MACRO results on atmospheric neutrino oscillations

A high energy primary cosmic ray interacts in the upper atmosphere producing a large number of pions and kaons, which decay yielding muons and muon neutrinos; the muons decay generating muon and electron neutrinos. These neutrinos are produced in a spherical shell at 10 − 20 km above ground and travel towards the Earth. The νμ can be detected by an underground experiment as muons, after their charged current interactions with the material inside or outside the detector (νμN → μ +− + ...). MACRO was a large area multipurpose underground experiment optimized to search for rare events in the cosmic radiation. The detector was located in the Gran Sasso laboratory, which is well shielded from downgoing cosmic ray muons by a mean rock thickness of 3700 m.w.e.. MACRO was composed of three sub-detectors: liquid scintillation counters for energy and T.O.F. measurements, limited streamer tubes for particle tracking and nuclear track detectors for magnetic monopoles and nuclearites searches. It had a modular structure: it was divided into six sections referred to as supermodules, each of which had a size of 12.6× 12× 9.3 m. A cross section of the detector is shown in Fig. 1 on the left, with the different event topologies of νμ interactions in or around the detector: Internal Upgoing (IU), Upgoing Stopping (UGS) and Internal Downgoing (ID), and Up throughgoing muons. Data were taken from March 1989 till April 1994 with the detector under construction and from the middle of 1994 till the end of 2000 with the full detector.