Status of the XENON100 Dark Matter Search

Transcription

1 Status of the XENON Dark Matter Search Guillaume Plante Columbia University on behalf of the XENON Collaboration Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, 211

2 XENON Collaboration Columbia Rice UCLA Zurich Coimbra LNGS SJTU Mainz Bologna MPIK NIKHEF Subatech Münster WIS Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

3 Direct Detection CRESST-I SIMPLE PICASSO COUPP Heat CDMS I, II SuperCDMS EDELWEISS I, II CRESST-II ROSEBUD IGEX GEDEON GENIUS GERDA CoGeNT Ionization ZEPLIN II, III XENON LUX WARP ArDM DarkSide Scintillation ZEPLIN I DEAP CLEAN XMASS DAMA/LIBRA KIMS Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

4 Why Xenon? Large mass number A ( 131), expect high rate for SI interactions (σ A 2 ) if energy threshold for nuclear recoils is low 5% odd isotopes ( 129 Xe, 131 Xe) for SD interactions No long-lived radioisotopes, Kr can be reduced to ppt levels High stopping power (Z = 54, ρ = 3gcm 3 ), active volume is self shielding Total Rate [evt/kg/day] 3 4 Si (A = 28) Ar (A = 4) Ge (A = 73) Xe (A = 131) Efficient scintillator ( 8% light yield of NaI), fast response 5 σ χn = 1 44 cm 2 Scalable to large target masses Nuclear recoil discrimination with simultaneous measurement of scintillation and ionization E th - Nuclear Recoil Energy [kev] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

5 Principle E ionization Xe + + e +Xe excitation Xe + 2 +e Xe Xe +Xe +Xe Xe nm triplet (27 ns) 178 nm singlet (3 ns) 2Xe 2Xe Bottom PMT array below cathode, fully immersed in LXe to efficiently detect scintillation signal (S1). Top PMTs in GXe to detect the proportional signal (S2). Distribution of the S2 signal on top PMTs gives xy coordinates( r < 3 mm) while drift time measurement provides z coordinate ( z < 3µm) of the event. Ratio of ionization and scintillation (S2/S1) allows discrimination between electron and nuclear recoils. Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

6 XENON: Design PMT HV and Signal Lines Anode Feedthrough Tube to Cooling Tower Top Veto PMTs Double-Wall Cryostat Upper Side Veto PMTs Bell Goal was to build a detector with a increase in fiducial mass and a reduction in background compared to XENON. All detector materials and components were screened in a dedicated low-background counting facility 161 kg LXe total mass consisting of a 62 kg target surrounded by a 99 kg active veto. 15 cm radius, 3 cm drift length active volume. Top Array PMTs PTFE Panels Side Veto PTFE Lining Lower Side Veto PMTs Bottom Array PMTs Top Mesh Anode Bottom Mesh Field Shaping Rings Cathode Bottom Veto PMTs TPC inner volume defined by 24 interlocking PTFE panels. Drift field uniformity ensured by 4 double field shaping wires, inside and outside the panels. Cathode at 6 kv, drift field of.533 kv/cm. Anode at 4.5 kv, proportional scintillation region with field 12 kv/cm. Custom-made low radioactivity HV feedthroughs. Aprile et al., arxiv: Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

7 XENON: Design PMT HV and Signal Lines Anode Feedthrough Tube to Cooling Tower Top Veto PMTs Double-Wall Cryostat Upper Side Veto PMTs Bell Goal was to build a detector with a increase in fiducial mass and a reduction in background compared to XENON. All detector materials and components were screened in a dedicated low-background counting facility 161 kg LXe total mass consisting of a 62 kg target surrounded by a 99 kg active veto. 15 cm radius, 3 cm drift length active volume. Top Array PMTs PTFE Panels Side Veto PTFE Lining Lower Side Veto PMTs Bottom Array PMTs Top Mesh Anode Bottom Mesh Field Shaping Rings Cathode Bottom Veto PMTs TPC inner volume defined by 24 interlocking PTFE panels. Drift field uniformity ensured by 4 double field shaping wires, inside and outside the panels. Cathode at 6 kv, drift field of.533 kv/cm. Anode at 4.5 kv, proportional scintillation region with field 12 kv/cm. Custom-made low radioactivity HV feedthroughs. Aprile et al., arxiv: Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

