Report

Neutron Beta Decay By: Christopher Hayes November 29, 2012 Fermi’s Original Theory • First Theory of beta decay developed by Fermi (1933) • Point-like four fermion vertex • Could be applied to any nucleus. Also works for neutron decay and muon decay • Historical document---First succesful theory of the creation of massive particles Muon Decay • Theory of muon decay is similar to neutron beta decay • Muon decay is a pure weak interaction ---defines the strength of the weak coupling constant • Precision of GF determined by precision of muon lifetime calculation First Order Calculation of Neutron Decay Rate • Treats neutron and proton as point-like Dirac particles which couple to W -• Feynman Diagram similar to muon decay • Matrix element exactly the same as muon decay Matrix Element Propagator for a massive vector boson: Weak force V-A vertex factor: Matrix Element: Decay Distribution and Lifetime Theory of Neutron Decay with Spectator Quarks • Presence of spectator quarks modifies the calculation of the matrix element at the quarkquark vertex • Generational mixing at the quark-quark vertex requires and additional factor of Vud from the CKM matrix Calculation of Lifetime Δ = 2.5% Further extensions of the theory of neutron beta decay • More sophisticated theory includes terms which couple directly to the neutron spin vector. • Particle Data Booklet lists values for a, A, B, D • Constants A,B, D determined by experiments with polarized neutrons • Nab experiment at SNS will measure the constants a, b.