PHY646 - Quantum Field Theory and the Standard Model
Even Term 2020 Dr. Anosh Joseph, IISER Mohali
LECTURE 47
Wednesday, April 15, 2020 (Note: This is an online lecture due to COVID-19 interruption.)
Topic: The Weak Interaction.
The Weak Interaction
The Beta Decay
The history of the weak interactions can be traced back to the beta decay process. The energetics of neutron decay indicates that there must be an unseen and electrically neutral particle. The conservation of spin in the neutron decay process requires that it must be a fermion. In 1930 Pauli postulated the existence of this particle and in 1932 Fermi named it as the neutrino (“the little neutral one” in Italian). The neutron decay process is
− n → p + e + νe, (1) where νe is the electron anti-neutrino. The astonishingly long neutron decay time (about 10 minutes) demands that the mediating interaction be very, very weak. We now know that the neutrinos indeed are an almost omnipresent indicator of any weak process. They are part of a family of fermions known as leptons. In Table. 1 we show the lepton family: it contains the trio corresponding to the charged particles: the electron, the muon, and the tau particle, and their respective neutrino cousins. The anti-leptons have the same mass and opposite charge to that of their leptonic counterparts.
The Fermi Model of the Weak Interaction
All fermions participate in the weak interaction. But many weak interaction processes have a strong or electromagnetic pathway, which dwarfs and thus masks the weak interaction contribution. Electrons repel one another weakly, just as they do electrically, but unless the impact parameter is PHY646 - Quantum Field Theory and the Standard Model Even Term 2020
− l νl ml (MeV) − e νe 0.511 − µ νµ 105.7 − τ ντ 1776.8
Table 1: The three generations (or flavors) of leptons, with the corresponding mass of the charged leptons. We do not yet know the masses of the neutrinos but the existence of mass (as well as their mass differences) is constrained by neutrino oscillation. on the scale of an atomic nucleus, the contribution of the weak interaction is effectively zero for any realistic system. However, processes involving neutrinos have no electromagnetic pathway, as is the case for neutron decay. While the ultimate description of the weak interaction will involve a mediator, as a first attempt, Fermi in 1933 proposed an interaction of the form