GIM mechanism

Decay of a meson into a muon-antimuon pair. The two Feynman diagrams with a virtual up quark and a charm quark have opposite signs and therefore almost cancel each other out. The factors and result from the Cabibbo mix .

{\ displaystyle {\ bar {K}} ^ {0}}

{\ displaystyle \ sin \ theta _ {\ mathrm {C}}}

{\ displaystyle \ cos \ theta _ {\ mathrm {C}}}

The GIM mechanism of particle physics (after Sheldon Lee G lashow , John I liopolus and Luciano M aiani ) explains why quarks with the same charge cannot be converted into one another due to the weak interaction (absence of neutral currents that change flavor ). Historically, it was the reason why the charm quark was predicted .

At that time it was known that the decay channel has a very small branching ratio , namely . This is far smaller than was expected at the time. To explain why the decay is so strongly suppressed, Glashow, Iliopoulos and Maiani hypothetically introduced the charm quark in 1970. ${\ displaystyle {\ bar {K}} ^ {0} \ to \ mu ^ {+} \ mu ^ {-}}$ ${\ displaystyle \ Gamma ({\ bar {K}} ^ {0} \ to \ mu ^ {+} \ mu ^ {-}) / \ Gamma _ {\ mathrm {dead}} ({\ bar {K} } ^ {0}) <1.5 \ cdot 10 ^ {- 6}}$

This has two important effects: On the one hand, the coupling of the state orthogonal to the electroweak interaction state to the charm quark prevents a flavor change in the lowest order (tree level). On the other hand, the charm quark induces another decay channel in higher orders, which is indistinguishable from the first and thus interferes with it . Because of the different signs of the two decay channels, this interference is destructive. This suppression of the neutral currents that change the flavor is known as the GIM mechanism. This provided an explanation for the small branching ratio. ${\ displaystyle d ^ {\ prime}}$ ${\ displaystyle s ^ {\ prime}}$

The first experimental proof of the charm quark (see J / ψ meson ) was finally achieved in 1974 by the SLAC -SP-017 collaboration and the E598 collaboration. With this prediction and the actual discovery of the charm quark, the GIM mechanism contributed to the acceptance of the standard model of elementary particle physics .

literature

Jean Iliopoulos: Glass show-Iliopoulos-Maiani mechanism . In: Scholarpedia . tape 5 , no. 5 , 2010, p. 7125 , doi : 10.4249 / scholarpedia.7125 .

Individual evidence

^ SL Glashow, J. Iliopoulos and L. Maiani: Weak Interactions with Lepton-Hadron Symmetry . In: Phys. Rev. D . tape 2 , 1970, p. 1285-1292 , doi : 10.1103 / PhysRevD.2.1285 .
↑ A. Barbaro-Galtieri et al. (Particle Data Group): Review of Particle Properties . In: Rev. Mod. Phys. tape 42 , 1970, pp. 87–200 , doi : 10.1103 / RevModPhys.42.87 ( cern.ch [PDF]).
↑ JE Augustin et al. (SLAC-SP-017 Collaboration): Discovery of a Narrow Resonance in e ⁺ e ^- Annihilation . In: Phys. Rev. Lett. tape 33 , no. 23 , December 2, 1974, pp. 1406-1408 , doi : 10.1103 / PhysRevLett.33.1406 .
↑ JJ Aubert et al. (E598 Collaboration): Experimental Observation Of A Heavy Particle J . In: Phys. Rev. Lett. tape 33 , no. 23 , December 2, 1974, pp. 1404-1406 , doi : 10.1103 / PhysRevLett.33.1404 .

[1] SL Glashow, J. Iliopoulos and L. Maiani: Weak Interactions with Lepton-Hadron Symmetry . In: Phys. Rev. D . tape 2 , 1970, p. 1285-1292 , doi : 10.1103 / PhysRevD.2.1285 .

[2] A. Barbaro-Galtieri et al. (Particle Data Group): Review of Particle Properties . In: Rev. Mod. Phys. tape 42 , 1970, pp. 87–200 , doi : 10.1103 / RevModPhys.42.87 ( cern.ch [PDF]).

[3] JE Augustin et al. (SLAC-SP-017 Collaboration): Discovery of a Narrow Resonance in e ⁺ e ^- Annihilation . In: Phys. Rev. Lett. tape 33 , no. 23 , December 2, 1974, pp. 1406-1408 , doi : 10.1103 / PhysRevLett.33.1406 .

[4] JJ Aubert et al. (E598 Collaboration): Experimental Observation Of A Heavy Particle J . In: Phys. Rev. Lett. tape 33 , no. 23 , December 2, 1974, pp. 1404-1406 , doi : 10.1103 / PhysRevLett.33.1404 .