Search for GUT Magnetic Monopoles with the MACRO Experiment at the Gran Sasso Lab

One of the primary aims of the MACRO detector at the Gran Sasso lab. (at an average depth of 3700 mwe) was the search in the penetrating cosmic radiation for the supermassive magnetic monopoles (MMs) predicted by Grand Unified Theories of the electroweak and strong interactions; the search was planned for a sensitivity level well below the Parker bound (∼ 10 cm s sr) and for a large range of velocities, 4 · 10 < β < 1, β= v/c. MACRO used three different types of detectors: liquid scintillators, limited steamer tubes and nuclear track detectors (NTDs; CR39 and Lexan) arranged in a closed-box modular structure, divided into a lower and an upper (“Attico”) part. The overall dimensions of the apparatus were 76.5×12×9.3m. MACRO took data from early 1989 till december 2000 [1]. The response of the three types of detectors to slow and fast particles was experimentally studied [2 4]. The use of three types of subdetectors with redundant electronics, in stand alone and combined modes, ensured redundancy of information, cross-checks and indipendent signatures for