  We present a measurement of the underground decoherence function using
multi-muon events observed in the MACRO detector at Gran Sasso at an average
depth of 3800 hg/cm**2. Muon pair separations up to 70 m have been measured,
corresponding to parent mesons with P_t < 1-2GeV/c. The accuracy of the
measurement is such that the possible contribution of rare processes, such as
mu + N ----> mu + N + mu- + mu+, can be experimentally studied. The measured
decoherence function is compared with the predictions of the hadronic
interaction model of the HEMAS Monte Carlo code. Good agreement is obtained. We
interpret this agreement to indicate that no anomalous P_t components in soft
hadron-Nucleus and Nucleus-Nucleus collisions are required by the MACRO
experimental data.
