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

128 5. Correlated background
Tag thr. evt. d P/D < 1.5 m evt. d P/D > 1.5 m ✏ d [%]
Mult. 2 3 16 84 ± 8
Mult. 4 2 10 84 ± 11
Mult. 6 1 9 90 ± 9
Table 5.4: Expected number of background events and background rejection
e ciency (✏ d ) with prompt/delayed distance cut, for di↵erent IVT
multiplicity condition. The background rejection e ciency is independent
from the IVT multiplicity condition, suggesting the systematic related to
the background rejection is negligible.
tel-00821629, version 1 - 11 May 2013
shown a distance between prompt and delayed bigger than 1.5 m and are
thus rejected by the distance cut. The remaining background amount to 3
events, corresponding to ⇠ 2 % of the IVT sample. The rejection e ciency
(✏ d ) is estimated to be 84 ± 20 %, the rather big statistical uncertainty is
due to the low statistics available by selecting events with only one o↵
time window.
The number of IVT events selected o↵-time and the background rejection
e ciency are summarised in Tab. 5.4, for di↵erent IVT multiplicity condition.
The rejection e ciency is found constant within statistical uncertainty,
suggesting the systematic related to background rejection, introduced by
the choice of the IVT multiplicity, is negligible.
¯⌫ e +IV
/n background
The ¯⌫ e +IV /n background does not present time nor spatial correlation
between prompt and IV energy depositions, as summarised in Tab.
5.3. The spatial and the time distributions between prompt and IV energy
depositions could then be used as discriminants in order to separate such
background from the IVT FN. The spatial correlation has not been evaluated
yet, since it requires an IV vertex reconstruction algorithm, which
is currently under development. However, the design of the IV has been
optimised for tagging high energy deposition from muons and not to detect
point-like energy deposition, as the ID. The IV vertex reconstruction algorithm
is then not expected to show good vertex resolution and consequently
an high background discrimination e ciency is not expected. On the other
hand, the pulse start time is available for both ID and IV waveforms. The
time distribution is expected to show better discrimination e ciency than
the spatial distribution, since the time is measured with an uncertainty of
the order of