ZEPLIN-III Second Science Run &Beyond& Beyond
Henrique Araújo Imperial College London
On behalf of the ZEPLIN-III Collaboration: Edinburgh University (UK), Imperial College London (UK), ITEP-Moscow (R(Ri)ussia), LIP-CiCoim bra (Pt(Portuga l) STFC Rutherford Appleton Laboratory (UK)
Darkness Visible, 2-6 August 2010, IOA Cambridge Direct WIMP detection: how hard can it be?
{ You are looking at a snapshot of n-body simulations of the Milky Way’s DM halo (Via Lactea I) 2 The exppgerimental challenge
{ Low-energy particle detection is easy ;) E.g. Microcalorimetry with superconducting TES Detection of keV particles/photons with eV FWHM!
{ Rare event searches are also easy ;)
E.g. Super-Kamiokande contains 50 kT water Cut to ~20 kT fiducial mass (self-shielding)
{ But doing both is hard! Small is better for signal collection Large is better for shielding background
{ Doh: andhd there is no trigger, and no control of luminosity… Low energy nuclear recoils
{ Elastic scatter off nucleus: F N o F N z Decreasing, featureless spectrum of low-energy recoils (<~100 keV) z Rate depends on target mass & spin, WIMP mass & spin, DM halo, … z Neutrons are irreducible background
* { Inelastic scatter off nucleus: F N o F N o F N J z Short-lived, low-lying excited states (easier signature?)
z 129Xe(3/2+ĺ1/2+) + J (40 keV), 73Ge(5/2+ĺ9/2+) + J (13 keV) z Neutrons are irreducible background
* { Inelastic dark matter (iDM): F N o F N z If there is no elastic channel, then kinematic effects may ensure that “particles will scatter at DAMA but not at CDMS” (Smith & Weiner 2001) z Recoil spectrum with threshold (mass splitting, G) z Neutrons are irreducible background The WIMP-search technology zoo
Ionisation Detectors
Targets: Ge, Si, CS 2, CdTe CoGeNT, DRIFT, GENIUS, Light & Ionisation HDMS, IGEX, NEWAGE Heat & Ionisation Detectors BlBolometers Targets: Xe, Ar Targets: Ge,Si ionisation ArDM, LUX, WARP, CDMS, EDELWEISS XENON, ZEPLIN Q cryogeni(50ic (<50 mK) cold (LN2)
SitilltScintillators BlBolome ters
Targets: NaI, Xe, Ar Targets: Ge, Si, Al 2O3, TeO2 ANAIS, CLEAN, DAMA, CRESST-I, CUORE, CUORICINO DEAP, KIMS, LIBRA , NAIAD, XMASS, ZEPLIN-I Bubbles & Droplets Light & Heat Bolometers CF3Br, CF3I, C3F8, C4F10 Targets: CaWO 4, BGO, Al2O3 COUPP, PICASSO , SIMPLE CRESST, ROSEBUD cryogenic (<50 mK) ZEPLIN-III: 2 -phase xenon TPC
Readout of scintillation light and iitiionisation charge with array of 31 photomultipliers
Gaseous xenon Liquid xenon
photomultipliers
Sensitivity to single ionisation electrons! Edwards et al., arXiv:0708.0768 Secondary Scintillation Primary Scintillation S1 (electroluminescence) S2
6 Single electron sensitivity in S2
1e probability v S1 (photoionisation) 1e
~30 phe
Sensitivity: Quantum of ionisation measured in >kg LXe targets Nuclear recoil detection threshold ~0.2 keVnr
Calibration: Edwards et al., Astroparticle Phys. 30 (2008) 54 Absolute ionisation yields, electroluminescence, for study of single-electron emission in ZEPLIN-II trigger thresholds, electron lifetime, etc 7 ZEPLIN-III design features
{ Good light collection for scintillation
z Photomultippqliers immersed in the liquid
z Slab geometry (35 mm LXe height, 390 mm dia)
{ Improved discrimination
z ‘Open plan’ target, no extraction grids
z High field operation (4 kV/cm)
z Precision 3D position reconstruction
{ Low background construction
z Copper construction, low background Xe
12 kg LXe
8 ZEPLIN-III: Entrails
