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

136 5. Correlated background
Source stat. bkg. ✏ IV T ✏ t ⇢ t total
[%] [%] [%] [%] [%] [%]
Uncertainty 9.5 62.1 15.1 1.3 8.2 65.1
Table 5.9: Break down of statistical and systematic uncertainties.
Fit const. slope 2 /dof R FN (R FN )/R FN
model [1/(3 MeV)] [1/(3 MeV) 2 ] [cpd] [%]
tel-00821629, version 1 - 11 May 2013
Flat 33.5 ± 4.7 – 9.4/9 0.54 ± 0.07 14
Slope 10.7 ± 15.1 1.5 ± 0.9 7.3/8 0.33 ± 0.16 48
Table 5.10: Summary of FN spectral parameter and FN background rate
for the flat and the sloped linear model.
summarised in Tab. 5.10. The flat linear model gives
2 /ndf =9.4/9 and a
fit probability of ⇠ 40 %, while the sloped linear model gives 2 /ndf =7.3/8
and a fit probability of ⇠ 50 %. Since the sloped linear model shown a better
agreement with the data, it is assumed as FN spectral shape. A total rate
of (0.33 ± 0.16) cpd is found. The rate relative uncertainty of ⇠ 50 % is in
agreement with the expected total error estimated in Tab. 5.9.
5.4.5 Systematics uncertainty on the IVT threshold
The IVT sample and its related background have been obtained by moving
the multiplicity condition from 2 PMT hits to 4 and 6 PMT hits. The results
obtained with 4 and 6 PMT hits are shown in Fig. 5.20 and summarised
in Tab. 5.11. Increasing the multiplicity threshold the tagging e ciency
decrease, then the number of tagged events and the expected background
decrease. The spectral shape does not change by changing the IVT condition.
The FN rate change up to 25 %, however, such increase is within
uncertainty.
The choice of the tag multiplicity threshold do not change FN rate and
shape results, suggesting the systematical uncertainties related to the IVT
strategy are negligible.