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, 3S1 → 3P1 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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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).
Blyth, D.; Fry, J.; Fomin, N.; Alarcon, R.; Alonzi, L.; Askanazi, E.; Baeßler, S.; Balascuta, S.; Barrón-Palos, L.; Barzilov, A.; Bowman, J. D.; Birge, N.; Calarco, J. R.; Chupp, T. E.; Cianciolo, V.; Coppola, C. E.; Crawford, Chrisopher B.; Craycraft, K.; Evans, D.; Fieseler, C.; Frlež, E.; Garishvili, I.; Gericke, M. T. W.; Gillis, R. C.; Grammer, K. B.; Greene, G. L.; Hall, J.; Hamblen, J.; Hayes, C.; Iverson, E. B.; Kabir, Md Latiful; McCrea, Mark; Milburn, R.; Simmons, F.; Sprow, Aaron P.; and Tang, E., "First Observation of P-Odd γ Asymmetry in Polarized Neutron Capture on Hydrogen" (2018). Physics and Astronomy Faculty Publications. 618.