Photon Beam Polarimetry Using Detection of e⁺e^- Pairs at GlueX

Bethe-Heitler e⁺e^- pair production provides an important method for determining photon beam polarizations. Conventional techniques are pair production on atomic electrons (triplet polarimeter, TPOL) and pair production on a nuclear target with detection of one lepton. For the Charged Pion Polarizability (CPP) experiment at GlueX there is a need for a precision cross-check on TPOL that can be synchronous with data taking. In this talk we present a new technique and result for measurement of linear beam polarization through the detection of both e⁺and e^- in the forward drift chambers and calorimeters of the GlueX detector. The polarized form of the Bethe-Heitler cross-section shows that the analyzing power for the azimuthal distribution of e⁺e^- is 60% in the kinematics of the CPP experiment. Using a neural net for e/π separation, we analyzed the GlueX 2018-01 data set and measured an average beam polarization of 0.325 ± 0.007. This result is within 2σ of the polarization from the TPOL measurement. Obtaining linear beam polarization from e⁺e^- events produced during data taking can be applied in photo-production experiments where detection of e⁺e^- pairs is achieved with good particle ID.