MIT Senior Research Scientist Frank Taylor on two types of current interactions, experiments with predictive power, and charge-conjugation parity
What is the Weinberg angle and what does it describe? Why is neutrino-nucleon scattering hard to study? Senior Research Scientist at MIT Department of Physics, Frank Taylor talks on the experimental success in studying neutrinos.
When you take a beam of neutrinos and have them smash against nucleons, which are constituents of the nuclei that make up atoms, an interaction happens, which is called the weak interaction. The weak interaction has very interesting symmetry properties. By studying the nature of the neutrino-nucleon scattering and measuring the angles of the debris that comes out from this scattering, you can study properties of the weak interaction. There are two kinds of particles that are exchanged in this interaction. When a neutrino interacts with a nucleon, either a proton or a neutron, it exchanges what is called a heavy gauge boson, that’s either called a Z-boson (Z for zero charge, neutral) or a charged particle, which is called a W-boson.
By studying two different interactions and measuring their relative strength one can then use what is called radiative corrections, which are computations of higher-order interactions, where the W- and the Z-bosons interact with neutrinos and with quarks inside the nucleon. By studying these higher-order corrections you can then make a prediction about another particle. That other particle happened to be the top quark. The top quark was something which was not observed until experiments were done at Fermilab outside of Chicago. There, protons were collided with antiprotons with sufficient energy to directly produce the top quark.
I was involved in a very early experiment to try and see how one kind of neutrino changes into another kind of neutrino, but subsequent measurements have measured neutrino mixing or neutrino oscillations, as it’s called. This was something that was predicted by Bruno Pontecorvo, who was a very interesting physicist. He predicted a long time ago, it was in the thirties or early forties, that neutrinos could oscillate into other kinds of neutrinos. He predicted that it is possible that an electron neutrino could oscillate and change its “skin”, so to speak, into another kind of neutrino. That prediction was made a long, long time ago and it was a marvelous thing. People had searched for it and searched for it, but never found it. Then over the last twenty years or so neutrino oscillations were really found.