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

tel-00724955, version 1 - 23 Aug 2012
5
110 Analysis of Run 8
events in [−1000, +15] ∨ [+35, +1000] ms [+15, +35] ms
ER ≥ 50 keV Q < 0.6 23 (6.3%) 0 (0%)
Q ≥ 0.6 245 (67.5%) 11 (68.8%)
ER < 50 keV Q < 0.6 11 (3.0%) 4 (25.0%)
Q ≥ 0.6 84 (23.2%) 1 (6.2%)
Table 5.5: Number of events outside and inside the coincidence interval in 4
different regions defined by the conditions ER ≥ 50 keV or ER < 50 keV and
Q ≥ 0.6 or Q < 0.6.
determine which kind of events they are.
The resolution lines for the Q-plot are defined in Section 3.1.5, with input variables
as listed in Appendix A.1. The results for the 16 events at ER < 250 keV and
∆tcoinc ∈ [+15, +35] ms is shown in Figure 5.13. Most of the events are enclosed in
the γ-region band around Q ∼ 1 and are likely to be γ-particles, except for 4 events.
Although, there is no condition on the bolometer multiplicity in this Q-plot, all
events are the coincidences of the muon veto with only 1 individual bolometer.
These 4 events would not have been removed under the condition of multiple hits
to exclude neutron-like events.
The distribution of coincidence events inside the coincidence window can be compared
to the one outside over ∆twin = ±1 s, which is pure uncorrelated background.
The comparison can be made defining 4 regions with the conditions ER ≥ 50 keV
or ER < 50 keV and Q ≥ 0.6 or Q < 0.6, see Table 5.5. In brackets is the percentage
of the selection compared to the total number of events. The ratio of events
for the region (ER < 50 keV, Q < 0.6) and (ER < 50 keV, Q ≥ 0.6) is 4 1 inside
∆tcoinc and 11 84 outside. Therefore the events inside the coincidence region
∆tcoinc ∈ [+15, +35] ms are not a background like distribution. These events are
clearly identified as muon-induced neutrons.
Information from the veto can also be added to have more detail on the coincidences.
A 3-D visualization has been implemented in the C++ based ROOT
software. The position of the hit on a veto module is from the study made in Section
4.5.1. The results for the 4 events are shown in Figure 5.14. Except for the
one with the lowest ER, the muon track is very clear, and they are all likely to be
muon-induced events. For the other one the track is not so evident, it is not possible
to decide if it is not a muon.
Examples of events with ER ≥ 250 keV are shown in Figure 5.15. Events at
high energy are as different as a single bolometer with a very high or relatively low
energy (top and bottom left), or few bolometers which totalize a high or relatively
low energy (top and bottom right). The second type of events, for which the recoil
energy is the sum of the recoil energy of the bolometers, are particularly interesting.
The individual bolometers, which constitute a coincidence event, may fall into the
region of interest for WIMPS at ER < 250 keV, especially when quality cuts exclude
the second bolometer. The results of the individual contribution is shown in Figure
5.16. Three events fall in the nuclear recoil band with (ER = 35 keV, Q = 0.33),
(ER = 34.9 keV, Q = 0.47) and (ER = 16 keV, Q = 0.22). Note that the first two
are associated with one or two bolometers of some tens of MeV, clearly a muon,
and the other one with two bolometers of hundreds keV. Muon-induced background