The LUX Experiment - Background Model and Physics Goals. D. Malling April APS, Denver, CO

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1 The X Experiment - Background Model and Physics Goals D. Malling April APS, Denver, CO

2 X Background Goals Background goal: <1 WIMP-like event in 100 kg fiducial in 300 days Dominated by PMTs (material selection difficult, proximity to active region) Counting goals: ensure contributions from other materials subdominant to PMTs D. Malling April APS Denver, CO April 13,

3 X Backgrounds Program Monte Carlo Material Screening Generic ER Modeling for Small Internals Precision ER Modeling for Major Internals Precision NR Modeling for Major Internals In Situ Measurement S1/S2 Yields and Collection Efficiencies Activation Isotopes Predicted Low-Energy Background D. Malling April APS Denver, CO April 13,

4 non Backgrounds Require low-energy single-scatter nuclear recoil in central (100 kg) fiducial volume X PMT ER backgrounds (0-25 kevee) with unoptimized 100 kg fiducial (dashed) WIMP search energy window: 5-25 kevnr (1-5 kevee) Gammas required to traverse length of detector, deposit <5 kevee energy, exit again without scattering; suppression seen at right Another >2 orders of magnitude rejection of ER events based on S2/S1 ratio Neutron generation 1 n/day from radioisotopes; NR events rejected by multiple scatter signatures DRUee = cts/kevee/kg/day D. Malling April APS Denver, CO April 13,

5 Background Sources Internal components (PMTs, cryostats, panels, etc.) Managed primarily through material screening External radioactivity and muoninduced backgrounds External rates reduced to <x1/10 internal rates by 3.5 m thick water shield (300 tonnes) D. Malling April APS Denver, CO April 13,

6 Background Modeling All Monte Carlo modeling performed using Geant4-based XSim package [NIM A (2012)] Highly faithful X detector geometry -- allows for detailed studies of backgrounds from individual internals / radioisotopes Integrates NEST code for S1/S2 yields [JINST 6 p10002 (2011)] D. Malling April APS Denver, CO April 13,

7 Background Modeling Detailed ER / NR modeling performed for all large internals -- yields energy spectrum in detector as function of position, radioisotope concentration Generic activity library created to accommodate >100 small components -- whole regions of detector modeled with different radioisotopes Generic library sets required screening ULs, in addition to adding to total expected ER D. Malling April APS Denver, CO April 13,

8 Counting / Modeling Results Counting results [Bq total in detector] Component Facility Amount in X 226 Ra ( 238 U) 232 Th 40 K Other PMTs + bases SOLO ( 60 Co) Ti Oroville 231 <0.085 <0.185 < ( 46 Sc)* HDPE SOLO 34 kg <0.21 <0.1 PTFE SOLO 25 kg <0.12 <0.03 Stainless steel SOLO 10 kg ( 60 Co) Cu SOLO 1100 kg 1.4 ( 60 Co)** Insulation SOLO 8 kg 0.9 < Kr -- <0.038 kg -1 Projected activity in 100 kg unoptimized fiducial [cts/kev/kg/day] Component ER (x10-3 ) NR (x10-6 ) * Measured from activation at Surface Lab; extrapolated for time underground ** Estimated from exposure at 1 km altitude PMTs + bases Other 0.1 (<0.4) 0.2 (<0.2) Total 0.6 (<0.9) 0.3 (<0.3) 10-3 cts/kevee/kg/day = 0.4 evts / day BEFORE ER rejection (1-5 kevee, 100 kg fiducial) D. Malling April APS Denver, CO April 13,

9 Projected ER Spectra XSim predicted ER background activity (internals only) in 100 kg unoptimized fiducial 10 2 Simulation Only cts / kev ee / kg / day Energy deposited [kev ee ] Solid lines are positive measurements from SOLO counting; Dashed lines are ULs from SOLO counting D. Malling April APS Denver, CO April 13,

10 Projected ER Spectra m Sum cts / kev ee / kg / day (11.8 d) 129m (8.9 d) (36 d) 10 5 (5.4 d) Energy deposited [kev ee ] D. Malling April APS Denver, CO April 13,

11 Projected ER Spectra 10 0 Sum cts / kev ee / kg / day m (11.8 d) 129m (8.9 d) 127 (36 d) Energy deposited [kev ee ] D. Malling April APS Denver, CO April 13,

12 Physics Reach Cross section [cm 2 ] (normalised to nucleon) Gaitskell,Mandic,Filippini WIMP Mass [GeV/c 2 ] ZEPLIN III (2011) 319 day x 4.2 kg XENON100 (2012) 225 day x 34 kg X 0 BG 300 day x 100 kg D. Malling April APS Denver, CO April 13,