Measurement of the angular distribution of γ-rays after neutron capture by 139La for a T-violation search

Takuya Okudaira,Yutaka Yamagata,Kenji Sakai,Shunsuke Endo,Masanori Hino,Hirohiko M Shimizu,Shusuke Takada,Katsuya Hirota,Takayuki Oku,Takumi Sato,Kohei Ishizaki,Tamaki Yoshioka,Masaaki Kitaguchi,Takamasa Momose,Tomoki Yamamoto,J Koga ,T Ichihara ,T Shima ,T Iwata ,T Nakao ,C Haddock ,K Asahi ,A Kimura ,Izumi Ito

doi:10.1051/epjconf/201921909001

Abstract

Parity violating effects enhanced by up to 106 times compared to proton-proton scattering have been observed in several neutron capture induced compound nuclei. This enhancement is explained as an interference between an s-wave and a p-wave amplitude (s-p mixing). Theory predicts that this mechanism can also enhance T-violating effects. For estimation of this enhancement in a promising candidate nucleus, we measured the angular distribution of γ-rays in the (n,γ) reaction induced in a 139La target. Experiments were performed at J-PARC, using a germanium detector array at an intense neutron beam. Analysis of the observed angular dependence around the 0.74 eV p-wave resonance allowed us to determine the ratio of the partial p-wave neutron width to the total neutron width in the entrance channel of the compound nucleus. We also report our development of a 3He spin filter needed as an epithermal-neutron polarizer for the T-violation search.

Highlights

Parity violating effects enhanced by up to 106 times compared to proton-proton scattering have been observed in several neutron capture induced compound nuclei
Tiny parity violation caused by the hadronic weak interaction is observed as a helicity dependence of the cross section of p-p scattering in experiments using a polarized proton beam and an unpolarized proton target, and the size of this fundamental parity violating asymmetry is approximately 10−7 [1,2,3]
We are aiming to search for T-violation beyond that predicted by the Standard Model, for which several experiments and developments are in progress at Japan Proton Accelerator Research Complex (J-PARC)

Summary

Strategy to search for T-violation in compound nuclei

It is well known that the weak interaction violates parity symmetry. Tiny parity violation caused by the hadronic weak interaction is observed as a helicity dependence of the cross section of p-p scattering in experiments using a polarized proton beam and an unpolarized proton target, and the size of this fundamental parity violating asymmetry is approximately 10−7 [1,2,3]. The expression on the right side relates the T-violating effect to the (enhanced) P-violating cross section difference σP measured via transmission of the two neutron helicity states through the unpolarized target (due to the C term in Eq (1)). The enhancement of the P- and T-violation is based on the assumption of the s-p mixing and is derived from the statistical interpretation of the compound nuclear model It can be tested by measuring the angular distribution of the (n,γ ) reaction using polarized neutrons. This paper reports the result of a measurement of κ(J ) for the target nucleus 139La and the development of a neutron polarizer for the T-violation search. An experiment to measure the (n,γ ) reaction using polarized neutrons for verification of the compound nuclear model is reported in Ref. An experiment to measure the (n,γ ) reaction using polarized neutrons for verification of the compound nuclear model is reported in Ref. [8]

Formalism of the angular distribution of γ -rays

Experimental setup

Development of 3He spin filters at JAEA