Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
Atmosphere
Detector
Earth
θz ν
20 km
ν
Cosmic Ray
10,000 km
Cosmic Ray
Figure 2.4: Scheme of Super-K atmospheric experiment. A down-going neutrino (θz ∼
0 o ) travels through the atmosphere above the detector (a distance of about 20 km),
whereas an up-going neutrino (θz ∼ 180 o ) has traveled through the entire Earth (a
distance of about 13000 km). Hence a measurement of number of neutrinos as a
function of the zenith angle yields information about their numbers as a function of
the traveled distance.
The expected fluxes
The main mechanism of production of the atmospheric neutrinos is given by the
following chain of reactions:
p(α, ...) + Air( 16 O, 14 N, mainly) → π ± (K ± ) + X
π ± (K ± ) → µ ± + νµ(¯νµ)
µ ± → e ± + νe(¯νe) + ¯νµ(νµ)
(2.6)
Atmospheric neutrinos can be observed directly in large mass underground detectors
predominantly by means of their CC interactions:
νe(¯νe) + A → e − (e + ) + X ,
νµ(¯νµ) + A → µ − (µ + ) + X (2.7)
but not distinction between neutrinos and anti-neutrinos can be made in Cerenkov
type detectors because they are not sensitive to the charge of the charged leptons
produced in the CC reactions of Eqs(2.7). Nevertheless, one can distinguish
electrons from muons since their Cherenkov rings observed by the phototubes
are sharp for muons whereas those produced by electrons are diffuse.
Naively, from the reaction chain (2.6) one would expect to have two atmospheric
muon neutrinos (or antineutrinos) for every electron neutrino (or antineutrino).
Actually, one has to take into account the lifetime differences of π ± , K ± and µ ±
as well as the differences in their spectra and the fact that the reaction chain
(2.6) though dominant is not the only one. The νµ/νe ratio also depends on
neutrino energy and on the zenith angle of neutrino trajectory. Calculations of
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