Study of hadron interactions in a lead-emulsion target

The 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. The 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. The 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 the 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.