Weak interaction

Weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four fundamental interactions of nature, along with strong interaction, electromagnetic force, and gravitation. 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 (or the emission of electrons by neutrons or positrons by protons in atomic nuclei) and the associated radioactivity. It is a non-contact force, called weak because the typical field strength is 10⁻¹¹ times the strength of the electromagnetic force and some 10⁻¹³ times that of the strong force, when forces are compared between particles interacting in more than one way.

The weak force was originally (in the 1930s) described by Fermi's theory of a contact four-fermion interaction, that is to say, a "force" without range.

Properties

The weak interaction affects all left-handed fermions and right-handed anti-fermions. Other than gravity, it is the only force affecting neutrinos. The weak interaction is unique in a number of respects:

1. It is the only interaction capable of changing flavour.
2. It is the only interaction which violates P or parity-symmetry (because it exclusively acts on left-handed particles). (As a consequence it is also the only one which violates CP symmetry.)
3. It is mediated by massive 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/c²), their mean life is about 3×10⁻²⁵ seconds.

The Feynman diagram for beta-minus decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W⁻ boson

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, or one up quark and 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 antineutrino. Since high-energy electrons are beta radiation, this is called a beta decay.

Due to the short life of the bosons that mediate weak interactions, weak decays are much more unlikely, hence much slower than strong or electromagnetic decays. For example, an electromagnetically decaying neutral pion has a life of about 10⁻¹⁶ seconds; a weakly decaying charged pion lives about 10⁻⁸ seconds, a hundred million times longer. A free neutron lives about 15 minutes, making it the unstable subatomic particle with the longest known mean life.

Weak isospin is to the weak interaction what color charge is to the strong interaction, and what mass is to gravity. Weak isospin is a quantum number; particles not involved in the weak interactions have a value of 0. Other elementary particles have weak isospin values of -1/2 or 1/2. As is the case with electric charge, these two possible values are equal except for sign. Weak isospin is conserved: the sum of the weak isospin numbers of the particles exiting a reaction equals the sum of the weak isospin numbers of the particles entering that reaction.

Interaction types

There are three basic types of weak interaction vertices (up to charge conjugation and crossing symmetry). Two of them involve charged bosons, intermediate vector bosons, they are called "charged current interactions". The third type is called "neutral current interaction".

• A charged lepton (such as an electron or a muon) can emit or absorb a W boson and convert into a corresponding neutrino.
• A down-type quark (with charge −¹⁄₃) can emit or absorb a W boson and convert into a superposition of up-type quarks. Conversely, an up-type quark can convert into a superposition of down-type quarks. The exact content of this superposition is given by the CKM matrix.
• Either a lepton or a quark can emit or absorb a Z boson.

Two charged-current interactions together are responsible for the beta decay phenomenon. The neutral current interaction was first observed in neutrino scattering experiments in 1974 and in collider experiments in 1983.

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-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 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 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.

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.

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, 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 massless gauge boson fields similar to the photon and a complex scalar Higgs field doublet. The gauge bosons are associated with a SU(2) ×U(1) gauge symmetry. However, at low energies, one of the Higgs fields acquires a vacuum expectation value and the gauge symmetry is spontaneously broken down to the U(1) symmetry of electromagnetism. This symmetry breaking would produce three massless Goldstone bosons but they become integrated by three of the photon-like fields through the Higgs mechanism, giving them mass. These three fields become the W⁺, W^– and Z bosons of the weak interaction, while the fourth gauge field which remains massless is the photon of electromagnetism.

Although this theory has made a number of predictions, including a prediction of the masses of the Z and W bosons before their discovery, the Higgs boson itself has never been observed. Producing Higgs bosons is a major goal of the Large Hadron Collider at CERN.

See also

• Electroweak interaction
• Standard model — the received theory of the elementary particles and the fundamental interactions other than gravity.
• Weakless Universe — the postulate that weak interactions are not anthropically necessary

References

General readers

• R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume. ISBN 9780132366786. 
• B.A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Press. ISBN 0-8018-7971-X. 

Texts

• D.A. Bromley (2000). Gauge Theory of Weak Interactions. Springer. ISBN 3-540-67672-4. 
• G.D. Coughlan, J.E. Dodd, B.M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists (3rd ed.). Cambridge University Press. ISBN 978-0521677752. 
• D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. 
• G.L. Kane (1987). Modern Elementary Particle Physics. Perseus Books. ISBN 0-201-11749-5. 
• D.H. Perkins (2000). Introduction to High Energy Physics. Cambridge University Press. ISBN 0-521-62196-8. 

External links

• Georgia State Univ. page on fundamental interactions.
• Nobel Prize in Physics:
  • 1957 Citation.
  • 1979 Citation.
  • 1980 Citation.