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

5.7 Conclusion 157
5.7 Conclusion
tel-00821629, version 1 - 11 May 2013
In this chapter the correlated background a↵ecting the ¯⌫ e search in DC has
been discussed. The correlated background is produced by muons interacting
in the detector or in its surrounding, whose prompt and delayed come
from the same physical process. Such background produces a prompt-like
and a delayed-like events which show similar time and spatial correlations
as ¯⌫ e signal, i.e time correlation within 100 µs and spatial correlation within
few tens of cm.
Most of the correlated background is reduced < 0.2 % by applying a 1000 µs
veto time upon an identified muon interacting in the ID or in the IV. The
remaining background is estimated by dedicated analysis.
Two contributions to the remaining correlated background have been identified,
FN and SM. Any indications of further contributions have not been
found within the sensibility provided by the current data, but are not excluded
a priori.
The FN component have been separated by the SM by requiring the time
correlation between the prompt and the delayed events to be > 10 µs. A
low energy sample has been selected with an e ciency of 33 ± 5%bya
tag strategy based on the number of PMT hit in the IV. FN are IVT if the
ID prompt candidate is found in coincidence with an IV energy deposition
presenting PMT multiplicity > 2. Two sources of background on the tagged
FN sample has been identified and reduced to ⇠ 14 %, obtaining a ⇠ 86 %
pure IVT FN sample. The remaining background has been measured in an
o↵-time window and subtracted in order to minimise the distortions on the
energy spectrum.
The SM component is obtained by requiring the time correlation between the
prompt and delayed events to be < 10 µs. The spectral shape at low energy
is obtained taking advantage from the delayed Michel electron spectrum,
selecting the delayed events at high energy in order to reduce contamination
from ¯⌫ e and FN. The prompt spectrum is found independent from the choice
of the energy window used to select the delayed event.
The total correlated background rate has been obtained by combining the
FN and the SM components, in order to reduce the statistical uncertainty.
The total correlated background rate is found to be (0.93 ± 0.26) cpd.
Consistent results have been obtained from an independent analysis based
on the OVT and also by analysing the high energy tail of the prompt spectrum.
Since the OV is observed to tag the correlated background with high purity,
the ¯⌫ e selection is performed by rejecting the events found in coincidence
with an OV hit, in order to reduce the background contamination.
A reduction of the correlated background of ⇠ 30 % is observed and a total
rate of (0.67 ± 0.20) cpd is obtained.