HIGH-THRESHOLD ANALYSIS: PARAMETRIC FIDUCIALIZATION
4.2 Spectrum Average Exposure Estimation
4.2.2 E ffi ciency Estimate
After accounting for the reduction in live time from our time period based cuts, we next turn to the event-based cuts and their effect on our exposure. For these cuts (such as one that selects well reconstructed events, or only allows events in a par- ticular portion of the detector) the experimental live time surrounding the event is perfectly valid and throwing it away would underestimate our sensitivity to WIMP interactions. There is a chance, however, that a valid WIMP-scattering event would be removed by removed by these cuts. We account for this using an “efficiency
curve”2or simply an efficiency. This is the probability, as a function of energy, of a true WIMP-interaction in our detector making it through all the cuts of our analysis and into the signal acceptance region and is denoted(E) in equation 4.1. To cal- culate this we use252Cf calibration data as a WIMP-scattering proxy and examine how our various cuts remove this population. Quantitatively, for theEth energy bin we can calculate the efficiency of a cut (or series of cuts) as follows:
E = nPassingE
nTotalE (4.2)
where nPassingE are the number of NR-band calibration events that pass the cut and nTotalE are the total number of NR-band252Cf events in the bin. There were a select number of cuts where, for various reasons,252Cf calibration data could not be used to calculate the cut efficiency. These are summarized below, but for a much more detailed description of their treatment please see [75].
Non-252Cf Efficiency Cuts:
• Trigger EfficiencyFor a variety of reasons, there is some probability that an event that otherwise reconstructs perfectively well, does not induce a hard- ware trigger in our iZIP array. As discusses in section 3.5, we require that all events that make it into our signal region need to have issued a valid trig- ger. As a result, this population of events has, by construction, a trigger efficiency of unity. To get an unbiased population that would actually allow us to measure the efficiency of this cut, we turn to multiple-scattering events in WIMP-search data. During WIMP-search operation, if any iZIP issues a trigger the entire detector array is read out and reconstructed. By examining non-primary events that reconstruct well, but do not issue a trigger we can measure our energy dependent trigger efficiency.
• Single-scatter Cut252Cf data has a much higher probability of causing scat- tering events in multiple detectors than true WIMP interactions, and as a re- sult it is not a good candidate to measure the efficiency of this cut. Naively, the efficiency of this could could be assumed to be 1. After all the probability of a real WIMP event being removed by this cut seems vanishingly small. In
2This would probably be better named “WIMP acceptance probability curve”, but the term efficiencyis very strongly ingrained in our collaboration, and it would be difficult for me to avoid using that word.
• Ionizationχ2As describe in section3.3, early in the run it was discovered that certain series suffered from contamination of random events (used to build our OF noise template) resulting in problematically high or low χ2 values.
These were removed by the random triggerχ2cut (cGoodRandomChi2_V53_HT).
Importantly, this cut was applied before the ionizationχ2 cut was developed.
As a result of these removed series, the ionizationχ2 between252Cf calibra- tion and WIMP search is systematically different. As the ionization pulse- shape does not depend on an event being an electron- or nuclear-recoil the
133Ba calibration was used as a proxy instead.
• Good Phonon Start TimeAs described in section 3.4, this cut seeks to re- move events that are poorly reconstructed due to the pulse start time being too close to the edges of the OF search window (or even outside of it). This effects depends on event rate, and as a result, a high percentage of calibra- tion data is removed by it compared to WS data. Seeing as the criteria used to construct this cut are completely different than those used in the blinding definition, the blinded WIMP search data was thought to be a good proxy to the interactions that we would find in the signal region if it were unblinded.
As a result this is what was used.
• Muon VetoIn a sense, our muon veto cut is just a different kind of single- scatter cut, where the secondary detector is the active muon veto shield. As a result, we again turned to random traces from WIMP-search data to calculate its efficiency. By examining the fraction of random traces that were collected concurrently to a veto event (and thus removed by the cut) we can calculate the veto efficiency. This is again, not dependent on the interaction energy of the primary scatter, and a fixed value of∼98.6%.
The estimated efficiency for each detector after the time-period, quality, physics, and preselection cuts are shown in figure 4.1.
Figure 4.1: Efficiency curve estimates for each of the 10 HT detectors constructed to aid in our background and signal modeling. The errors include the systematic and statistical uncertainties. The notably reduced efficiency experienced by IT3Z1 and IT3Z3 is due to removing the half of each of those detectors closest to the installed
210Pb sources. [33]