Extracting the spectral function of ⁴He from a relativistic plane-wave treatment

The spectral function of ⁴He is extracted from a plane-wave approximation to the (e,e′ p) reaction using a fully relativistic formalism. We take advantage of both an algebraic "trick" and a general relativistic formalism for quasifree processes developed earlier to arrive at transparent, analytical expressions for all quasifree (e,e′ p) observables. An observable is identified for the clean and model-independent extraction of the spectral function. Our simple relativistic plane-wave calculations provide baseline predictions for the recently measured, but not yet fully analyzed, momentum distribution of ⁴He by the A1 Collaboration from Mainz. Yet in spite of its simplicity, our approach predicts momentum distributions for ⁴He that rival some of the best nonrelativistic calculations to date. Finally, we highlight some of the challenges and opportunities that remain, both theoretically and experimentally, in the extraction of quasifree observables.