Nuclear Modification of Two-hadron Correlations from HERMES

Semi-Inclusive DIS (SIDIS) of high energy leptons on nuclear targets has been used to study the nuclear medium modification of the quark fragmentation function. The possible interpretations and the implications on the parton energy loss are discussed. The first measurement of double-hadron production in deep-inelastic scattering within the nuclear medium is also presented. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or pre-hadronic scattering, and with a model based on a purely absorptive treatment of the final state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.