A Charming Story

This post is intended to give an overview of the motivation, purpose, and discovery of the charm quark.

The Problem

The conventional 3-quark (up, down, and strange) models of the weak interaction are inconsistent with weak selection rules. In particular, strangeness-changing $(\Delta S = 2)$ processes as seen in neutral Kaon oscillation $(K_0 \leftrightarrow \bar{K_0} )$ [1]. These processes should be smaller than the predictions obtained from the conventional 3-quark theory. There are two diagrams that contribute to neutral kaon oscillation [2].

In a 3-quark model, the fermion propagators can only be up quark propagators, they both give a positive contribution to the process, and it seems as though we are stuck with these $\Delta S = 2$ oscillations. It would be nice if we could somehow suppress these diagrams.

Solution

Introduce another up-type quark and one new quantum number called “Charm,” designed to counteract the effects of “Strangeness” carried by the strange quark. With some insight from the future, we will call this new up-type quark the charm quark.

Now, in our 4-quark model (up, down, strange, and charm), we have up and charm quark propagators a cancellation can in-principle occur. First proposed by Glashow, Iliopoulos, and Maiani, this mechanism would later become known as the “GIM Mechanism” [3]. The result is a suppression of these $\Delta S = 2$ processes which is exactly what we need to make the theory consistent with experiments.

Experimental Evidence

Amusingly, two different experiments reported the same resonance at nearly the same time. In 1974, both the Stanford Linear Accelerator [4] and the Brookhaven National Lab [5] both reported a resonance at 3.1 GeV. SLAC named this particle the $\psi$ , and Brookhaven named it the $J$ and thus the $J/ \psi$ particle was born. It turns out that the resonance they detected was “Charmonium,” a bound state of $c \bar{c}$ .