High resolution hypernuclear spectroscopy at Jefferson Lab, Hall A: The experimental challenge

The E94-107 experiment in Hall A at Jefferson Lab has started a systematic study of 1p-shell hypernuclei. Data have been taken on C-12, Be-9 and O-16 targets. The counting rate for hypernuclear electroproduction decreases dramatically as the scattering angle increases. Therefore, the electron scattering angle has to be as forward as possible to get high virtual photon flux and kaon angle has to be as close as possible to the virtual photon direction to minimize momentum transfer. In order to allow experiments at very forward angle in Hall A, two superconducting septum magnets were added to the High Resolution Spectrometers (HRS). The two magnets bend particles scattered at 6 degrees into each HRS, introducing only a small perturbation on the HRS optics thus preserving the excellent momentum resolution of the HRS. With the new setup a momentum resolution of 10(-4) FWHM on both HRS arms was obtained. One of the challenges of the experiment at very forward angle is the identification of very small peaks in the missing-energy spectrum; this requires a powerful Particle Identification (PID) system that provides unambiguous kaon selection.