Speaker
Description
The Charged and Neutral Pion Polarizability Experiments (CPP and NPP) at GlueX require a precision measurement of the linear polarization of the incident photon beam.The utilization of electron-positron ($e^+e^-$) pair production is a valuable technique for determining photon beam polarizations.Polarization determination from pair production on atomic electrons (the Triplet Polarimeter, TPOL technique) or on a nuclear target with the detection of a single lepton are reliable methods to this end. This talk will show a new technique and the subsequent result for measurement of linear beam polarization through detection of $e^+e^-$ pairs in the GlueX forward drift chambers and calorimeters. The analyzing power for the azimuthal distribution of $e^+e^-$ pairs accessible within GlueX kinematics is approximately $60\%$. Using a neural net designed for $e/\pi$ separation, the spring 2018 GlueX data set was analyzed and measured to have an average beam polarization of $0.349 \pm 0.009$. This result agrees with the TPOL measurement of polarization within $0.85$ standard deviations. This technique to derive linear beam polarization from $e^+e^-$ pairs obtained during data taking can be applied to photo-production experiments where detection of $e^+e^-$ pairs is achieved with good particle identification and good angular acceptance in the forward angles. This work was supported in part by the U.S. Department of Energy award DE-FG02-88ER40415 and by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177.
