Standard Model of Particles SPACE TIME Strong Interaction E&M Interaction The standard model consists of spin 1/2 quarks and leptons which interact through the interchange of gauge bosons. The electromagnetic force is mediated by the massless photon, which is the only stable boson. Thus the range is infinite. The coupling is very small, and so the electromagnetic interaction can be calculated perturbatively to a very high precision. The weak interaction is more like the electromagnetic than the strong interaction. It also has a weak coupling, but the W and Z are very heavy, which limits its range to about .2% of the radius of the neutron. The other property is that the W boson caries electric charge, and thus can convert an ‘up’ quark into a ‘down’ or a ‘strange’, and same for the leptons. The strong interaction is mediated by gluons with three separate charges labeled by color. The gluon does not interact with leptons, but does with quarks and even other gluons. so what you end up with is that even the ground state is a messy tangle of gluons and quark- anti-quark pairs called the ‘sea’. This multiplies the force and strongly confines quarks and gluons inside of the nucleon. So if both the strong and weak forces are short-range, how do neutrons interact? By the exchange of quark - anti-quark pairs or mesons, which has the range of 1-2 fm. The interesting physics happens at the vertex, which can involve weak or strong interactions. That is what we will be discussing today. Now, besides their obvious quantum mechanical differences fermions are a lot like cars, so let me take you on an auto-mechanical tour of the standard model. Weak Interaction Hadronic Interaction (residual nuclear force)

Symmetries Continuous Symmetries space-time translation rotational invariance Lorentz boosts gauge invariance Noether’s Theorem continuous symmetries correspond to conserved quantities energy-momentum angular momentum center-of-momentum electric charge Discrete Symmetries parity P : x  -x time T : t  -t charge C : q  -q particle P12: x1  x2 exchange Discrete Theorems spin-statistics theorem CPT theorem position symmetry conserved momentum