1. Lepton physics & weak interactions

2. The neutrino hypothesis Given that you can only measure What physics observations/measurements prompted the addition of the neutrino? How was the neutrino hypothesis confirmed?

3. The neutrino hypothesis Electronics Many PMTs Liquid scintillator Fission reactor Shielding

4. The neutrino hypothesis Neutrino source (Fission reactor) neutron rich nuclides Hydrocarbon scintillator “target” E  = 9.1 MeV Light - PMTs Coincidence expt (a)  [10  s]  (b) Reines & Cowan (Nobel Prize) (a) (b)

5. The neutrino hypothesis

6. The two neutrino hypothesis

7. Neutrino helicity Right-handed particles Left-handed particles (Not arbitrary assignments!

8. Neutrino helicity Eu-152 Sm-152 * Sm-152  E1  0 1 0

9. Neutrino helicity Z I Sm sese sese s s

10. Neutrino helicity Z Z  Method: Measure the direction of the circular polarization of the outgoing photons by an asymmetry experiment using NaI Result: The neutrino emitted here has negative helicity!

11. Lepton helicity For electrons (e  ) For positrons (e + ) Not fundamental properties of e  (e + ); property of weak interaction - directly related to helicity of

12. Lepton families Arbitrary number assignment to conserve leptons - by family! Each lepton has its own anti-lepton

13. Lepton families ?? MEGA

14. Example:   decay   at rest Note helicities of neutrinos Note: helicity of e + due to neutrino helicities

15. Example:   decay   at rest Note helicities of neutrinos Note: helicity of e   due to neutrino helicities

16. Example:   decay   at rest Maximum momentum for e + Minimum momentum for e +