Cabibbo

Cabibbo refers to Nicola Cabibbo, an Italian physicist who introduced the concept of quark mixing in the weak interaction in 1963. He showed that weak charged-current processes act on a linear combination of down-type quarks, with a dominant contribution from the down quark and a smaller one from the strange quark. The strength of this mixing is encoded in the Cabibbo angle θ_C, with sin θ_C about 0.22 and cos θ_C about 0.97, which explains why strangeness-changing decays are suppressed relative to non-strange decays in hadrons.

Cabibbo's proposal, known as the Cabibbo theory, successfully described a broad class of weak decays and laid the groundwork for a unified treatment of flavor in the weak interaction. The idea later provided the impetus for a more complete description of quark mixing.

In 1973, Kobayashi and Maskawa extended the framework to three generations of quarks, resulting in the CKM matrix. The Cabibbo angle is the 2x2 submatrix angle that relates the first two generations of quarks and remains a fundamental parameter within this larger mixing description.

Legacy: The Cabibbo angle remains a central concept in flavor physics and precision tests of the Standard Model. The term “Cabibbo” is widely used to denote quark mixing in the light-quark sector and continues to influence the interpretation of weak decays and CP violation studies.