High statistics measurement of the underground muon pair separation at Gran Sasso - art. no. 032001

We present a measurement of the underground decoherence function using mult i-muon events observed in the MACRO detector at Gran Sasso at an average de pth of 3800 hg/cm(2). Muon pair separations up to 70 m have been measured, corresponding to parent mesons with P-perpendicular to less than or equal t o 1-2 GeV/c. Improved selection criteria are used to reduce detector effect s mainly in the low distance separation region of muon pairs. Special care is given to a new unfolding procedure designed to minimize systematic error s in the numerical algorithm. 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 decohe rence function is compared with the predictions of the hadronic interaction model of the HEMAS Monte Carlo code. Good agreement is obtained. We interp ret this agreement to indicate that no anomalous P-perpendicular to compone nts in soft hadron-nucleus and nucleus-nucleus collisions are required by t he MACRO experimental data. Preliminary comparisons with other Monte Carlo codes point out that the uncertainties associated with the hadronic interac tion model may be as large as 20%, depending on the energy. MACRO data can be used as a benchmark for future work on the discrimination of shower mode ls in the primary energy region around and below the knee of the spectrum. [50556-2821(99)03011-8].