( Proposal to Jefferson Lab PAC 30 ) Measurement of the Ratio R = σ L / σ T in Semi - Inclusive Deep - Inelastic Scattering July 6 , 2006

We propose to perform measurements of the ratio R = σ L /σ T of longitudinal to transverse cross sections in pion electroproduction. Our emphasis will be on measurements in the so-called semi-inclusive deep inelastic scattering region, with extensions into the exclusive region. Whereas inclusive scattering can not 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 depend on z, Q 2 , or p T , the transverse pion momentum. 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: They will allow us to study the inclusive-exclusive connection in pion electroproduction, a process where duality has recently been shown to be valid, and are a sine qua non for the interpretation of flavor decomposition by semi-inclusive deep inelastic scattering at a 12-GeV JLab (and other low-energy facilities). We will detect charged pions (π ±) in coincidence with scattered electrons from semi-inclusive (e, e π) reaction on LH2 and LD2 targets at energies 6.6, 8.8 and 11.0 GeV, and beam currents of 50 µA. The Hall C HMS spectrometer and the projected SHMS with its first-generation detector package will be used for electron and meson detection.