Background Model Status and Improvements for the LUX Experiment Dark Matter Search Brian Tennyson Yale University On behalf of the LUX Collaboration LU APS April Meeting 2015 — Baltimore, MD —12 April 2015 The Large Underground Xenon Experiment Low-radioactivity titanium cryostat Copper shield PMT array Active xenon volume 59 cm 49 cm 370 kg total xenon mass 250 kg active liquid xenon 118 kg fiducial mass PTFE reflector panels and field cage PMT array Copper shield 2 How the LUX Detector Works 3 Signal and Background in LUX • Dark matter signal we’re looking for: Single-scatter nuclear recoils (NR) with energy less than 25 keVnr (~5.3 keVee) • Our backgrounds: Events that will mimic a dark matter response in the detector, such as: • • Neutrons that scatter once in the detector • Electron recoil (ER) events, statistically leaking (0.4%) into NR area of parameter space (The focus of this talk) We construct models for candidate WIMP signals and these expected backgrounds and use a Profile Likelihood Ratio (PLR) analysis to achieve our science result 4 ER Background Model • • Needs to include: • Sources of gamma radiation near the detector (i.e. construction materials) • Sources of beta radiation and gamma radiation within the xenon itself (e.g. activated xenon lines) Values and limits for these activities are reached by counting activity in the construction material before construction 5 Sources of activity in model (mBq/unit) Units Amount 226R 232Th 40K 60Co 46S 85Kr 127Xe 220Rn 222Rn PMTs PMT 122 a 9.5 2.7 66 2.6 c - - - - - PMT Bases Base 122 1.4 0.13 1.2 0.03 - - - - - Reflector Panels kg 9.3 5.0 1.3 - - - - - - - Field Shaping Rings kg 28 0.5 0.8 - 0.3 - - - - - Field Ring Supports kg 33.5 0.5 0.35 - - - - - - - Field Grids kg 4.5 1.4 0.23 0.4 1.4 - - - - - Grid Supports kg 15.5 3.0 1.0 - - - - - - - Cryostats (Ti) kg 231 0.37 0.8 1.6 - 4.4 - - - - PMT Mounts (Cu) kg 169 2.2 2.9 - 1.7 - - - - - Copper Shields kg 414 2.2 2.9 - 1.7 - - - - - Weir (Cu) kg 3.2 0.4 0.2 - 0.17 - - - - - Superinsulation kg 2.2 73 14 640 - - - - - - Thermal Insulation kg 6.0 130 55 100 - - - - - - Xenon kg 370 - - - - - 6 0.0013 0.49 0.007 0.049 How the background model is generated: This flowchart represents the steps taken in constructing the ER background model. Energy Deposition Only Simulations Each step has can be (and has been) checked for consistency. Optical Simulations (only events with energy below 20 keVee) Data Processing Chain PDFs for PLR 7 Updates to the background model • An earlier version of the background model was constructed for our original science result in 2013 • Our work represents an improvement over this model in the following ways: • • Increased simulated statistics to 10x live time • Added step of processing simulated background data through same data processing chain as real data These additions to the process increase our confidence in the model’s ability to accurately predict the backgrounds observed in the LUX detector 8 Background Model: Xenon External Backgrounds 118 kg fiducial volume, with an S1 signal < 50 phe S2 Counts/phe Counts/phe S1 10 16 14 8 12 10 6 8 4 6 4 2 2 0 5 10 15 20 25 30 35 40 0 45 50 S1 Area (phe) 0 500 1000 1500 2000 2 Counts/10cm Counts/cm 50 40 20 10 10 5 20 25 4000 4500 5000 S2 Area (phe) 20 15 15 3500 25 30 10 3000 R2 Z 0 2500 30 35 40 45 Z Position (cm) 9 0 50 100 150 200 250 300 350 400 Radius Squared (cm2 ) Background Model: Xenon Internal Backgrounds 118 kg fiducial volume, with an S1 signal < 50 phe S2 Counts/phe Counts/phe S1 8 7 10 6 8 5 6 4 3 4 2 2 1 0 0 5 10 15 20 25 30 35 40 0 45 50 S1 Area (phe) 0 500 1000 1500 2000 2500 3500 4000 4500 5000 S2 Area (phe) R2 Z 2 8 Counts/10cm Counts/cm 3000 7 6 12 10 5 8 4 3 6 2 4 1 0 10 15 20 25 30 35 40 45 Z Position (cm) 10 0 50 100 150 200 250 300 350 400 Radius Squared (cm2 ) Future Work • • For our re-analysis of the first science run: • Comparing to data and updating the model as need. • Continue to scrutinize and perform more checks on the model • (If time allows) Increase the simulated background statistics • Expand energy range to allow for analysis to include of nonWIMP dark matter signals In preparation for analysis of our ongoing second science run: • Understand how activities in the detector will change (decay away of activated Xenon, for example) 11 Summary • Signals and backgrounds in the LUX Detector • ER background model generation • Improvements to the model • Extension of background model for further analysis and second science run 12
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