NPDGamma

$\vec n + p \to d + \gamma$

NPDGamma is an ongoing effort to measure the parity-violating asymmetry in polarized cold neutron capture on parahydrogen.

Polarized neutrons of energies 2 meV -15 meV are incident on a liquid parahydrogen target. The neutrons are captured, forming Deuterium and releasing a gamma ray with an energy of 2.2 MeV (the binding energy of Deuterium), which is subsequently detected. The detector array consists of 4 rings of 12 detectors each, where each ring is concentric around the neutron beam. The polarization of the incoming neutron beam is then reversed and the resulting gamma-ray is emitted in a different direction.

The measured quantity is the difference in the number of gamma rays emitted between the two neutron spin states. This difference (divided by the sum) forms a parity-violating asymmetry, A_γ. By studying the correlation between the spin of the incoming neutron (its polarization) and the direction of the emitted gamma ray, one primarily probes f_π, the long-range coupling constant used to describe the hadronic weak interaction.

The first phase of the NPDGamma experiment was carried out at Los Alamos National Lab in 2006, but did not have the statistical sensitivity to test the theoretical prediction (A_γ~-5x10^-8).

The next phase of the NPDGamma experiment will run at the Spallation Neutron Source at Oak Ridge National Lab. Data taking is projected to commence in early 2010.

References

1. A. Csoto, B. F. Gibson (1997). "Parity conserving gamma asymmetry in n-p radiative capture". Physical Review C 56 (631). arXiv:nucl-th/9704063. doi:10.1103/PhysRevC.56.631. 
2. Lauss, Bernhard and others (2006). "Precision measurement of parity violation in polarized cold neutron capture on the proton: The NPDgamma experiment". AIP Conf. Proc. 842: 790–792. doi:10.1063/1.2220383. 

External links

• Experiment Wiki