Experiment Research
eg1-DVCS run period
E05-113: Semi-Inclusive Pion Production with a Longitudinally Polarized Target at 6GeV
E05-114: Deeply Virtual Compton Scattering at 6 GeV with polarized target and polarized beam using the CLAS detector
An important goal of JLab is to provide a detailed three-dimensional picture of the proton in terms of its quark and gluon constituents and to understand how this complex structure leads to its well known properties such as mass, spin and magnetic moment. Of special interest are topics such as how the quarks orbit around each other in the very small confined volume of the proton and how quarks and gluons of specific momentum and type interact with each other.
The eg1-DVCS experiments are the first comprehensive study in Hall B specifically focusing on this task using both a 6 GeV polarized electron beam and a polarized proton target. In this run period, the polarization directions will be parallel or anti-parallel to the beam direction. The polarization direction of the beam is reversed frequently (30 times per second), while the target polarization direction is changed once every few days. As in the earlier e1-DVCS experiment, the experimental setup uses the standard Hall B CEBAF Large Acceptance Spectrometer (CLAS) with the addition of a fine-grained calorimeter to detect forward-going photons. The polarized proton target has been used previously in the eg1 and eg4 run periods. The dynamic nuclear polarization technique is used to produce highly polarized protons in ammonia crystals immersed in very cold liquid helium and a high magnetic field.
Due to its large acceptance, many reactions can be measured simultaneously in CLAS. The common feature for the eg1-DVCS run period is that at least one scattered electron will be detected at a relatively large angle, in order to probe the short-distance structure of the proton. The two principal observables are the differences in scattering probability with different orientation of the beam and target spins. The DVCS reaction involves only a proton and a photon in the final state (in addition to the scattered electron), and it probes specific correlations in the proton. The semi-inclusive deep-inelastic scattering (SIDIS) reaction involves many pions in the final state (of which usually only one is detected), and it is sensitive to the transverse (sideways) motion of quarks in the proton. In addition, SIDIS can provide information about how the "up" quarks and "down" quark spins line themselves up with the way the proton as a whole is spinning about its axis (similar to the Earth rotating around the north and south poles).