8 XENON: PMT Arrays 242 low activity Hamamatsu R852-6-Al 1 square PMTs, 98 tubes on top (QE 23%), in concentric circles and enclosed in a PTFE structure, 8 high QE ( 33%) tubes on bottom, on a rectangular grid to maximize photocathode coverage, 64 tubes in the LXe veto, in two rings, alternating inward and down (up) to allow them to view the top, bottom and sides of the veto volume. Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

9 XENON: Shield XENON installed in a passive shield to suppress external backgrounds: 2 cm of H 2 O to moderate neutrons produced in the cavern rock 2 cm Pb (inner 5 cm with low radioactivity Pb) to stop gamma rays 2 cm polyethylene to moderate neutrons produced in the Pb 5 cm Cu to stop gamma rays from the polyethylene Shield cavity continuously purged with N 2 to keep 222 Rn level < 1Bq/m 3. Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

10 XENON: Gran Sasso Lab Outside Buildings Tunnel Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

11 XENON: Typical Low Energy Event Amplitude [V] TPC.15 S1: 14.1 pe S2: pe Veto 1.5 Amplitude [V] Drift Time [µs] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, 211 / 37

12 XENON: Typical Low Energy Event [pe] S2 PMTi Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

13 XENON: ER Background ] kev day Rate [events kg Pb 28 Tl Co Cs 54 Mn 228 Ac 6 Co 4 K 214 Bi data (Fall 29, no veto cut) MC (total) 85 MC ( Kr, 12 ppt) 222 MC ( Rn, 21 µbq/kg) MC ( Xe 2ν ββ, T =2.11 y) 1/2 214 Bi 28 Tl Energy [kev] ] d kg Rate [events kevee CoGeNT CDMS XENON kevee scale needed below 9 kev CRESST DAMA/LIBRA IGEX XENON XENON (target volume) XENON (fiducial volume) Energy [kevee] Measured and predicted ER background without veto cut and for a 3 kg fiducial volume. Measured ER background (9.6 3 eventskev 1 kg 1 d 1 ) is in good agreement with Monte Carlo predictions (no tuning). Background from materials is dominated by PMTs. ER background level in fiducial volume (< 6 3 eventskev 1 kg 1 d 1, with veto coincidence cut) is lower than XENON (.6eventskeV 1 kg 1 d 1 ). Details in Aprile et al., Phys. Rev. D 83, 821, 211 Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

14 Electronic recoil band calibration performed with high energy gammas from 6 Co (1.17 Mev, 1.33 MeV). log(s2/s1) XENON: Calibration Nuclear recoil band calibration performed with 3.7 MBq (22 n/s) AmBe neutron source. Since WIMPs are expected to elastically scatter off of nuclei understanding the behavior of single elastic nuclear recoils in Xe is essential. log(s2/s1) Background in the energy region of interest is due to low energy Compton scatters from high energy gamma rays or β decays Co AmBe 5 Nuclear Recoil Equivalent Energy [kevnr ] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

15 Nuclear Recoil Equivalent Energy Nuclear recoil equivalent energy E nr is obtained from the S1 signal E nr = S1 L y,er 1 L eff (E nr ) S er S nr L y,er, light yield of electron recoils from 122 kev γ rays S er, S nr, scintillation light quenching due to drift field Relative scintillation efficiency L eff L eff (E nr ) = L y,nr (E nr ) L y,er (E ee = 122keV) L eff Sorensen 29 Lebedenko 29 Arneodo 2 Akimov 22 Aprile 25 Aprile 29 Bernabei 21 Chepel 26 Manzur 2. 1 Nuclear Recoil Energy [kev] The uncertainty in L eff at low energies is the largest systematic uncertainty in the reported results from LXe WIMP searches at low WIMP masses. Recent measurement performed at Columbia University, lowest energy measured 3 kev. Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

