==
The weak interaction is unique in a number
of respects:
It is the only interaction capable of changing
the flavor of quarks (i.e., of changing one
type of quark into another).
It is the only interaction that violates P
or parity-symmetry. It is also the only one
that violates charge-parity CP symmetry.
It is mediated (propagated) by force carrier
particles that have significant masses, an
unusual feature which is explained in the
Standard Model by the Higgs mechanism.Due
to their large mass (approximately 90 GeV/c2)
these carrier particles, termed the W and
Z bosons, are short-lived with a lifetime
of under 10−24 seconds. The weak interaction
has a coupling constant (an indicator of interaction
strength) of between 10−7 and 10−6, compared
to the strong interaction's coupling constant
of 1 and the electromagnetic coupling constant
of about 10−2; consequently the weak interaction
is weak in terms of strength. The weak interaction
has a very short range (around 10−17 to
10−16 m). At distances around 10−18 meters,
the weak interaction has a strength of a similar
magnitude to the electromagnetic force, but
this starts to decrease exponentially with
increasing distance. At distances of around
3×10−17 m, a distance which is scaled up
by just one and a half decimal orders of magnitude
from before, the weak interaction is 10,000
times weaker than the electromagnetic.The
weak interaction affects all the fermions
of the Standard Model, as well as the Higgs
boson; neutrinos interact through gravity
and the weak interaction only, and neutrinos
were the original reason for the name weak
force. The weak interaction does not produce
bound states nor does it involve binding energy
– something that gravity does on an astronomical
scale, that the electromagnetic force does
at the atomic level, and that the strong nuclear
force does inside nuclei.Its most noticeable
effect is due to its first unique feature:
flavor changing. A neutron, for example, is
heavier than a proton (its sister nucleon),
but it cannot decay into a proton without
changing the flavor (type) of one of its two
down quarks to an up quark. Neither the strong
interaction nor electromagnetism permit flavor
changing, so this proceeds by weak decay;
without weak decay, quark properties such
as strangeness and charm (associated with
the quarks of the same name) 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 W− boson which
is then converted into an electron and an
electron antineutrino. Another example is
the electron capture, a common variant of
radioactive 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., flavor 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
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. 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 (symbol T3), which serves as a quantum
number and governs how that particle behaves
in the weak interaction. Weak isospin plays
the same role in the weak interaction as does
electric charge in electromagnetism, and color
charge in the strong interaction. All left-handed
fermions have a weak isospin value of either
+​1⁄2 or −​1⁄2. For example, the
up quark has a T3 of +​1⁄2 and the down
quark −​1⁄2. A quark never decays through
the weak interaction into a quark of the same
T3: Quarks with a T3 of +​1⁄2 only decay
into quarks with a T3 of −​1⁄2 and vice
versa.
In any given 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 a νμ (+​1⁄2)
and a μ+ (as a right-handed antiparticle,
+​1⁄2).Following the development of the
electroweak theory, another property, weak
hypercharge, was developed. It is dependent
on a particle's electrical charge and weak
isospin, and is defined by:
Y
W
=
2
(
Q
−
T
3
)
{\displaystyle \qquad Y_{W}=2(Q-T_{3})}
where YW is the weak hypercharge of a given
type of particle, Q is its electrical charge
(in elementary charge units) and T3 is its
weak isospin. Whereas some particles have
a weak isospin of zero, all spin-​1⁄2
particles have non-zero weak hypercharge.
Weak hypercharge is the generator of the U(1)
component of the electroweak gauge group.
== Interaction types ==
There are two types of weak interaction (called
vertices). The first type is called the "charged-current
interaction" because it is mediated by particles
that carry an electric charge (the W+ or W−
bosons), and is responsible for the beta decay
phenomenon. The second type is called the
"neutral-current interaction" because it is
mediated by a neutral particle, the Z boson.
=== Charged-current interaction ===
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 a corresponding
neutrino (with a charge of 0), where the type
("flavor") of neutrino (electron, muon or
tau) is the same as the type of lepton in
the interaction, for example:
μ
−
+
W
+
→
ν
μ
{\displaystyle \mu ^{-}+W^{+}\to \nu _{\mu
}}
Similarly, a down-type quark (d with a charge
of −​1⁄3) can be converted into an up-type
quark (u, with a charge of +​2⁄3), by
emitting 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 a down-type quark, for example:
d
→
u
+
W
−
d
+
W
+
→
u
c
→
s
+
W
+
c
+
W
−
→
s
{\displaystyle {\begin{aligned}d&\to u+W^{-}\\d+W^{+}&\to
u\\c&\to s+W^{+}\\c+W^{-}&\to s\end{aligned}}}
The W boson is unstable so will rapidly decay,
with a very short lifetime. For example:
W
−
→
e
−
+
ν
¯
e
W
+
→
e
+
+
ν
e
{\displaystyle {\begin{aligned}W^{-}&\to e^{-}+{\bar
{\nu }}_{e}~\\W^{+}&\to e^{+}+\nu _{e}~\end{aligned}}}
Decay of the 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 the neutron into
a proton. Because of the energy involved in
the process (i.e., the mass difference between
the down quark and the up quark), the W−
boson can only be converted into an electron
and an electron-antineutrino. At the quark
level, the process can be represented as:
d
→
u
+
e
−
+
ν
¯
e
{\displaystyle d\to u+e^{-}+{\bar {\nu }}_{e}~}
=== Neutral-current interaction ===
In neutral current interactions, a quark or
a lepton (e.g., an electron or a muon) emits
or absorbs a neutral Z boson. For example:
e
−
→
e
−
+
Z
0
{\displaystyle e^{-}\to e^{-}+Z^{0}}
Like the W boson, the Z boson also decays
rapidly, for example:
Z
0
→
b
+
b
¯
{\displaystyle Z^{0}\to b+{\bar {b}}}
== Electroweak theory ==
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 the presence of three massive
gauge bosons (W+, W−, Z0, the three carriers
of the weak interaction) and the massless
photon (γ, the carrier of 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
gauge bosons – each similar to the photon
– forming a complex scalar Higgs field doublet.
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.
This symmetry-breaking would be expected to
produce three massless bosons, but instead
they become integrated by the other three
fields and acquire mass through the Higgs
mechanism. These three boson integrations
produce the W+, W− and Z0 bosons of the
weak interaction. The fourth gauge boson is
the photon of electromagnetism, and remains
massless.This theory has made a number of
predictions, including a prediction of the
masses of the Z and W-bosons before their
discovery. 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 125–127 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, the Higgs boson was tentatively
confirmed to exist.
== Violation of symmetry ==
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 mirror-reflected copy of the experimental
apparatus. This so-called law of parity conservation
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 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, 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 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 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 limited circumstances. Despite
its rarity, it is widely believed to be the
reason that there is much more matter than
antimatter in the universe, and thus forms
one of Andrei Sakharov's three conditions
for baryogenesis.
== See also ==
Weakless Universe – the postulate that weak
interactions are not anthropically necessary
Gravity
Nuclear force
Electromagnetism
