DVCS@JLab

Deeply Virtual Compton scattering in Hall A

E07-007 and E08-025 experiments

New QCD factorization theorems establish that exclusive reactions such as H(e,e'γ)p and H(e,e'π⁰)p, in the deeply virtual limit of large Q², W, and -t< Q² are determined (in leading twist, or leading term in an expansion in inverse powers of Q²) by a new class of quark and gluon matrix elements, called Generalized Parton Distributions (GPDs). The GPDs correlate the transverse spatial distribution of the struck parton (quark or gluon) with the light-cone momentum fraction of the parton in the target.

Hall A experiment E00-110 measured the dependence of the helicity dependent H(e,e'γ)p cross section at x_Bj=0.36 and Q² = 1.5, 1.9, and 2.3 GeV². The results are consistent with dominance of the leading twist ℑ[BH*DVCS] term. The experiment also measured the helicity independent cross section at Q² = 2.3 GeV². The latter results show evidence for strong contributions from both the Real part of the BH*DVCS interference and the pure |DVCS|² terms (as well as the |BH|² term).

E12-06-114 was approved by the JLab PAC30 in August, 2006. With a new calorimeter, the DVCS and deep virtual π⁰ measurements in x_Bj,Q² to (0.7,9.0 GeV²) will be extended. In PR07-007, approved by the JLab PAC31 in January, 2007, DVCS measurements at fixed Q², x_Bj and two different beam energies (6 and 4 GeV) are proposed. The kinematic structure of the ep→ epγ cross section will allow to separate the Real part of the BH*DVCS interference from the pure |DVCS|² term at all three Q² values of the previous E00-110. Rosenbluth L/T separations of the ep→ epπ⁰ cross section, which is crucial to testing factorization in this channel will be obtained.