The Monopole, Astrophysics and Cosmic Ray Observatory (MACRO) is an underground muon detector at Gran Sasso, which is now adding to the evidence for neutrino oscillations.
Fully operational since 1994-95, MACRO’s bread-and-butter physics is the detection of cosmic-ray muons, but its ultimate objective is to search for new phenomena and to pick up particles from cosmic sources such as supernovae. In its search for cosmic signals, MACRO is assisted by the EAS-TOP array on the mountain 1400 metres above.
MACRO intercepts particles which pierce the overhead rock shield. 77 metres long, 12 metres across and 9 metres high, the detector is divided lengthways into six modules. The bottom half of the detector is composed of seven layers of crushed rock absorber interspersed with streamer tubes, together with an outer cladding of scintillator and streamer tube detectors and a box-like top layer with scintillator and streamer chamber walls and roof running the length of the detector.
While magnetic monopoles continue to be elusive, a bonanza for MACRO is the study of muons produced by neutrino interactions inside the detector, confirming an intriguing effect seen in other detectors. These studies show a marked difference between the signals due to upward- and downward-moving neutrinos.
Multistage decays
On the Earth’s surface, muon-like neutrinos (as distinct to electron-like) mainly result from the decay of particles produced by high-energy cosmic rays hitting nuclei high up in the atmosphere. These reactions produce kaons and pions, which themselves decay to produce muons, which in their turn decay. In these multistage decays, the end result should be that there are twice as many neutrinos producing muons as producing electrons.
However, the detectors see fewer such muons than expected. MACRO, showered by other cosmic muons from above, cannot isolate those downward muons that are due to neutrinos, but does see a clean signal due to muon neutrinos arriving from below, which have passed right through the Earth before hitting the detector.
There are considerably fewer of these upward muons than expected, underlining the suggestion that muon neutrinos “oscillate” on their way through the Earth, changing into other neutrino types which do not produce muons.
