ZEPLIN: The Hunt for Dark Matter Blair Edwards Imperial Co ege - PowerPoint PPT Presentation

ZEPLIN: The Hunt for Dark Matter Blair Edwards Imperial Co � ege London & Rutherford Appleton Laboratory

Dark Matter Why?? Most of our Universe is “ missing ” . What is the other 95 % made of? All current evidence is from the gravitational e ff ects DM has on baryonic matter we can observe. WIMPs are best motivated candidate for CDM.

Detection with Two-phase Xenon Interaction with Xenon atom Xe + -> Prompt Scintillation, S1 signal Ionisation +Xe Ionisation electrons drifted to extraction Electron/nuclear recoil region by electric field Xe 2 + -> Electroluminescence, S2 signal Excitation +e - (recombination) Xe * Xe ** + Xe S2 +Xe 140 mm Xe 2 * ~85 µ s 175nm 175nm Triplet Singlet S1 27ns 3ns 2Xe 2Xe

Detection with Two-phase Xenon Interaction with Xenon atom Xe + -> Prompt Scintillation, S1 signal Ionisation +Xe Ionisation electrons drifted to extraction Electron/nuclear recoil region by electric field Xe 2 + -> Electroluminescence, S2 signal Excitation +e - (recombination) Xe * Xe ** + Xe S2 +Xe Ionization Secondary Scintillation Primary (S2) 140 mm Xe 2 * ~85 µ s (S1) 175nm 175nm Triplet Singlet S1 27ns 3ns 2Xe 2Xe

ZEPLIN-II: Operations Searching for very rare and small signals requires a very low background. To achieve this the detector is located within a shielding castle, in the Palmer UG lab at Boulby mine. After a commisioning period, a 31 - day high - quality data set was acquired. A blinded analysis procedure was carried out, meaning only 10 % of the background data was available for detector characterisation.

ZEPLIN-II: Results Signal (AmBe) and • Detector calibrated for expected signal and Background ( 60 Co) search - box set. Di ff erence in S2/S1 ratio Calibrations provides discrimination power. 500 400 • Blind science dataset analysed ➙ Unexpected 300 200 background population observed from Rn 100 90 S2/S1 80 progeny events located at the walls of the 70 60 50 40 detector. 30 20 0 5 10 15 20 25 30 35 40 energy, keV ee 500 400 10 2 300 Science 200 S2/S1 100 90 S2/S1 80 dataset 70 60 10 50 40 30 20 1 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 energy, keV ee energy, keV ee

ZEPLIN-II: Science Result • Number of events expected in the box calculated from calibrations and background data. • W e observed 29 events with a total expectation of 28.6 ± 4.3 events. • Giving a limit on cross - section with a minimum of 6.6 x 10 - 7 pb at M D = 65 GeV . -40 10 Cross-section [cm 2 ] (normalised to nucleon) “ First Limits on WIMP nuclear recoil signals in ZEPLIN - II... ” -41 10 G.J. Alner et al., Astroparticle Physics 28 ( 2007 ) 287 - 302 -42 10 21.88 / 18 constant 133.7 4.588 19.64 / 20 mean 0.7272 0.2989E-02 P1 1.852 0.6302 sigma 0.1056 0.2616E-02 P2 27.57 2.335 Limit Plot 10 2 10 2 P3 0.3748 0.1502E-01 -43 P4 0.1939 0.1507E-01 10 events/bin 10 10 -44 DATA listed top to bottom on plot 10 ZEPLIN I (2005) ZEPLIN II (Jan 2007) result CDMS (Soudan) 2004 + 2005 Ge (7 keV threshold) CDMS 2008 Ge XENON10 2007 (Net 136 kg-d) Roszkowski/Ruiz de Austri/Trotta 2007, CMSSM Markov Chain Monte Carlos (mu>0): 68% contour Roszkowski/Ruiz de Austri/Trotta 2007, CMSSM Markov Chain Monte Carlos (mu>0): 95% contour -45 1 1 10 1 2 3 10 10 10 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 log(S2/S1) 5-10keV ee log(S2/S1) 5-10keV ee WIMP Mass [GeV/c 2 ] radius, a.u

