Hadronic fragmentation initiated by pointlike probes

Isotopic ratios of fast mesons produced in the target-fragmentation r\'egion in reactions initiated by pointlike probes should resemble those seen in ordinary hadronic reactions when the probe removes sea partons from the target but should show dramatic changes once the probe knocks out valence partons. We use simple parton-fusion models to predict the shapes and magnitudes of inclusive meson spectra (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\ensuremath{-}}$,${K}^{+}$,${K}^{\ensuremath{-}}$) in the target-fragmentation region of $\ensuremath{\mu}p$, $\ensuremath{\nu}p$, and $\overline{\ensuremath{\nu}}p$ semi-inclusive scattering and in the inclusive production of mesons in association with a massive lepton pair: $\mathrm{pp}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}\mathrm{MX}$ or $p\overline{p}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}\mathrm{MX}$. Our predictions are in agreement with the limited existing data on these reactions and indicate areas where experiments may shed light on the correlated distribution of several partons in the nucleon.