Sensitivity and Backgrounds of the LUX Dark Matter Search

Transcription

1 Sensitivity and Backgrounds of the LUX Dark Matter Search 1

2 LUX Goal: Direct Detection of Dark Matter WMAP 5-year data (2008) gives matter densities (Ω) based on best fit to Λ-CDM cosmological model: Ω total = / (flat Universe) Ω nb = / Ω baryon = / Ω Λ = / Ω ν < WMAP Assuming Ω nb is composed of Weakly Interacting Massive Particles (WIMPs): relic density of ~3 WIMPs / liter: (Mx = 100 GeV/c 2 ) In case you missed it... Talk by T. Shutt, 11:21 am Saturday (and others) V 0 ~ 230 km/s 2

3 LUX Detector building on established technology from XENON10 / Zeplin See talk by L. Kastens (Yale) (E13, 3:42 pm Saturday) I m going to talk about the background model and how LUX will reach its predicted sensitivity... Construction Materials: Cu cryostat PTFE SS flange/feedthrough (Cu shield) PMTs are dominant contribution other components reduced to << PMTs 3

4 External Background Moderation: Watershield 2.5 m H2O shielding equivalent to ~25 cm Pb for γ external γ contribute <2% of projected BG moderates external neutrons too => subdominant ALSO: possibility of tagging internally generated neutrons, and >10 MeV punch-through neutrons... Installation in DUSEL Homestake Davis cavern (summer/fall 2008) LUX See talk by S. Dazely (LLNL) (E13, 3:30 pm Saturday) 4

5 Current Sensitivity of Dark Matter Direct Detection Building on established technology: XENON10 (2007) Exposure: 5.4 kg fiducial, 58 days live (BG limited) 136 kg-days after cuts/nr acceptance Sensitivity (100 GeV WIMP): 8.8x10-44 cm 2 <1 evt/kg/month LUX (2009) discovery potential! Exposure: 100 kg fiducial, 300 days live Project >10 4 kg-days after cuts/nr acceptance Sensitivity (100 GeV WIMP): 7x10-46 cm 2 Expect <1 ER background event during exposure (assuming XENON10 discrimination performance) Comparison: SuperCDMS (2013) SNOLab: Gross Ge Mass 25 kg (x 50% fid mass+cut acceptance) Limit set for 1000 days live x 7 SuperTowers Cross-section [cm 2 ] (normalised to nucleon) LUX (2009) ZEPLIN II WIMP Mass [GeV/c 2 ] Gaitskell,Mandic,Filippini Edelweiss I CDMS II (2006) WARP XENON10 (2007) CDMS II@Soudan ( ) SuperCDMS@SNOLab (2013) XMASS (2009) This plot has a limited number of current and projected results. Please see 5

6 Discrimination in Xe: Electron Recoil vs Nuclear Recoil ~4 kevr single scatter nuclear recoil phe 595 phe 12 data from XENON10 50% acceptance phe/sample phe x10 S1 S2 neutrons γ,β kevr 4 2! 0.10 µs " µs 137 Cs AmBe!)*+)"!(),-).//01+*2/0)! #!!$ &&'!( log 10 (S2/S1) 2.5 γ,β 6-)-070/+892 &&'"( 2 neutrons #!!% &&'&( kev r [QF=0.19] 6! " #! #" $! $" %! %" 3045

7 Primary Challenge: Manage Backgrounds (LUX will do this, very niiize) Table assumes 100 kg fiducial mass, 300 days live expect 2 BG events in 300 days live conservative! 7 ER background goal (before discrimination)

8 Backgrounds (γ) From Detector Construction Cu: 10 ppt U / 12 ppt Th / 18 ppb K [~1% of total ER BG budget] Cosmogenic activation of Cu (sea-level: 2.1 mbq/kg 60Co would be ~43% of ER BG budget) need to limit surface exposure to < 12 months Teflon: < 13 ppt U / < 39 ppt Th / 22 ppb K [~1% of total ER BG budget] XENON10 upper limits x10 lower U/Th EXO measured x1000 lower SS: activities from off-the-shelf feedthrough are high, but will be sub-dominant (low mass, and shielded by Cu) PMTs: Dominant source of ER BG: activities ~ x10 3 higher (per unit mass) calcs and Monte Carlo assume: U/Th/K/Co 18/17/30/8 (mbq/pmt). All PMTs surveyed so far have shown lower values [cm] log10(dru) [cm] above: Monte Carlo of (dominant) PMT activity in LUX Goal 8

9 Backgrounds (neutron) from Detector Construction PMTs are the primary (internal) source of neutron background: fission [~1.5% of goal, > 6 MeV prompt γ ] log10(dru) (α,n) [higher yield for lighter elements] Calcs assume U/Th/K/Co = 18/17/30/8 (mbq/pmt) => 1.5 neutrons / yr / PMT Monte Carlo assumes 5 neutrons / yr / PMT (very conservative) NOTE: Teflon (α,n) sub-dominant (10% of goal) assuming EXO measurements for U/Th [cm] [cm] Goal x20 reduction in FV with multiple scatter veto 9

10 Other Potential Intrinsic Backgrounds Potential sources of radioactivity that could be dissolved/contained in Xe / detector components: Xe is clean! 85 Kr (τ 1/2 = 10 y) => reduced to < 2 ppt by CWRU charcoal column [2 ppt => ~0.4 μbq/kg => 0.8x10-4 dru], ~10% of ER BG budget... but also an upper limit U/Th => materials selected for U/Th safely below targets Rn => need to worry about Rn (next slide) Atmospheric gases => H2O, CH4, CO2... removed by high T getter 3 H 14 C 40 K }naked β emitters. H,C removed by getter. materials selected for low 40 K. Cosmogenic activation of Xe sub-dominant for any reasonable surface exposure (τ 1/2 ~ 10 days) External backgrounds which are sub-dominant (dru rates quoted for 0-50 kevee): p-p solar ν => ~1.2x10-5 dru [~1.5% of LUX bg goal] 136 Xe ββ => current limit τ 1/2 = 0.8x10 22 would give ~3x10-6 dru [0.3% of LUX bg goal] Thermal neutron radiative capture on Xe => several coincident γ s (~7 MeV) NOTE: assessments conservatively assume ER rejection is only 99.4% 10

11 238U / 232Th chain: Rn is main concern 6 naked β candidates in U/Th chains, all below Rn 220 Rn (τ=55s) daughters less of a problem Only one really dangerous: 214 Pb 6% naked β (1 MeV) XENON10 (α counting) : ~5 mhz in detector => 1.6 mbq (~3 α per Rn decay) assuming 1.6mBq α rate in LUX (and pessimistic assumption that all α s are from 222 Rn) [10% of ER BG goal]... without any attempt to reduce/screen This was not studied in XENON10; starting an alpha screening program at CWRU 11

12 Conclusions LUX 300 kg (100 kg fiducial) Xe detector will build on established Xe technology Dominant activity is from PMTs have screened 12 PMTs so far, all show lower U/Th (by ~ x3) than our conservative assumptions conservative assumption of 99.4% ER versus NR discrimination conservative BG model predicts 1 ER and 1 NR event in signal window, for 100 kg fiducial and 300 days live Expect sensitivity to (100 GeV) WIMP dark matter of 7x10-46 cm 2 (interaction rate ~ 4 events) x100 below current exclusion limits discovery potential! Installation at Homestake Sanford Lab 4850ft will take place in Summer/Fall Operation of detector underground begins Dec

13 Detection Simulation 100 kg fiducial / 300 days running Based on projected ER background and WIMP Mass 100 GeV/c 2 13

14 Thank You 14