Hadron Formation in Nuclei in DIS

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive DIS has been studied by the HERMES experiment at DESY with a 27.5 GeV positron beam. A large reduction of the differential multiplicity of charged hadrons from krypton relative to that from deuterium is observed. The reduction is larger than that seen in previously published HERMES data on nitrogen. The data are compared to two theoretical models. Both well describe the reduction of the multiplicity ratio at low values of the virtual photon energy ν and at high values of the fractional energy transfer z to the hadron. In order to obtain a better understanding of the hadronization process and possibly be able to distinguish between the two theoretical models the multiplicity ratio on the krypton data has been evaluated separately for pions, kaons and protons. (author)