Etude de bruit de fond induit par les muons dans l'expérience ...

tel-00724955, version 1 - 23 Aug 2012
Conclusion
The sensitivity of a WIMP experiment is set by its rate of background events
which are indistinguishable from WIMP candidates. Neutrons induced either
directly by cosmic ray muons or indirectly by electromagnetic processes in the shower
of a muon have to be considered as a limiting background source. The active muon
shield, so called muon veto, of the EDELWEISS-II experiment is made to reject
muon-induced background, by associating cosmic ray muons in the vicinity of the
experiment with neutron recoil events in the bolometers.
The EDELWEISS-II muon veto has already run for three years and is continuously
taking data. Despite some discrepancies with the simulation, the analysis of
the events yields a good detection and identification of muon candidates and their
showers.
Coincidences between the muon veto system and the bolometers are of great
interest to reduce and understand the muon-induced background. The first analysis
of coincidences between the muon veto and the bolometer systems during Run 8 is
successful. Although the definition of the time of the event in the analysis B makes
the coincidence events to be artificially centered at 25 ± 10 ms, the EDELWEISS-II
experiment does identify coincidences between bolometer events and muon candidates
of the veto as a clear excess compared to the expected accidental rate. The
coincidence rates are
for ER < 250 keV, Γ Run8
coinc = 0.043 ± 0.015 events·(kg·d) −1
for ER ≥ 250 keV, Γ Run8
coinc = 0.09 ± 0.02 events·(kg·d) −1
(6.20)
(6.21)
which can be compared and are in agreement with the simulated rate at low energy.
However, the analysis was not as complete as it could have been, since there
was no overall clock on the experiment. Since each data acquisition had its own
independent clock, defining the time of the different sets of bolometers has been
difficult, and periods have been removed regardless of the quality of the data. The
lack of one global clock for the entire system was a huge problem during Run 8.
For Run 10, a new device to dispatch a unique time for all sub-systems was
installed. However, in contrast to Run 8, there was no calibrated data available
yet for Run 10. The second analysis is based on the “ntp” files, which are the
online pre-processed data, usually used for the monitoring. Using the ntp files,
the rate of bolometer was 10 times higher and the coincidences with muon veto,
if any, completely overwhelmed in background. To perform the analysis, a drastic
cut has been applied: only events with EH > 30 keV and EI > 30 keV are kept.
The advantage of the cut is that it removes all low ionization and low heat events,
without applying a complex bolometer period analysis as the one which has been
performed for the analysis of Run 8. The disadvantage is that it will also cut any
low Q, low ER muon-induced neutron-like events, as the ones found in Run 8.
The analysis of the bolometers with the ntp is not meant to be a full standard
reference analysis, but it allows to have a first look into the bolometer data and to
further look for coincidences with the muon veto. And the analysis of coincidences
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