Extracting neutron polarization observables using the CLAS start counter

The existence of hexaquark states has far-reaching consequences, such as our understanding of quark structure, and the mechanisms involved inside neutron stars[1]. Predicted in 1964[2], and recently discovered, the simplest non-trivial hexaquark, is the d*(2380), an “excited deuteron” state. The deuteron, comprised of a proton and neutron, can be excited to this state during deuteron photo-disintigration reactions with high photon energies (Eγ ~500-600 MeV). Several other bound/quasibound N-N* dibaryonic states can also be studied in this reaction. However, the world dataset of deuteron photo-disintegration has significant gaps in terms of photon energy coverage, angular coverage, and particularly in measurements of polarization observables. To address this problem, we have utilized experimental data from the CEBAF large acceptance spectrometer (CLAS) in a unique way.

CLAS was a many-component detector housed in Hall B of Jefferson Lab, a world leading international facility. One such component, the start counter, consisting of a set of scintillators surrounding the beamline, was used to determine the start time of an event originating in the target via photo-induced reactions. A novel approach that exploits the start counter as a nucleon polarimeter is implemented by this project. We will show analysis that has led to measurements of neutron polarization in deuteron photodisintegration between beam energies of 0.6 and 2.2 GeV, with a wide angular range, covering N-N* reaction dynamics in second and third resonance region, and providing exciting new insights into hexaquark studies.

[1] I. Vidana, M. Bashkanov, D. P. Watts, and A. Pastore, Phys. Lett. B 781, 112 (2018).

[2] F. J. Dyson and N.-H. Xuong, Phys. Rev. Lett. 13, 815 (1964).