16 New Measurement of L eff : Detector Cubicsensitivevolumewithsix2.5x2.5cmHamamatsu R SEL High QE PMTs Built a new special purpose LXe detector with maximized scintillation light detection efficiency Calibration with 122 kev γ rays from a 57 Co source gives a light yield of L y = ±.9(stat) ±.44(sys) pe/kevee with a resolution (σ/e) of 5% Very high light yield, enables a measurement with a low energy threshold Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

17 New Measurement of L eff : Setup 2 H(d, n) 3 He d ϕ = π 2 n LXe EJ31 θ θ EJ31 ] d Rate [events ns γ n o θ = ± 1. kev 5 Accidentals Accidentals Time of Flight [ns] Record fixed-angle elastic scatters of monoenergetic neutrons tagged by organic liquid scintillators with n/γ discrimination ] 8 o θ = ± 1. kev Use 2 H(d,n) 3 He 2.5 MeV neutrons from a compact sealed-tube neutron generator Recoil energy is fixed by kinematics E r 2E n m n M Xe (m n +M Xe ) 2 (1 cosθ) d Rate [events pe Scintillation Signal [pe] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

18 New Measurement of L eff : Result L eff Sorensen 29 Lebedenko 29 Horn 211 (FSR) Horn 211 (SSR) Aprile 25 Aprile 29 Chepel 26 Manzur 2 Plante Nuclear Recoil Energy [kev] Lowest energy (3 kev) and most precise L eff direct measurement achieved to date. For details see Plante et al., Phys. Rev. C 84, 4585, 211 Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

19 XENON: Data Taking 29/2 12 Science Data (run_8) 8 Live Days Science Data (run_7) Calibration (AmBe) 6 Calibration ( Co) /27 11/26 12/26 1/25 2/24 3/26 4/25 5/25 Date [mm/dd] Results from 11.2 days unblinded data in Aprile et al., Phys. Rev. Lett. 5, 13132, (2). Data taken in the first half of 2, blinded region of interest,.9 live days Results from the blind.9 days in Aprile et al., Phys. Rev. Lett. 7, 13132, 211 Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

20 XENON: Data Selection Basic quality cuts Designed to remove noisy events, events with unphysical parameters. Very high acceptance. S1 coincidence cut S2 threshold cut Fiducial volume cut Because of the high stopping power of LXe, fiducialization is an extremely effective way of reducing background. Fiducial volume chosen: 48 kg super-ellipsoid S2 saturation cut Signal/Noise cut Scatter cuts Designed to remove events with multiple interactions (multiple S2s), with delayed coincidences (multiple S1s) or misidentified S1s. S1 single peak cut S2 single peak cut Veto cut Advanced cuts Designed to remove events which fail consistency checks, e.g. mismatch in positions from different algorithms, or with S1 PMT patterns not consistent with their position in the TPC S1 PMT pattern cut S2 width consistency cut Position reconstruction cut Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

21 XENON: L eff Parametrization Leff.35 Arneodo 2 Bernabei 21.3 Akimov 22 Aprile Chepel 26 Aprile 29 Manzur 2.2 Plante Energy [kevnr] L eff parametrization is an average of the direct fixed angle neutron scattering L eff measurements New measurement of L eff at low energies added to the data used to obtain the parametrization Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

22 XENON: Nuclear Recoil Acceptance Keep acceptance high (think discovery) Data quality cuts acceptance estimated from AmBe and 6 Co calibration data, MC simulations, and ERs outside the WIMP search energy range Decided a priori to use Profile Likelihood approach and test both background only and signal+background hypotheses No S2/S1 rejection cut in the Profile Likelihood approach DefineabenchmarkWIMP region for a parallel cuts-based analysis 99.75% ER rejection line S2 threshold NR band lower 3-σ /S1)-ER mean Acceptance (S2 log b S1 [PE] mχ 5GeV mχ = GeV mχ = 7GeV Energy [kevnr] S1 [PE] % rejection -3σ NR acceptance Energy [kevnr] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

