Results from the LUX Experiment Sally Shaw DMUK Meeting UCL, 18th - PowerPoint PPT Presentation

Results from the LUX Experiment Sally Shaw 
 DMUK Meeting UCL, 18th January 2016 1

Large Underground Xenon Detector 2

The LUX Collaboration 3

The Black Hills 4

Sanford Lab, South Dakota 5

Direct Detection of WIMPs WIMP-nucleon scattering: • Spin Independent: scalar, coherent across nucleus, σ ∝ A 2 • Spin Dependent: axial vector, needs unpaired nucleon detector exposure particle/nuclear physics astrophysics Need a medium that produces something detectable after a Need a low background nuclear recoil, and if possible a environment, well shielded way to discriminate between from cosmic rays and with signal (DM) and background minimal radioactivity ( 𝛿 ,e-,n) 6

LUX TPC 250 kg liquid xenon, 122 PMTs, 4850 ft underground 7

Signal and Background Nuclear recoil (NR) tracks: 
 dense, high recombination, low extraction → S2/S1 lower � Electron recoil (ER) tracks: 
 cylindrical, distance between ions much shorter than e- thermalisation length → S2/S1 higher 
 99.8% electron recoil (ER) nuclear recoil (NR) discrimination 8

Calibrating LUX - Electron Recoils (ER) Tritium (CH 3 T) - ER band, Ly, Qy arXiv:1512.03133 performed every 3 months CH 3 T injected via gas system distributes uniformly within LXe E max = 18.6 keV 
 <E> = 5.9 KeV 83m Kr → position reconstruction, S1 & S2 corrections, field mapping 
 rubidium source left to decay, then 83 Rb build-up of krypton injected into LUX J = 5/2- � performed weekly T 1/2 = 86.2 d 83m Kr J = 1/2- � T 1/2 = 1.83 h E = 32.2 keV J = 7/2+ � 83 Kr T 1/2 = 154.4 ns 
 32.2 keV E = 9.4 keV 9.4 keV J = 9/2+ � 𝛿 -rays → intrinsic Xe isotopes → photon and electron detection e ffi ciencies (g 1 & g 2 , via Doke Plot) 9

Calibrating LUX - Nuclear Recoils (NR) Deuterium-deuterium fusion neutrons: 
 mono-energetic at 2.45 MeV 
 fired horizontally into the TPC Neutrons pass through an air-filled acrylic conduit, collimated to 1° Beam aligned 15.5 cm below the liquid level within the LUX active region (maximise double scatters) � Cuts applied ensure 95% of neutrons are within 4% of 2.45 MeV and are well constructed � Validated extensively with Monte Carlo arXiv:1608.05381 10

Calibrating LUX - Nuclear Recoils (NR) Lowest ever energies for liquid xenon! Light yield (L y ) Charge yield (Q y ) measured down to measured down to 1.1 keV nr 0.7 keV nr NR band characterised Results confirmed validity of with much better statistics past NR calibrations (AmBe, than previously 252 Cf) arXiv:1608.05381 11

LUX Timeline reanalysis limit 1 st science run starts grid conditioning published (95-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 85-day limit published 2 nd science run ends LUX decommissioned founded arXiv:1512.03506 arXiv:1310.8214 2016 (Feb): spin-dependent limits Improvements due to: Low energy threshold 3 keV → 1.1 keV - additional 10 days of data - improved background model - improved data processing algorithms - → improved efficiency 12 arXiv:1602.03489

LUX Timeline reanalysis limit 1 st science run starts grid conditioning published (95-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 85-day limit published 2 nd science run ends LUX decommissioned founded e - ! e - ! Effects of grid conditioning between 1 st and 2 nd science run: • Extraction field increased fro .. kV/cm to .. kV/cm • Electron extraction efficiency increased from ~49% to >70% • Time and drift dependent field distortions • These found to be consistent with a build up of charge on PTFE wall 13

