Measuring Neutron Polarisation in Deuteron Photo-disintegration with the CLAS Start Counter

Deuteron photo-disintegration (γd → np) is a reaction that represents the simplest case in which nuclear and hadron physics models can be tested. Despite this, associated polarisation analyses are limited in terms of angular coverage and energy ranges, especially in observables related to the recoil neutron. This is largely due to a lack in dedicated polarimetry equipment, and represents a roadblock in global progress to understand high-energy phenomena such as hexaquarks, and quark-gluon degrees of freedom.
To address this problem, this PhD thesis pioneers a new methodology for the parasitic measurement of nucleon polarisation using kinematic reconstruction of (spin-dependent) nucleon-nucleus scattering of reaction products, prior to their detection in large acceptance particle detector apparatus. Following this novel approach, which requires no dedicated polarimeter, a determination of the double polarisation observable, (neutron) Cx', from deuteron photo-disintegration is presented, using Jefferson Lab’s CLAS detector. The analysis utilises the (n,p) charge exchange reaction in CLAS’s "start counter" (plastic scintillator) to determine the final state neutron polarisations. The results present the first ever data for this observable above 0.7 GeV (photon beam energy) and significantly extend the angular range of the world data set. This new data is largely statistically consistent with the previous measurement of neutron Cx′ by Bashkanov et al. (2023) in the overlapping energy range of 0.4-0.7 GeV.
It is planned for the statistical accuracy of the presented result to be increased by the inclusion of additional data. The analysis herein serves as a key proof of concept for future applications, including a recommended similar analysis to be implemented with data from the more modern CLAS12 detector. This paves the way for a plethora of additional analyses using existing data sets that would provide crucial new constraints for hadron and nuclear physics.