GIM mechanism

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In quantum field theory, the GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism) is the mechanism through which flavour-changing neutral currents (FCNCs) are suppressed in loop diagrams. It also explains why weak interactions that change strangeness by 2 (ΔS = 2 transitions) are suppressed, while those that change strangeness by 1 (ΔS = 1 transitions) are allowed, but only in charged current interactions.

Rare leptonic decay of the neutral Kaon predicated on the GIM mechanism

History[edit]

The mechanism was put forth by Sheldon Lee Glashow, John Iliopoulos and Luciano Maiani in their famous paper "Weak Interactions with Lepton–Hadron Symmetry" published in Physical Review D in 1970.[1]

At the time the GIM mechanism was proposed, only three quarks (up, down, and strange) were thought to exist. Glashow and James Bjorken predicted a fourth quark in 1964,[2] but there was little evidence for its existence. The GIM mechanism however, required the existence of a fourth quark, and the prediction of the charm quark is usually credited to Glashow, Iliopoulos, and Maiani.

Description[edit]

The mechanism relies on the unitarity of the charged weak current flavor mixing matrix, which enters in the two vertices of a one-loop box diagram involving W boson exchanges. Even though Z0 boson exchanges are flavor-neutral (i.e. prohibit FCNC), the box diagram induces FCNC, but at a very small level. The smallness is set by the mass-squared difference of the different virtual quarks exchanged in the box diagram, originally the u-c quarks, on the scale of the W mass.

The smallness of this quantity accounts for the suppressed induced FCNC, dictating a rare decay, , illustrated. If that mass difference were ignorable, the minus sign between the two interfering box diagrams (itself a consequence of unitarity of the Cabibbo matrix) would lead to a complete cancellation, and thus a null effect.

References[edit]

  1. ^ S.L. Glashow; J. Iliopoulos; L. Maiani (1970). "Weak Interactions with Lepton–Hadron Symmetry". Physical Review D. 2 (7): 1285. Bibcode:1970PhRvD...2.1285G. doi:10.1103/PhysRevD.2.1285.
  2. ^ B.J. Bjorken; S.L. Glashow (1964). "Elementary particles and SU(4)". Physics Letters. 11 (3): 255–257. Bibcode:1964PhL....11..255B. doi:10.1016/0031-9163(64)90433-0.

Further reading[edit]