1. 'Helicity of Neutrinos' Shibata lab. TAMORI Midori Dec 12th, 2006 M. Goldhaber et al. Physical Review 109 (1958) 1015-1017 Contents 1. Introduction 2. Experiment 3. Result 4. Summary

2. 1. Introduction the projection of spin to the direction of momentum spin Helicity can either be +1 or -1. helicity : : momentum : spin spin Spin of neutrino is 1/2.

3. If a gamma ray is emitted in the momentum direction, its energy is increased by Doppler effect, which is needed for resonance scattering on. (see a later page) 2. Experiment is used. orbital electron capture γ kinematics z e no circular polarization Sm ν γ It decays by orbital electron capture. Neutrino is emitted in the direction opposite to. Gamma ray is emitted from excited state of Sm.

4. z Sm ν e no circular polarization Sm e no circular polarization ν If circular polarization is observed, and if The neutrino helicity can be measured this way. Conservation of angular momentum ½ ½ ½ ½ m=0 Sm ν e circular polarization ½ ½ m=+1 j=1 γ Sm e circular polarization ν ½ ½ m=-1 j=1 γ

5. Pb Fe scatterer NaI PMT Experimental arrangement 10cm magnetic field: up or down direction (every 3 minutes) If γ rays and magnet have spin of same direction, γ rays pass through the magnet. The γ rays are resonant-scattered. resonance scattering γ s p d 1 2 3 4 Fe Spin of electrons in Fe atom and γ parallel : electron spin cannot flip antiparallel : electron spin flips Cross section of Compton scattering for antiparallel is larger than parallel. Only Doppler shifted (forward emitted) gamma rays can have resonance scattering. ν 960keV

6. 840keV 960keV 122keV Non-resonant background 3. Result The horizontal axis is Bjorken scale in logarithmic scale. The dashed curve is the value based on a quark-parton model. : counting rate with magnetic field pointing down (spin of electrons in Fe up) : pointing up Helicity of neutrino is negative estimation pulse height counts : spin of electrons in Fe up, Compton cross section is small. The lager transmission of gamma rays in Fe.

7. 4. Summary my comment: If helicity of neutrino is always -1, it means that neutrino travels at light velocity. It then means that neutrino mass is 0. So, neutrino helicity is related to neutrino mass. ● The objective of this experiment is to determine the helicity of neutrino. ● Orbital electron capture of was used: ● Gamma rays emitted in the momentum direction were selected by means of resonance scattering. ● Helicity of gamma ray was measured with a Compton polarimeter of Fe atom. ● The helicity of gamma ray was negative, which indicates.

8. spin of electrons in Fe atom : down spin of gamma ray : up spin of electrons in Fe atom : up spin of gamma ray : down cross section for antiparallel is larger than parallel. 960keV 840keV 960keV 122keV Non-resonant background pulse height counts magnetic field electron in Fe gamma ray Compton cross section smalllarge small Spin of electrons in Fe atom and γ cross section for antiparallel is larger than parallel.