Chiral Symmetry Breaking

QCD is the theory of quarks. Yet, because of confinement we don’t see individual quarks. Instead we see protons and neutrons. QCD also predicts other particles, known as pions. These are quark-anti-quark composites and are the lightest particles in the spectrum. They decay through the weak force, which is why we don’t see them every day, but if we restrict attention to just the strong force then it’s important to understand the physics of the pion and its relatives. This physics is largely constrained by symmetry. The pions should rightly be viewed as massless Goldstone bosons for spontaneously broken chiral symmetry (even though pions have a mass of 100 million electron volts!). This perspective allows us to write down an effective theory for the low-energy dynamics of pions, known as the chiral Lagrangian, from which we can recover the proton and neutron as a soliton, known as a Skyrmion.