Authors

Abstract

Background: Energetic quarks in nuclear deep-inelastic scattering propagate through the nuclear medium.Processes that are believed to occur inside nuclei include quark energy loss through medium-stimulated gluonbremsstrahlung and intranuclear interactions of forming hadrons. More data are required to gain a more completeunderstanding of these effects.Purpose: To test the theoretical models of parton transport and hadron formation, we compared their predictionsfor the nuclear and kinematic dependence of pion production in nuclei.Methods: We have measured charged-pion production in semi-inclusive deep-inelastic scattering off D, C, Fe,and Pb using the CLAS detector and the CEBAF 5.014-GeV electron beam. We report results on the nuclear-todeuterium multiplicity ratio for + and as a function of energy transfer, four-momentum transfer, and pionenergy fraction or transverse momentumthe first three-dimensional study of its kind.Results: The + multiplicity ratio is found to depend strongly on the pion fractional energy z and reachesminimum values of 0.67 0.03, 0.43 0.02, and 0.27 0.01 for the C, Fe, and Pb targets, respectively. Thez dependencies of the multiplicity ratios for + and are equal within uncertainties for C and Fe targets butshow differences at the level of 10% for the Pb-target data. The results are qualitatively described by the GIBUUtransport model, as well as with a model based on hadron absorption, but are in tension with calculations basedon nuclear fragmentation functions.Conclusions: These precise results will strongly constrain the kinematic and flavor dependence of nuclear effectsin hadron production, probing an unexplored kinematic region. They will help to reveal how the nucleus reactsto a fast quark, thereby shedding light on its color structure and transport properties and on the mechanisms ofthe hadronization process.