First Observation of $P$-odd $\gamma$ Asymmetry in Polarized Neutron Capture on Hydrogen

We report the first observation of the parity-violating gamma-ray asymmetry $A_{\gamma }^{np}$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A_{\gamma }^{np}$ isolates the $\mathrm{\Delta }I=1$, ${{}^{3}S}_1\to {{}^{3}P}_1$ 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 $A_{\gamma }^{np}=[-3.0\pm 1.4(\mathrm{stat})\pm 0.2(\mathrm{syst})]\times {10}^{-8}$, which implies a DDH weak $\pi NN$ coupling of $h_{\pi }^1=[2.6\pm 1.2(\mathrm{stat})\pm 0.2(\mathrm{syst})]\times {10}^{-7}$ and a pionless EFT constant of $C^{{{}^{3}S}_1\to {{}^{3}P}_1}/C_0=[-7.4\pm 3.5(\mathrm{stat})\pm 0.5(\mathrm{syst})]\times {10}^{-11}{\mathrm{MeV}}^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

Figure 1

A schematic vertical cut view of the NPDGamma experiment on the FnPB; for details see text.

Figure 2

Plot of a typical detector voltage signal as a function of time bin for eight 60 Hz neutron pulses. The proton pulse was not delivered to the spallation target in the 2nd pulse resulting in a dropped pulse. The peak yield in the 3rd pulse is 1% low because the phosphorescence tail from the second pulse is missing. The rising (falling) edges of the pulses correspond to the choppers opening (closing). The pedestal from the $\beta$-delayed $\gamma$’s of ${}^{28}\mathrm{Al}$ is shown. Finally, the fiducial time interval (27 time bins wide) is shown in pulse seven (time bins 253 to 279).

Figure 3

Histogram of hydrogen asymmetries ($\sim 1/30$ of all the data) for a typical detector before (left) and after (right) the cuts described in the text have been applied. Note the different $x$-axis scale on the right panel. The distinct side lobes in the uncut data correspond to SS in which one or more dropped pulses occurred.

Figure 4

$h_{\pi }^1$ from theoretical estimates or calculations (blue) and this work (red).