PHYSICS · Subjective · Fermi University Physics · Particle Physics & Quantum Fields · Electroweak theory and the Standard Model · Electroweak theory and the Standard Model
Particle Physics, Ch The Standard Model as a table you can predict from
Problem
The free neutron n (quark content udd, mass 939.565MeV/c2, charge 0) is unstable. The proton has mass 938.272MeV/c2, the electron 0.511MeV/c2, and the charged pion 139.57MeV/c2. (a) Using the four questions (which vertices are available, which final states are allowed by charge, baryon number, lepton number and available energy, which interaction is needed, and so which lifetime band is expected), work out the neutron's dominant decay channel. State explicitly why n→pπ− is not available. (b) Commit to an order-of-magnitude lifetime band for this channel before looking up the measured value, using the muon (2.197μs) and the tau (2.9×10−13s) as the weak-decay lifetimes already met. (c) The measured mean lifetime of the free neutron is τn=878.4s. Compare this with the band committed to in part (b), and explain, without introducing anything about decay rates as a number, why the table's four questions do not rule out such a long lifetime.