  The influence of the nuclear medium on the production of charged hadrons in
semi-inclusive deep-inelastic scattering has been studied by the HERMES
experiment at DESY using a 27.5 GeV positron beam. The differential
multiplicity of charged hadrons and identified charged pions from nitrogen
relative to that from deuterium has been measured as a function of the virtual
photon energy \nu and the fraction z of this energy transferred to the hadron.
There are observed substantial reductions of the multiplicity ratio R_M^h at
low \nu and at high z, both of which are well described by a
gluon-bremsstrahlung model of hadronization. A significant difference of the
\nu-dependence of R_M^h is found between positive and negative hadrons. This is
interpreted in terms of a difference between the formation times of protons and
pions, using a phenomenological model to describe the \nu- and z-dependence of
R_M^h.