23 XENON: Nuclear Recoil Acceptance 1 S1 [PE] WIMP spectrum Photoelectron spectrum Efficiency corrected Acceptance mχ 5GeV mχ = GeV mχ = 7GeV Energy [kevnr] 5 3 pe 4 pe Resolution near threshold is dominated by statistics in photon detection efficiency. For each WIMP mass, convert the energy spectrum to a S1 spectrum in photoelectrons, apply the S2 acceptance function, convolve the spectrum with a Poisson distribution. cs2 [pe] Preliminary cs1 [pe] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

24 XENON: Background Prediction Expected background: 48 kg fiducial mass, 99.75% electronic recoil rejection,.9 live days Statistical leakage (from electronic recoil events) 1.14 ±.48 events, estimated from the non-blinded electronic recoil band from background Acceptance S1 [PE] Energy [kevnr] Dominated by 85 Kr (Kr concentration 7ppt) due to a previous leak.4 S1 [PE] Anomalous leakage.56±.25 events, estimated using data and MC from 6 Co and background Neutron prediction from MC.11 ±.8 events, muon-induced fast neutrons and neutrons from (α, n) reactions and spontaneous fission Total 1.8±.6 events in.9 days /S1)-ER mean (S2 log b % rejection -3σ NR acceptance Energy [kevnr] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

25 XENON: Unblinding 6 events in the signal region after unblinding, inspection reveals 3 are due to electronic noise Population of noise events near threshold, leaks into signal region Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

26 XENON: Unblinding.4.2 S1 [PE] /S1)-ER mean (S2 log b Energy [kevnr] 6 events in the signal region after unblinding, inspection reveals 3 are due to electronic noise Population of noise events near threshold, leaks into signal region Remove population with noise post-unblinding cut, 3 candidate WIMP events remain Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

27 XENON: Event Distribution.4 S1 [PE] Radius [cm] /S1)-ER mean (S2 log b z [cm] Energy [kevnr] candidate events clearly inside the 48 kg fiducial volume Radius Probability (Poisson) to observe 3 or more events when expecting 1.8±.6 is 28% Profile Likelihood analysis does not yield a significant signal excess either, background-only hypothesis has a p-value of 31% [cm ] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

28 Construct the Likelihood function L = L1 (σ, Nb, ǫs, ǫb, Leff, vesc ; mχ ) L2 (ǫs ) L3 (ǫb ) L4 (Leff ) L5 (vesc ) Main term contains only one parameter of interest, the signal cross-section σ, other parameters are nuisance parameters and profiled out Additional terms constrain the nuisance parameters in the main term Makes use all observed events in the WIMP search data, no sharp S2/S1 discrimination cut, energy distribution Allows systematic uncertainties to be incorporated in a consistent manner Guillaume Plante S2 [PE] XENON: Profile Likelihood Approach S1 [PE] More details in Aprile et al., Phys. Rev. D 84, 523, 211 Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

29 XENON: Limit ] 2 WIMP-Nucleon Cross Section [cm DAMA/Na CoGeNT DAMA/I CDMS (211) CDMS (2) XENON (S2 only, 211) EDELWEISS (211) XENON (211) observed limit (9% CL) Expected limit of this run: ± 1 σ expected ± 2 σ expected XENON (2) Trotta et al. -45 Buchmueller et al WIMP Mass [GeV/c ] Strongest limit to date over a large WIMP mass range, challenges the interpretation of CoGeNT and DAMA signals as being due to low mass WIMPs Results recently published Aprile et al., Phys. Rev. Lett. 7, 13132, 211 Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

30 XENON: Current Data Taking Status Kr concentration reduced by more than 5 ( 5% background reduction) Improved S2 trigger with lower trigger threshold New dark matter run started early 211, 16 live days accumulated Much more ER calibration data, already 16 live days 6 Co and 18 live days 232 Th Science Data (run_) Live Days Calibration (AmBe) 6 Calibration ( Co) 232 Calibration ( Th) 3/1 5/1 7/1 8/31 /31 Date [mm/dd] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

