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

5.5 Stopping Muon analysis 139
5.5.1 High energy light noise
tel-00821629, version 1 - 11 May 2013
The light noise cut applied for the ¯⌫ e searches (Q max /Q tot < 0.09 and
RMS(Tstart) < 40 ns) are optimised for low energy events and do not
reject HELN in an e cient way. However, LN shows negligible time correlation
between prompt and delayed events and a pure sample is then selected
by performing o↵-time selection in [1000, 1100] µs time window, rescaling
the sample to [0, 10] µs time window used for on-time selection.
Fig. 5.21, 5.22 and 5.22 show comparison between on-time (black) and o↵time
(violet) selection.
The Fig. 5.21 (top left) shows the prompt energy spectrum. The low energy
part of the on-time selection, < 3 MeV, shows a spectral shape similar to the
one produced by natural radioactivity gamma, while it appear to be rather
flat > 3 MeV. The low energy spectrum is well reproduced by the o↵-time
HELN selection, suggesting the HELN is due to accidental coincidence from
a natural radioactivity gamma and a high energy light noise emitted from
some PMT. O↵-time coincidence with an higher energy prompt candidate,
> 12 MeV, are also observed, suggesting some accidental coincidence between
two HELN events or between 12 B decay products and HELN.
The Fig. 5.21 (top right) shows the delayed energy spectrum. There is less
HELN contamination in the low energy part of delayed spectrum, < 40 MeV,
while the o↵-time selection reproduce well the higher part of the spectrum,
suggesting more contamination.
The time distributions are shown in Fig. 5.21 (bottom). The o↵-time selection
shows a flat distribution as expected for uncorrelated random coincidences,
while the on-time sample shows the correlated distribution from
SM, with a time constant of 1.9 ± 0.2 µs, defined by the muon life time.
The vertex distributions for prompt and delayed events are shown in Fig. 5.22.
The on-time prompt distributions (top left and right) shows clear excess of
events below the detector chimney, as expected by SM, while the o↵-time
distribution shows events distributed more widely in the detector, as expected
from natural radioactivity gammas. The delayed vertex distribution
(bottom left and right) shows an excess below the chimney and some others
peaks at fixed positions. Such peaks are well reproduced by the o↵-time
selection and are due to LN. The packed distribution is an artefact of the
vertex reconstruction algorithm, which reconstruct the LN vertex close to
the PMT responsible of the light emission.
Finally plots in Fig. 5.23 show the Q max /Q tot variable as a function of the
prompt (left) and delayed (right) energy. The Q max /Q tot variable is used together
with the RMS(Tstart) as low energy light noise discriminant for the
signal selection. Physical events are selected by requiring Q max /Q tot < 0.09
for the prompt and Q max /Q tot < 0.06 for the delayed candidate. The
Q max /Q tot ratio is expected to have a 1/E behaviour since the total deposited
charge is proportional to the energy. However, such 1/E behaviour