β- (electron) decay.
β+ (positron) decay.
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β- (electron) decay. |
β+ (positron) decay. |
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 CP
symmetry.
• It is propagated by gauge bosons that have significant
masses, an unusual feature which is explained within the
Standard Model by the Higgs mechanism.
(A quote from Wikipedia)
Neutral Currents
Weak decays fall into the categories of leptonic, semi-leptonic, and hadronic. “The semi-leptonic decay of a hadron is a decay caused by the weak force in which one lepton (and the corresponding neutrino) is produced in addition to one or more hadrons. An example for this can be K0 → e- + π+ + an anti-electron-neutrino. This is to be contrasted with purely hadronic decays, such as K0 → π+ + π−, which are also mediated by the weak force.” [Quote from Wikipedia.] Of course, a leptonic decay would be, for instance, the decay of a μ− into an electron, a muon neutrino, and an electron anti-neutrino. Nothing but leptons. [In the literature, the term weak charge is used specifically to refer to the coupling of particles to the Z0 boson.]
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Semileptonic decays involve both leptons and quarks, while hadronic decays involve only quarks. |
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In 1956, graduate student George Sudarshan realized that the operator in the weak matrix element must have the form of a polar vector minus an axial vector, with the two having essentially equal strength, to produce the maximal parity violations that were then being seen experimentally in weak decays. |
To me personally as a teenager the most baffling thing about the weak process was that only states of left-handed chirality participate in weak processes! The weak interaction completely ignores right-handed chirality states! Boy, I didn't know anything... many more shocks were to come. Although the weak interaction violates both P and C symmetry to the maximum amount physically and mathematically possible, it turns out to dash the hopes of physicists by exhibiting almost perfect PC symmetry. There is a tiny, tiny little bit of direct PC violation, but as we have seen, it is almost vanishingly small. What's going on??
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Because it factors out of the expression for the lifetime of the muon, and everything else is a physical constant, the Fermi constant GF is known to incredible precision: GF/(ℏc) = 1.1663787 × 10-5 GeV-2. This gives a weak coupling constant gw (weak charge) of about 0.65 (Gaussian units) and an αw of about 1/30. So the weak interaction is significantly stronger than the electromagnetic interaction, although we don't see it that way in observed processes, due to the huge mass of the weak bosons.
 For both neutrinos and quarks, the mass eigenstates are mixtures of the flavor eigenstates! The weak interaction does not respect flavor. However, the situation, while extreme for neutrinos, is relatively minor for quarks. |
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 The choice of d, s and b is an arbitrary convention. |
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The CKM matrix describes the mixing of the quark mass eigenstates into the observed weak eigenstates. The choice of usage of down-type quarks in the definition is a convention, and does not represent a physically preferred asymmetry between up-type and down-type quarks. Other conventions are equally valid: The mass eigenstates of the up-type quarks can equivalently define the matrix in terms of u, c and t. Since the CKM matrix is unitary, its inverse is the same as its conjugate transpose, which the alternate choices use; it appears as the same matrix, in a slightly altered form.
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Right-handed helicity
neutrinos would be "sterile," not interacting except
gravitationally. Observations from the W-MAP and Planck
satellite observatories strongly support only 3 neutrino
types, but there have still been extensive and intensive
searches for sterile neutrinos, since they would offer an
immediate solution to the Dark Matter puzzle. Theories
beyond the standard model often predict sterile neutrinos,
but there is no reliable prediction of their mass, other
than "larger than the left-handed neutrinos."
Many studies of deep-inelastic scattering using neutrino beams instead of electron beams have been made over the years. Despite some misleading results at various times, the neutrino results agree remarkably well with the electron results, as regards details of parton distributions in the nucleon.
A nice summary of weak interaction phenomena...recommended reference!