Etude et impact du bruit de fond corrélé pour la mesure de l'angle ...

After the Sun, the other main natural source of neutrinos is the earth atmosphere.
An intense flux of cosmic rays, primarily protons, arrives on the high atmosphere producing
a huge number of secondaries, in particular pions. These particles then decay in flight via
± ! µ ± +⌫ µ (⌫ µ ). The produced muons again decay according to µ ± ! e ± +⌫ e (⌫ e )+⌫ µ (⌫ µ ).
The typical energy spectrum of atmospheric neutrinos starts at about hundred MeV and extends
up to 2.4several Measuring GeV. neutrino oscillation parameters 29
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Figure 1.7: Figure Di erent 2.7: Flight flight distances, between the their creation production point andpoint the SKand detector SuperKamiokande,
for neutrinos for neutrinos producedcreated in cosmic in atmosphere. ray interactions with the Earth atmosphere.
The measurement of light collected ofby the the atmospheric PMTs if theneutrino lepton stops oscillation into the requires detector. aEven di erent if technique
from thatthe used lepton for energy solar neutrinos, is not strongly mainly correlated because to the the neutrino atmosphere energy cannot it allowsbe considered
as a point-like to handle source the energy at fixedependence distance, of like theitoscillation was probability. case of the Sun. Neutrinos can be
generatedSK provided any point of 1998 the the atmosphere, first firm evidence thus neutrinos of of flavour the same transition energy born at the
same timecomparing can travel thevery expected di erent number distances of events before with thereaching observed the ones, detector as a func-and this gives
di erent oscillation of the zenith probabilities angle. SK(see observed figurethat 1.7). there Toare study twice the as many oscillation downward probability it is
going ⌫ µ than upward going ⌫ µ , as shown in Fig. 2.8 [96]. The hypothesis
that ⌫ µ have interacted crossing the earth is not reliable because the earth
19
is nearly transparent for neutrinos with energy of about few GeV and a
similar behaviour should have also been found for ⌫ e . Moreover, since no
excess of the electron neutrino flux has been found, the observed oscillation
is attributed to the transition ⌫ µ ! ⌫ ⌧ .
Accelerator experiments
Experiments using neutrino produced at accelerator have also been performed
to confirm the results obtained by SK.
The K2K experiment in Japan used ⌫ µ produced from a pulsed beam at
KEK and the SK detector placed at about 250 km. The average neutrino
energy is slightly above 1 GeV. Given the mass squared di↵erence of about
2.5 ⇥ 10 3 eV 2 measured with atmospheric neutrino by SK, the oscillation