Helicity of Neutrinos

1. Helicity of Neutrinos ニュートリノのヘリシティ M.Goldhaber, L. Grodzins and A. W. Sunyar Brookhaven National Laboratory Phys. Rev. 109 (1958) 1015. Contents 1. Introduction 2. Principle of experiment 3. Experimental method 4. Result 5. Summary Shibata Lab. Akihito Fukuda

2. 1. Introduction History of neutrino If we can measure the helicity of neutrino, the type is decided. 1903 Rutherford : Discovery of α-ray , β-ray , γ-ray Experiment by Goldhaber, Grodzins and Sunyar 1904 Chadwick : Discovery of continuous spectrum of β-ray 1931 Pauli : Neutrino hypothesis 2 • Rutherford: Discovery of α-ray , β-ray , γ-ray • 1904 Chadwick : Discovery of continuous spectrum of β-ray • 1931 Pauli : Propounding of neutrino hypothesis • Fermi : Propounding that β-decay is three-body problem 1954 Reines : Detection of neutrino 1956 Yang, Lee : Parity violation in weak interaction Helicity is defined as p σ Measurement of the helicity of electron by Frauenfelder p σ -Matrix In this experiment, the helicity of neutrino was measured.

3. 2. Principle of experiment * , , Electron Capture 839keV 961keV * * If , If , If we measure the helicity of γ-ray, the helicity of neutrino is determined.

4. 3. Experimental method ・, the source, was produced in the Brookhaven reactor. The helicity of γ-ray is measured by Compton scattering. ・Electromagnet was alternately magnetized in the up or down directionevery 3 minutes. Forwardγ-rays are selected by resonant scattering. ・9 runs with length from 3 to 9 hours were carried out. ・As a further check, 3 runs were carried out with a shorter magnet with the source on top of the magnet. I will explain about resonant scattering and measurement of γ-ray helicity with Compton polarimeter.

5. Resonant scattering of γ-rays In order to select forward γ-ray, resonant scattering was used. (共鳴散乱) : recoil energy × 0 0 Energy of γ-ray Doppler shift For resonant scattering : energy of γ-ray that can be spent for excitation of nucleus Condition of resonant scattering

6. Measurement of γ-ray helicity with Compton polarimeter ・The γ-ray enters into the magnetized iron. Fe ・Cross-section of Compton scattering is larger if electron and γ-ray spins are antiparallel. ・The counting rate varies depending on the direction of the magnetization by Compton scattering. : counting rate with the magnetic field pointing up (The spin of electron pointing down) : counting rate with the magnetic field pointing down (The spin of electron pointing up) ・If the γ-ray are 100% circularly polarized, we expect an effect of with an accuracy of 10%. up down

7. 4. Result ・An effect was observed in channel B. ・In the last 3 runs also a negative helicity was found, the circular polarization being . ・The helicity of γ-ray is negative.

8. 5. Summary ・The helicity of neutrinos was necessary to decide the type of the β-interaction. ・The helicity of the neutrino was determined by the measurement of γ-ray from the electron capture of . ・A combined analysis of circular polarization and resonant scattering of γ-rays measured the helicity of the neutrino. ・To select forward γ-ray, the resonant scattering was used. ・We had γ-ray Compton-scattered, and Circular polarization of γ-ray was measured by the difference of counting rate. ・The helicity of γ-ray was measured by Compton scattering. ・The helicity of forward γ-ray was negative. ・The resonant-scattered γ-ray was detected. ・It indicates that the helicity of neutrino is negative; ・That the helicity of γ-ray is negative( was found, and that the helicity of neutrinos is negative ( was determined.

9. 以下、補足スライド

10. δ and : magnetic moment electric current anti δ + para para δ anti Fe peripheral electron : 2 density : →