  Topological and kinematical characteristics of hadron interactions have been
studied using a lead-emulsion target exposed to 2, 4 and 10 GeV/c hadron beams.
A total length of 60 m $\pi^-$ tracks was followed using a high speed automated
emulsion scanning system. A total of 318 hadron interaction vertices and their
secondary charged particle tracks were reconstructed. Measurement results of
interaction lengths, charged particle multiplicity, emission angles and momenta
of secondary charged particles are compared with a Monte Carlo simulation and
appear to be consistent. Nuclear fragments emitted from interaction vertices
were also detected by a newly developed emulsion scanning system with
wide-angle acceptance. Their emission angle distributions are in good agreement
with the simulated distributions. Probabilities of an event being associated
with at least one fragment track are found to be greater than 50% for beam
momentum $P > 4$ GeV/c and are well reproduced by the simulation. These
experimental results validate estimation of the background due to hadron
interactions in the sample of $\tau$ decay candidates in the OPERA $\nu_{\mu}
\to \nu_{\tau}$ oscillation experiment.
