  The influence of the nuclear medium on lepto-production of hadrons was
studied in the HERMES experiment at DESY in semi-inclusive deep-inelastic
scattering of 27.6 GeV positrons off deuterium, nitrogen and krypton targets.
The differential multiplicity for krypton relative to that of deuterium has
been measured for the first time for various identified hadrons ($\pi^+$,
$\pi^-$, $\pi^0$, $K^+$, $K^-$, $p$ and $\bar{p}$) as a function of the virtual
photon energy $\nu$, the fraction $z$ of this energy transferred to the hadron,
and the hadron transverse momentum squared $p_t^2$. The multiplicity ratio is
strongly reduced in the nuclear medium at low $\nu$ and high $z$, with
significant differences among the various hadrons. The distribution of the
hadron transverse momentum is broadened towards high $p_t^2$ in the nuclear
medium, in a manner resembling the Cronin effect previously observed in
collisions of heavy ions and protons with nuclei.
