For the particles discovered so far, we now had a pretty detailed map of the weak, the electromagnetic and the strong interactions. Only one thing was not quite right in the picture presented so-far: on average, three out of every thousand KLong mesons decay into just two pions, and thus they violate conservation of PC symmetry, as I described in Chapter 7. Which force is responsible for this phenomenon?

What makes this problem particularly difficult is that KLong is the only particle in which this peculiar force has manifested itself up till now. KLong is composed of two quarks, strange and down. Apparently, quarks exert forces on each other that are not quite PC symmetric. Where could such a force come from? Until now, our mathematical scheme has not included such a force; all our equations have automatically been PC symmetric. Several ways of repairing our model, by adding a PC violating force, such that the KLong decay could be explained, have been invented. In any case, more ‘auxiliary particles’ would be needed to transmit the new force.

It turns out not to be possible to repair our model by invoking yet another Yang–Mills field. The spin 1 particles always preserve PC symmetry (could this be why the violation of PC symmetry is so tenuous?)