Quark fragmentation to pi+-, pi0, K+-, p and anti-p in the nuclear environment

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.