Measurement of charged-pion production in deep-inelastic scattering off nuclei with the CLAS detector

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 gluon bremsstrahlung and intranuclear interactions of forming hadrons. More data are required to gain a more complete understanding of these effects.Purpose: To test the theoretical models of parton transport and hadron formation, we compared their predictions for 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-to-deuterium multiplicity ratio for ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ as a function of energy transfer, four-momentum transfer, and pion energy fraction or transverse momentum---the first three-dimensional study of its kind.Results: The ${\ensuremath{\pi}}^{+}$ multiplicity ratio is found to depend strongly on the pion fractional energy $z$ and reaches minimum values of $0.67\ifmmode\pm\else\textpm\fi{}0.03, 0.43\ifmmode\pm\else\textpm\fi{}0.02$, and $0.27\ifmmode\pm\else\textpm\fi{}0.01$ for the C, Fe, and Pb targets, respectively. The $z$ dependencies of the multiplicity ratios for ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ are equal within uncertainties for C and Fe targets but show differences at the level of $10%$ for the Pb-target data. The results are qualitatively described by the GiBUU transport model, as well as with a model based on hadron absorption, but are in tension with calculations based on nuclear fragmentation functions.Conclusions: These precise results will strongly constrain the kinematic and flavor dependence of nuclear effects in hadron production, probing an unexplored kinematic region. They will help to reveal how the nucleus reacts to a fast quark, thereby shedding light on its color structure and transport properties and on the mechanisms of the hadronization process.

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