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

5.6 Estimation of correlated background for ✓ 13 measurement 151
The comparison between the prompt energy spectrum obtained from the
two SM selections is shown in Fig. 5.30. The spectra are normalised to the
same number of events in [15, 30] MeV. Within the statistical uncertainty
of the SM sample selected with the low delayed energy (⇠ 10 %), the two
SM prompt spectra are found in agreement. The SM spectrum obtained
by selecting the delayed events at high energy is then equivalent to the one
obtained from the low energy delayed.
5.6 Estimation of correlated background for ✓ 13
measurement
The FN spectral shape obtained implementing a tagging strategy based on
the IV has been found in agreement with a linear model. The linear fit to the
prompt energy spectrum provides a positive slope of (1.5 ± 0.9)/(3 MeV) 2 ,
with a
2 /dof =7.3/8. The integral of the fitted spectral shape provides a
FN rate of (0.33 ± 0.16) cpd with a relative uncertainty of 48 %.
The SM spectral shape 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 SM spectrum is found in agreement with a linear model. A
linear fit to the spectrum provides a positive slope of (0.4 ± 1.3)/(3 MeV) 2 ,
with a
2 /dof =6.9/7. The SM rate is found to be (0.62 ± 0.20) cpd with
a relative uncertainty of 37 %.
Since the FN and the SM spectral shapes are well described by a linear
model, the two components could be combined in order to reduce the statistical
uncertainty on the total correlated background rate. The combined
spectra is shown with the fitted linear model and its 1 uncertainty in
Fig. 5.31. The linear fit provides a positive slope of (1.3 ± 1.4)/(3 MeV) 2
and a
2 /dof =4.9/8. The total correlated background rate is found to be
(0.93 ± 0.26) cpd with a relative uncertainty of 28 %.
The total correlated background shape have also been obtained by analysing
the high energy tail of the prompt spectrum in Sec. 5.2. The high energy
analysis does not separate FN and SM components providing an estimation
of the total correlated background rate and shape. Such analysis has been
repeated without applying the OVV, in order to compare the results with
the combined FN and SM analysis. A linear fit to the high energy tail of
the prompt spectrum provides a negative slope of ( 0.6 ± 0.5)/(3 MeV) 2
and a
2 /dof = 72/66. Assuming the shape does not change at low energy,
the total correlated background rate of (1.17 ± 0.15) cpd is obtained extrapolating
the fitted shape at low energy.
Even if shape and rate obtained from the high energy analysis are compatible
with the results obtained by combining FN and SM shapes, the extrapolatel-00821629,
version 1 - 11 May 2013