Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

A Measurement of the Parity Violating Gamma-ray Asymmetry in the Capture of Polarized Cold Neutrons on Protons



Thomas R. Gentile, M T. Gericke, R. Alarcon, S. Balascuta, L. Barron, C S. Blessinger, J D. Bowman, R D. Carlini, W.C. Chen, T E. Chupp, C. Crawford, S. Covrig, M Dabaghyan, N. Formin, S J. Freedman, R C. Gillis, G L. Greene, F W. Hersman, T. Ino, Gordon L. Jones, B Lauss, M Leuschner, W R. Lozowski, R Mahurin, Y Masuda, J Mei, G S. Mitchell, S Muto, H Nann, S A. Page, S I. Penttila, W D. Ramsay, A. Salas-Bacci, S Santra, Katelynn A. Sharma, P.-N. Seo, E I. Sharapov, T B. Smith, William M. Snow, W S. Wilburn, V Yuan


The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry A_gamma with respect to the neutron spin direction of gamma-rays emitted in the reaction n + p -> d + gamma using the capture of polarized cold neutrons on the protons in a liquid parahydrogen target. One expects parity-odd effects in the hadronic weak interaction (HWI) between nucleons to be induced by the weak interaction between quarks. A_gamma in n + p -> d + gamma is dominated by a I = 1, 3S1 - 3P1 parity-odd transition amplitude in the n-p system. The first phase of the measurement was completed at the Los Alamos Neutron Science Center spallationsource (LANSCE) with the result A = (1.2 +/- 2.1 (stat) +/- 0.2 (sys)) x 10^-7. We also report the first measurement of an upper limit for the parity allowed left right asymmetry in this reaction,with the result A_LR = (1.8 +/- 1.9 (stat) +/- 0.2 (sys)) x 10^-7. In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of the parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and the LANSCE results. The asymmetry has an estimated size of 5 x 10^-8 and the aim of the NPDGamma collaboration is to measure it to 1 x 10^-8. The second phase of the measurement will be performed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.
Physical Review C (Nuclear Physics)


asymmetry, gamma-ray, neutron, parity-violating, polarized, weak interactions


, T. , Gericke, M. , Alarcon, R. , Balascuta, S. , Barron, L. , Blessinger, C. , Bowman, J. , Carlini, R. , Chen, W. , Chupp, T. , Crawford, C. , Covrig, S. , Dabaghyan, M. , Formin, N. , Freedman, S. , Gillis, R. , Greene, G. , Hersman, F. , Ino, T. , Jones, G. , Lauss, B. , Leuschner, M. , Lozowski, W. , Mahurin, R. , Masuda, Y. , Mei, J. , Mitchell, G. , Muto, S. , Nann, H. , Page, S. , Penttila, S. , Ramsay, W. , Salas-Bacci, A. , Santra, S. , Sharma, K. , Seo, P. , Sharapov, E. , Smith, T. , Snow, W. , Wilburn, W. and Yuan, V. (2011), A Measurement of the Parity Violating Gamma-ray Asymmetry in the Capture of Polarized Cold Neutrons on Protons, Physical Review C (Nuclear Physics) (Accessed April 16, 2024)
Created January 31, 2011, Updated February 19, 2017