Update of Experiment E 12 - 06 - 104 Measurement of the Ratio R = σ L / σ T in Semi - Inclusive Deep - Inelastic Scattering July 01 , 2010

1 Executive Summary E12-06-104 seeks to perform measurements of the ratio R = σ L /σ T of longitudinal to transverse cross sections in pion electroproduction. The emphasis will be on measurements in the so-called semi-inclusive deep inelastic scattering region, where essentially no information exists on the value of R, with extensions into the exclusive region. Whereas inclusive scattering cannot distinguish between the quark flavor, there is great promise in flavor decompositions of regular parton distributions through semi-inclusive deep inelastic scattering and of generalized parton distributions through deep exclusive scattering. For the latter, the ratio R = σ L /σ T asymptotically scales like Q 2 , the four-momentum transfer squared, at fixed Bjorken x. For the former, the ratio R is assumed to be similar to that of deep inelastic scattering. Surprisingly, the latter assumption has never been thoroughly checked. Obviously, with less and less energy available to produce mesons, as is the case close to the exclusive limit (where the elasticity z → 1), this assumption must locally fail. In fact, the ratio R for semi-inclusive pion electroproduction may at low energies very well depend both on z and p T , the transverse pion momentum, beyond the well-known dependence on x and Q 2 of R in deep inelastic scattering. To measure this behavior, for the first time, is the subject of this proposal, for which we request 40 days of beam time. The proposed measurements are both of fundamental and of practical value: Fundamental since the ratio R in semi-inclusive deep inelastic scattering has never been properly measured; Practical because many semi-inclusive deep inelastic scattering experiments at a 12-GeV JLab (and other low-energy facilities) requiring knowledge on the ratio R as input. Beyond this, the new data will allow further study of the inclusive-exclusive connection in pion (and kaon) electroproduction, and the onset of factorization in terms of a quark scattering process followed by a quark-hadron fragmentation proceed. We will detect charged pions (π ±) in coincidence with scattered electrons from the semi-inclusive (e, e π) reaction on 10 cm extended LH2 and LD2 targets at beam energies of 6.6, 8.8 and 11.0 GeV, and beam currents of up to 50 µA. The Hall C HMS spectrometer and the projected SHMS with its first-generation detector package will be used for electron and pion detection. Members of this collaboration have spearheaded the now-flourishing semi-inclusive deep inelastic scattering …