particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, the Glashow–Iliopoulos–Maiani (GIM) mechanism is the mechanism through which flavour-changing neutral currents (FCNCs) are suppressed in loop diagrams. It also explains why

weak interaction In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...

s that change

strangeness In particle physics, strangeness (symbol ''S'') is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions that occur in a short period of time. The strangeness of a ...

by 2 (Δ''S'' = 2 transitions) are suppressed, while those that change strangeness by 1 (Δ''S'' = 1 transitions) are allowed, but only in charged current interactions. It is named after physicists

Sheldon Glashow Sheldon Lee Glashow (, ; born December 5, 1932) is a Nobel Prize-winning American theoretical physicist. He is the Metcalf Professor of Mathematics and Physics at Boston University, and a Eugene Higgins Professor of Physics, emeritus, at Harv ...

John Iliopoulos John (Jean) Iliopoulos (Greek language, Greek: Ιωάννης Ηλιόπουλος; 1940) is a Greeks, Greek physicist. He is the first person to present the Standard Model of particle physics in a single report. He is best known for his predictio ...

and

Luciano Maiani Luciano Maiani (born 16 July 1941) is a Sammarinese physicist. He is best known for his prediction of the charm quark with Sheldon Glashow and John Iliopoulos (the "GIM mechanism"). Academic history In 1964 Luciano Maiani received his degree in ...

History

The mechanism was put forth in a famous paper by ; at that time, only three

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

s ( up, down, and

strange Strange may refer to: Fiction * Strange (comic book), a comic book limited series by Marvel Comics * Strange (Marvel Comics), one of a pair of Marvel Comics characters known as The Strangers * Adam Strange, a DC Comics superhero * The title c ...

) were thought to exist.

James Bjorken James Daniel "BJ" Bjorken (June 22, 1934 – August 6, 2024) was an American theoretical physicist. He was a Putnam Fellow in 1954, received a BS in physics from MIT in 1956, and obtained his PhD from Stanford University in 1959. Bjorken was a ...

and Glashow nowiki/>had previously predicted a fourth quark, 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 The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including th ...

is usually credited to Glashow, Iliopoulos, and Maiani (initials "G. I. M.").

Description

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 In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , an ...

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'',

K_L\to\mu^+\mu^-

, illustrated in the figure. 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.

History

Description

References

Further reading