Parity violation in neutron-proton capture-The NPDGamma experiment

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

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10 Scopus citations


The NPDGamma collaboration has recently completed the first phase of a measurement to determine the size of the weak nucleon-nucleon interaction from cold neutron capture on a liquid hydrogen target. In the framework of the nearly 30 year old DDH model [B. Desplanques, J.F. Donoghue, B.R. Holstein, Annals of Physics 124 (1980) 449], the measured process is explained in terms of the weak pion-nucleon coupling, while the framework of modern effective field theory parameterizes the measured process in terms of the 3S1 - 3P1, long range transition (essentially the Danilov parameter ρt) [S.L. Zhu et al., Nuclear Physics A 748 (2005) 435; C.-P. Liu, Phys. Rev. C 75 (2007) 065501]. The couplings in terms of either model are directly proportional to the parity violating up-down asymmetry in the angular distribution of gamma rays with respect to the neutron spin direction in the reaction over(n, ⇒) + p → d + γ. The asymmetry has a predicted size of 5 × 10- 8 and the aim of the NPDGamma collaboration is to measure it to 20%. The first phase of the measurement was completed at the Los Alamos National Laboratory Neutron Science Center Spallation Source with a preliminary result of (- 1.1 ± 2.1 stat . ± 0.2 sys .) × 10- 7. Here, we report on the measurements and the results obtained so far. The experiment is currently being installed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, for the remainder of its run time.

Original languageEnglish (US)
Pages (from-to)239-243
Number of pages5
JournalNuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Issue number2-3
StatePublished - Dec 1 2009


  • Cold neutrons;
  • Hadronic weak interaction
  • Parity violation;

ASJC Scopus subject areas

  • Nuclear and High Energy Physics
  • Instrumentation


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