Solution - 16 “detectors”… 4 bins in time, 4 bins in z Treat each bin as a separate detector, with its own calibration and model! NEST models tuned to data by adjusting E-field and Fano factor 14

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] 3.2 3 33 keVnr 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 15 S1 (phd)

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] 3.2 3 33 keVnr 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 16 S1 (phd)

Salting the Data Traditional blinding can mask rare backgrounds and pathologies -> rarely seen in dark matter community � Instead, we “salted” the data Fake NR events created from uncorrelated S1s and S2s in Tritium (ER) data Collaboration-wide Turing test done to ensure no one could identify the salt Key to unsalting only known by two people 17

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] 3.2 3 33 keVnr 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 18 S1 (phd)

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] B 3.2 C 3 33 keVnr A 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 19 S1 (phd)

3 Bad Events 80% of light in single top PMT consistent with energy - deposited outside of TPC, and light leaked through gap near edge of PMT array � � Concentrated under a few top PMTs time structure consistent with - gas scintillation emission event came < 1s after high rate - � Do not correspond to interactions within the TPC: 
 develop loose post-unsalting cuts to target these pathologies cuts were checked for a flat and high signal acceptance on calibration data 20

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] B 3.2 C 3 33 keVnr A 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 21 S1 (phd)

4 9.8 keVee 8.7 7.5 6.3 3.8 5.2 4 3.6 2.9 3.4 1.7 log 10 [S2 (phd)] 3.2 3 33 keVnr 2.8 27 2.6 21 2.4 15 3 9 2.2 0 10 20 30 40 50 22 S1 (phd)

Run 4 Limit reanalysis limit 1 st science run starts grid conditioning published (93-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 83-day limit published 2 nd science run ends LUX decommissioned founded arXiv:1608.07648 23

End-of-LUX Calibrations / R&D D-D Calibrations for low energy measurements & LZ R&D: • short pulse ( S2-only ) • D 2 O reflector ( 1-4 keV, 8 B ν region) 
 • Injection campaigns • Xe-131m ( CH 3 T replacement for LZ ) • C-14 ( new energy range ), • Rn-220 ( skin calibration for LZ ) • Ar-37 ( low E K,L,M peaks, signal physics ), • 24

reanalysis limit 1 st science run starts grid conditioning published (95-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 85-day limit published 2 nd science run ends LUX decommissioned founded 25

Combining the Science Runs Treat the 1 st as the 17th detector/exposure segment, and treat nuisance parameters in PLR as independent, except for Lindhard k factor (measured with DD data) 1 2 Livedays 95 332 Spatial 
 2 (r,z) 3 (r, ɸ , z) Co-ordinates Corrections x, y, e- lifetime x, y Xe-127, 𝛿 sources, β 𝛿 sources, β sources, Backgrounds sources, B-8 ν ,wall B-8 ν ,wall Fiducial mass (kg) 145.4 105.4, 107.2, 99.2 98.4 FV no longer a simple cylinder - calculate using Decayed away by the start of acceptance of uniform 83m Kr events and multiply the 2 nd run (half life ~36 days) by total LXe mass for each time bin 26

Combined Run 3 + Run 4 reanalysis limit 1 st science run starts grid conditioning published (95-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 85-day limit published 2 nd science run ends LUX decommissioned founded arXiv:1608.07648 27

It’s not over yet! reanalysis limit 1 st science run starts grid conditioning published (93-day) 332-day limit published 427-day limit published LUX funded 2013 (April) 2014 (Jan) 2015 (Dec) 2016 (July) 2008 2016 (Oct) 2006 2012 2013 (Nov) 2014 (Sept) 2016 (May) 2016 (Sep) collaboration 2 nd science run starts LUX moved underground 83-day limit published 2 nd science run ends LUX decommissioned founded Lots more interesting analyses and papers still to come from LUX → axions, spin-dependent run 4 limits, 83m Kr, DD, 134 Xe 0 νββ decay, position reconstruction, double e- capture, pulse shape discrimination, S2-only, inelastic DM, modulations… 
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