First Precision Measurement of the Parity Violating Asymmetry in Cold Neutron Capture on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>
Michael Gericke, S. Baeßler, L. Barrón-Palos, N. Birge, J. D. Bowman, J. R. Calarco, V. Cianciolo, C. E. Coppola, Christopher Crawford, N. Fomin, I. Garishvili, Geoffrey L. Greene, G. M. Hale, J. Hamblen, C. Hayes, Erik B. Iverson, M. L. Kabir, Mark McCrea, E. Plemons, A. Ramírez-Morales, P. E. Mueller, Ivan Novikov, S. Penttila, E. Scott, J. W. Watts, C. Wickersham
Abstract
We report the first precision measurement of the parity-violating asymmetry in the direction of proton momentum with respect to the neutron spin, in the reaction ^{3}He(n,p)^{3}H, using the capture of polarized cold neutrons in an unpolarized active ^{3}He target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the least well-understood aspects of electroweak theory. The measurement provides an important benchmark for modern effective field theory and potential model calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is A_{PV}=[1.55±0.97(stat)±0.24(sys)]×10^{-8}, which has the smallest uncertainty of any hadronic parity-violating asymmetry measurement so far.