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

72 3. The Double Chooz experiment
The likelihood is defined as:
L(X) = Y
f q (0; µ i ) Y
q i =0
q i >0
f q (q i ; µ i ) ⇥ f t (t i ; t (pred)
i
; µ i ) (3.12)
The first product concerns only PMTs that have not been hit, while the
second product concerns the remaining PMTs that have been hit (non-zero
charge q i reconstructed at time t i ). The two function f q and f t are the
charge and time probability density function obtained from MC simulation
and validated against physics and calibration data [66]. The event reconstruction
consists to find the set of event parameters X min which minimise
the negative log-likelihood function:
F (X = ln L(X) = X i
ln f q (q i ; X)+lnf t (t i ; X) =F q (X)+F t (X) (3.13)
tel-00821629, version 1 - 11 May 2013
The vertex reconstruction can be performed using only one of the two terms
in Eq. 3.13, F q to perform charge-only reconstruction, or F t to perform
time-only reconstruction, but using information from both charge and time
improves the vertex accuracy. The vertex reconstruction accuracy was evaluated
using 60 Co calibration source deployed at known positions along the
z-axis and the guide tube. Source positions were reconstructed with a spatial
precision of about 12 cm [80].
During the detector commissioning, a first preliminary validation of both
the vertex reconstruction algorithm and the liquid scintillator time response
was provided studying the time response of the scintillator. The scintillator
time response of Fig. 3.23 shows the characteristic time behaviour of the
liquid scintillator such as the fast excitation time (⇠ few ns) and the slower
de-excitation (⇠ few hundreds ns), it also shown the expected 4 ns time
spread around the peak (vertex resolution and read-out time spread) and
hint of PMT late pulse at about 60 ns.
3.8.3 Muon tagging and track reconstruction
Cosmic muons that reach the underground detector hall, cross the detector
and deposit large amounts of energy. More importantly, the muon-correlated
physics that follows the passage of a muon in the detector (mainly fast neutrons
and cosmogonical-produced isotopes) could mimic the ¯⌫ e signature.
For such reasons it is important to tag muons crossing the detector and
reconstruct their track.
Tagging a muon is not only about identifying a pure sample of cosmic-muons
going through the detector, but identifying the muon or any indirect traces
associated to the presence of cosmic-muons in the neighborhood of the detector.
Examples of indirect traces of the presence of a muon in or near the
detector are the observation of Michel electrons/positrons (whose beta spectrum
can reach ⇠ 60 MeV) and fast neutrons (whose detection is granted