D. Blyth, Arizona State University
J. Fry, University of Virginia
N. Fomin, University of Tennessee - Knoxville
R. Alarcon, Arizona State University
L. Alonzi, University of Virginia
E. Askanazi, University of Virginia
S. Baeßler, University of Virginia
S. Balascuta, Horia Hulubei National Institute for Physics and Nuclear Engineering, Romania
L. Barrón-Palos, Universidad Nacional Autónoma de México, Mexico
A. Barzilov, University of Nevada - Las Vegas
J. D. Bowman, Oak Ridge National Laboratory
N. Birge, University of Tennessee - Knoxville
J. R. Calarco, University of New Hampshire
T. E. Chupp, University of Michigan - Ann Arbor
V. Cianciolo, Oak Ridge National Laboratory
C. E. Coppola, University of Tennessee - Knoxville
Chrisopher B. Crawford, University of KentuckyFollow
K. Craycraft, University of Kentucky
D. Evans, University of Virginia
C. Fieseler, University of Kentucky
E. Frlež, University of Virginia
I. Garishvili, Oak Ridge National Laboratory
M. T. W. Gericke, University of Manitoba, Canada
R. C. Gillis, Oak Ridge National Laboratory
K. B. Grammer, Oak Ridge National Laboratory
G. L. Greene, University of Tennessee - Knoxville
J. Hall, University of Virginia
J. Hamblen, University of Tennessee - Chattanooga
C. Hayes, North Carolina State University
E. B. Iverson, Oak Ridge National Laboratory
Md Latiful Kabir, University of KentuckyFollow
Mark McCrea, University of KentuckyFollow
R. Milburn, University of Kentucky
F. Simmons, University of Kentucky
Aaron P. Sprow, University of KentuckyFollow
E. Tang, University of Kentucky


We report the first observation of the parity-violating gamma-ray asymmetry Anpγ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. Anpγ isolates the ΔI = 1, 3S13P1 component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured Anpγ = [−3.0 ± 1.4(stat )± 0.2(syst)] × 10−8, which implies a DDH weak πNN coupling of h1π = [2.6 ± 1.2(stat) ± 0.2(syst)] × 10−7 and a pionless EFT constant of C3S1 3P1/C0 = [−7.4 ± 3.5(stat) ± 0.5(syst)] × 10−11  MeV−1. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

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Notes/Citation Information

Published in Physical Review Letters, v. 121, issue 24, 242002, p. 1-7.

© 2018 American Physical Society

The copyright holder has granted the permission for posting the article here.

Due to the large number of authors, only the first 30 and the authors affiliated with the University of Kentucky are listed in the author section above. For the complete list of authors, please download this article or visit:

This group of authors is collectively known as the NPDGamma Collaboration.

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Funding Information

We gratefully acknowledge the support of the U.S. Department of Energy Office of Nuclear Physics through Grants No. DE-AC52-06NA25396, No. DE-FG02-03ER41258, No. DE-SC0014622, No. DE-AC-02-06CH11357, No. DE-AC05-00OR22725, and No. DE-SC0008107, the U.S. National Science Foundation through Grants No. PHY-1306547, No. PHY-1306942, No. PHY-1614545, No. PHY-0855584, No. PHY-0855610, No. PHY-1205833, No. PHY-1506021, No. PHY-1205393, and No. PHY-0855694, PAPIIT-UNAM Grants No. IN110410, No. IN11193, and No. IG1011016, CONACYT Grant No. 080444, the Natural Sciences and Engineering Research Council of Canada (NSERC) Grants No. SAPPJ/341298 and No. SAPEQ/375650, and the Canadian Foundation for Innovation (CFI).