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Externally produced events

External events are produced by neutrino interaction in the rock around the detector so the effective target is larger. The detector sensitivity depends on the surface and not on the active mass. Two kinds of events can be detected: a) Stopping muons: low energy muons produced from neutrino interaction in the rock and stopping inside the detector. In the interpretation of the data there are problems connected with the low energy neutrino cross-section and with a possible background due to large angle pions produced from the cosmic ray muons. b) Through-going muons: upward-going muons produced from neutrino interactions in the rock below the apparatus and crossing the entire apparatus. If the oscillation hypothesis is true and with the value suggested by internal events, it should be possible to see a clear signature in the angular distribution of the up-going muons. In fact vertical neutrinos (having a path length of the order of 13000 Km) should be attenuated (a factor 2 for maximum mixing) , while horizontal neutrinos (having a shorter path length) should not be attenuated. Data on up-going through-going muons come from Kamiokande, Superkamiokande (April 1997), IMB, Baksan and MACRO. The Baksan detector, located in the Baksan underground laboratory in Russia at a minimum depth of 850 hg/cm2, consists of 3150 liquid scintillator counters. MACRO consists of liquid scintillators and streamer-tube chambers. It is located in the Gran Sasso underground laboratory. The neutrino induced upward-going muons must be separated from a huge background due to down-going muons. The selection is based on the directionality of the Cerenkov light or on the time of flight measurement (Baksan, MACRO). Table [*] summarises the available statistics. The angular distribution of the data is at the moment not conclusive for the presence of oscillations.
 
Table: Upward-going muons data. The prediction is based on the Bartol neutrino flux. The theoretical uncertainty (17% ) is not included in the errors.
 
  IMB Kamiokande SuperKamiokande Baksan MACRO
data 617 364 267 558 351
$\frac {data}{prediction}$ 0.93 0.91 0.88 $\pm0.05_{stat}$ 1.0 $\pm0.04_{stat}$ 0.74 $\pm0.04_{stat}$
  $\pm0.04_{stat}$ $\pm0.05_{stat}$   $\pm0.08_{syst}$ $\pm0.06_{syst}$
 
 


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