Measurement of unpolarized semi-inclusive pi+ electroproduction off the   proton
  Semi-inclusive pi+ electroproduction on protons has been measured with the CLAS detector at Jefferson Lab. The measurement was performed on a liquid-hydrogen target using a 5.75 GeV electron beam. The complete five-fold differential cross sections were measured over a wide kinematic range including the complete range of azimuthal angles between hadronic and leptonic planes, phi, enabling us to separate the phi-dependent terms. Our measurements of phi-independent term of the cross section at low Bjorken x were found to be in fairly good agreement with pQCD calculations. Indeed, the conventional current fragmentation calculation can account for almost all of the observed cross section, even at small pi+ momentum. The measured center-of-momentum spectra are in qualitative agreement with high energy data, which suggests a surprising numerical similarity between the spectator diquark fragmentation in the present reaction and the anti-quark fragmentation measured in e+e- collisions. We have observed that the two phi-dependent terms of the cross section are small. Within our precision the cos(2phi) term is compatible with zero, except for low-z region, and the measured cos(phi) term is much smaller in magnitude than the sum of the Cahn and Berger effects. 
CLAS Collaboration: M. Osipenko, M. Ripani, G. Ricco, H. Avakian, R.   De Vita, et al