Progress toward a new measurement of the parity violating asymmetry in n→+p→d+γ

Abstract

We outline the motivation and conceptual design for a new experiment aimed at a 10-fold improvement in the accuracy of the parity-violating asymmetry A γ in the angular distribution of 2.2 MeV gamma rays from the n + p → d + γ reaction. This observable is primarily sensitive to the weak pion-nucleon coupling H 1 π . A proof-of-principle experiment using unpolarized low-energy neutron capture on polyethylene and an array of 12 CsI detectors operated in current mode has been performed. Results of this test experiment including the current mode signal, electronic noise and detector sensitivity to magnetic fields are reported.

Commissioning of the NPDGamma detector array: Counting statistics in current mode operation and parity violation in the capture of cold neutrons on B4C and Al-27

“…For each detector pair the resulting time bin physics asymmetries within a sequence were combined by calculating an error weighted average over the time of flight selected for analysis (13 ms through 34 ms). A 48 % 3 He polarization was calculated from the monitor ratios and was seen to be stable over the analyzed runs.…”

“…Due to the 1 mT holding field surrounding the experimental apparatus, the neutrons are polarized vertically after leaving the 3 He spin filter. While taking hydrogen data, the parity violating asymmetry in n-p capture is therefore seen in a difference of the number of γ rays going up and down.…”

“…It is the first experiment that was designed for the new pulsed high flux cold beam line, flight path 12, at LANSCE. NPDGamma will determine the very small weak pion-nucleon coupling constant f π in the nucleon-nucleon interaction [1][2][3][4][5]. This coupling constant is directly proportional to the parity-violating updown asymmetry A γ in the angular distribution of 2.2 MeV gamma rays with respect to the neutron spin direction in the reaction n → + p → d + γ.…”

“…The beam is transversely polarized by transmission through a polarized 3 He cell. Three 3 He ion chambers are used to monitor beam intensity and measure beam polarization through transmission ratios. A radio frequency spin flipper is used to reverse the neutron spin direction on a pulse by pulse basis.…”

Fundamental Neutron Physics

“…The NPDGamma experiment will measure the parity-violating directional gamma ray asymmetry A γ in the capture of polarized neutrons on protons [286,287]. The unique feature of this observable is that it is sensitive to the weak pion coupling f π , A γ = −0.11f π [288][289][290].…”