31 XENON1T: Future 1m 3 TPC, 2.4t LXe, 1t fiducial mass background reduction compared to XENON Low radioactivity photosensors m water shield Currently in design phase, construction 212 Approved for construction in Hall B at LNGS Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

32 Summary Science Data (run_) Live Days Calibration (AmBe) 6 Calibration ( Co) 232 Calibration ( Th) 3/1 5/1 7/1 8/31 /31 Date [mm/dd] XENON background goal achieved, detector is taking dark matter data Accumulated days exposure in 2, 3 events observed with an expectation of 1.8±.6 Set the most stringent limit to date on the WIMP-nucleon spin-independent cross section Already 16 days exposure accumulated with lower background level and improved trigger threshold, stay tuned for new results Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

33 Extras

34 hist Entries 2888 Mean x 78.9 Mean y RMS x RMS y XENON: Position Dependent Corrections y [mm] relative LCE Radius [mm] Z [mm] x [mm] Corrections from measurements with 137 Cs, AmBe (4 kev inelastic), 131m Xe (164 kev), with agreement better than 3%. S1 rz and S2 xy correction due to spatial dependence of the light collection. S2 z correction due to finite electron lifetime. Electron Lifetime [µs] /31/ 3/2/11 5/2/11 7/2/11 9/1/11 Date [mm/dd/yy].8 Concentration [ppb O eq.] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

35 New Measurement of L eff : Extracting L eff Rate [evts ns ] d o θ = ± 1. kev Time of Flight [ns] Rate [evts kev ] d o θ = ± 1. kev Energy [kev] Transform the MC recoil energy spectrum into a simulatedscintillationspectrumg usings1 = L y L eff,j E r, with gaussian energy resolution σ = R E r, PMT gain fluctuations, and applying trigger efficiency Extract the energy dependence of L eff by minimizing the χ 2 between the measured and simulated spectra, h and g χ 2( L eff,j,r j ) = N i= [ ( )] 2 hi g i Leff,j,R j σh,i 2 ( ) +σ2 g,i Leff,j,R j Resolution Parameter R ] d Rate [events pe o θ = ± 1. kev L eff o θ = ± 1. kev χ 2 = Scintillation Signal [pe] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

36 Uncertainty in L eff σl 2 eff = σl 2 eff,fit +( L eff ) 2σ 2 L y Ly + ( L eff ) 2σ 2 E nr Enr + ( L eff ) 2σ 2 ǫ ǫ + ( L eff ) 2σ 2 r g rg + ( L eff ) 2σ 2 r s rs The total uncertainty on L eff is computed from σ Leff,fit, uncertainty from the fit σ Ly, uncertainty 57 Co light yield σ Enr, spread in nuclear recoil energies σ ǫ, liquid scintillator cut efficiency uncertainty σ g, neutron generator position uncertainty σ s, liquid scintillator position uncertainty L eff / E nr is computed from a logarithmic fit to the measured values L eff / ǫ, L eff / r g, and L eff / r s are calculated from MC simulations At all energies, the dominant contribution is from σ Enr Below 6.5 kev, the second largest contribution is from σ ǫ. At 6.5 kev and above, the second largest contribution is from σ Leff,fit Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

37 XENON: Anomalous Leakage Two types of leakage from the ER band, statistical leakage and anomalous leakage We assume the ER band is Gaussian in log(s2/s1), fixed discrimination at 99.75% gives the expected statistical leakage Events with low non-gaussian S2/S1 also in gamma calibration data, e.g. 6 Co GXe S2 LXe E g One source for those anomalously leaking events is multiple scatter events where one or more scatter occurs in a charge insensitive region of the detector Use the S1 Pattern likelihood cut to remove events likely due to two energy deposits Compute the expected anomalous leakage in background using 6 Co as reference e e - - e - S1 2 S1 1 E d Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37

38 Example Noise Event.4 TPC Amplitude [V] Drift Time [µs] Amplitude [V].4 TPC S1: 3.7 pe cs1: 5.6 pe Drift Time [µs] Guillaume Plante Exploring Low-Mass Dark Matter Candidates - PITT PACC - November 146, / 37