Single Electron Emission First quantitative measurements of single ionisation electron emission in noble liquid detectors. Agreement of signal size with predictions from electroluminescence yield measurements provides strong evidence. Xe Number Density (x10 25 m -3 ) 5 5.5 6 6.5 7 7.5 16 Mean Single Electron Response, phe This work 14 Fonseca et al. (2004) [ 14 ] Aprile et al. (2004) [ 15 ] 12 10 8 6 4 2 0 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Pressure, bar

Single Electron Production Radial and depth distributions 4 suggest source/production 3.5 Electron rate, % 3 throughout the bulk of the liquid. 2.5 2 1.5 Clear link between number of 1 0.5 scintillation photons and rate of 0 0 10 20 30 40 50 single electron production. Energy, keV ➙ Likely source: Photoionisation What is being photoionised? Most probable candidates are impurities in the liquid. Although, none can be con fi rmed or ruled out. “ Measurement of single electron emission in two - phase xenon ” B. Edwards et al., arXiv:0708.0768v1 ( submitted to Astroparticle Phys. )

ZEPLIN-II: Related Work Alpha emanation from Getters S1 - S2 Anti - correlation • Alpha rates monitored and decay • Applied combination of S1 and time constants calculated. S2 signals to improve energy resolution. E* = S1 + k.S2 • Con fi rmed Rn emanating from • Used to resolve 40 keV SAES getters. inelastic feature from AmBe. • Results con fi rmed by experimental test.

ZEPLIN-III: Design Same operating principle as ZEPLIN - II, but with high -fi eld operation and surface - free geometry . 31 PMTs in the liquid phase provide better precision position reconstruction and better light yield . Low - background components and construction throughout. Ultra low background PMT upgrade in preparation. Detector fully shielded , with calibration source delivery system and detector levelling mechanism.

ZEPLIN-III: Commissioning Size of secondary signal as a function of electric fi eld can be predicted from parameterisations of di ff erent processes. Preliminary dataset to test this shows excellent agreement with prediction. Preliminary AmBe and 137 Cs calibrations appear to show improved separation of populations at higher fi eld.

ZEPLIN-III: Commissioning Size of secondary signal as a function of electric fi eld can be predicted from parameterisations of di ff erent processes. Preliminary dataset to test this shows excellent agreement with prediction. Preliminary AmBe and 137 Cs calibrations appear to show improved separation of populations at higher fi eld.

ZEPLIN-III: Commissioning Size of secondary signal as a function of electric fi eld can be predicted from parameterisations of di ff erent processes. Preliminary dataset to test this shows excellent agreement with prediction. Preliminary AmBe and 137 Cs calibrations appear to show improved separation of populations at higher fi eld.

ZEPLIN-III: Operations First science data run underway. ~ 300 kg.days of science data collected to date. ~ 600 - 700 kg.days will be collected. Aim: reach ~ 1 x 10 - 7 pb level. Daily monitoring carried out for detector tilt, energy calibrations, background rates and xenon purity.

Summary • ZEPLIN - II operated underground from 2005 to 2007. One tonne day of raw data collected, giving fi nal exposure of 225 kg.days. • ZEPLIN - II set a limit of 6.6 x 10 - 7 pb at M D = 65 GeV . • Additional work done highlighting areas of interest for future experiments: Single electron emission, alpha emanation and S1/S2 anti - correlation. • ZEPLIN - III fully commissioned and shielded. First science run underway , with ~ 300 kg.days of data acquired so far.

Thanks to the ZEPLIN-III Collaboration

ZEPLIN-II: Analysis 57 Co Energy and • Detector characterised with Position Calibrations calibrations and an unblinded sample of background data. Scintillation • Position reconstruction using a simple centroid method, giving resolution of about 1 cm. Electroluminescence • E ffi ciencies for all cuts carefully calculated. 1.1 Am-Be CAL/GEANT4 Calculated 1 Co-60 CAL/GEANT4 0.9 0.8 relative rate, keVee -1 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Radius 0 2 10 10 energy, keVee