9 Boulby Underground Laboratory We’re all in the gutter, but some of us are looking underground…
Depth1100m(2.8kmw.e.) 10 Shielding castle
{ Shielding against rock radioactivity { 30 cm hydrocarbon, 20 cm boxed lead { 105 attenuation for both neutrons and gamma-rays
11 Co-57 daily calibration
{ S1 calibration defines energy scale for electron (and nuclear) recoils (keVee, keVnr)
{ Good energy resolution byyp exploitin g S1-S2 anti-correlation Ionisation e- contribute either to S1 (recombination) or to S2 (charge extraction) V=5.4%@122keV in FSR, 6.7% with new array (preliminary)
VS1 VS2
VE* S2 fraction
12 Nuclear recoil calibration (Am-Be neutrons)
2-20 keVee NR x-yydstbuto distribution biased towards source location
WIMP BOX: 2-16 keVee (10-30 keVnr) PVP
5 keVee NR from neutron elastic scatter
13 FSR results – 7 events in search box
• 84 days from 27 Feb 2008 –128 kg*days (net) in acceptance region
• Predicted neutron elastic scatters in WIMP acceptance box: 1.2±0.6 events •Estimated ER leakage into WIMP acceptance box: 11.6±3.0(stat) events 14 FSR results
Lebedenko et al, PRD 80: 052010 (2009)
•Most probable WIMP signal P=0 •90% CL limit 3.05 events
• SI cross-section limit (W-N) 8.1x10-8 pb/nucleon near 60 GeV/c2 • SD cross-section limit (W-n) 1.0x10-2 pb/neutron near 60 GeV/c2 WIMP mass, GeV/c2
Lebedenko et al, PRL 103: 151302 (2009) 15 FSR results - iDM
Schmidt-Hoberg & Winkler, JCAP09(2009)010
Akimov et al., arXiv:1003.5626v2 (ZEPLIN-III)
16 Best discrimination in two-phase xenon
•Detector tilted steadily throughout run due to local geology (~1 mrad) • Tilt measurement to 0.06 mrad accuracy with one day’s calibration data
•7 events turn to 5 –now with no significant spatial bias •e/n-recoil discrimination improves to 1:7400 in 10-30 keVnr
17 Phase II Calibration
{ Automated Co-57 source delivery for improved reproducibility
{ Fibre-coupldled LED syst em for routi ne PMT caliblibtiration
{ Calibrated Am-Be neutron source for Leff new measurement { “phantom” grid above anode for position reconstruction
{ FWHM of 2 mm in the horizontal plane for Co-57
{ Tens of Pm in the vertical direction
simulation
18 Phase II Phototube upgrade
{ PMT gamma-rays limited sensitivity of first run
{ Custom design for ultra low-background tubes, pin-by-pin compatible
{ Assembly onto ZEPLIN-III array completed (eventually…)
19 Phase II Phototube upgrade
{ Factor ~30 improvement in gamma-ray activity per PMT (35-50 mBq/PMT)
{ Factor ~ 10 improvement in overall electron-recoil background
(depth) (depth)
20 Phase II Anti-coincidence veto
{ 52-ch (~3S) anti-coincidence veto replaces some hydrocarbon shielding { An important tool for neutron rejection as well as diagnostics { Inner Gd-loaded hydrocarbon surrounded by plastic scintillator modules { Delayed coincidence detection of capture gammas from Gd (and H)
J n
21 Phase II Anti-coincidence veto
For veto hardware description Akimov et al., Astropart. Phys. (2010)
22 Veto neutron tagging efficiency
{ Neutrons detected by radiative capture in Gd-loaded plastic { 70% veto efficiency for internal neutrons
23 Veto gamma-ray tagging efficiency
{ 30% tagging efficiency for low-energy gamma-rays hitting ZEPLIN-III { Useful signal-free sample of dominant background: more robust prediction
prompt gammas
accidental coincidences
24 Second Science Run Sensitivity
{ Neutron bk: 0.5 evt/year in 5-50 keVnr, vetoed to 0.15 evt/year { Gamma bk: 14x lower, bett er cont rol of syst emati cs with new ve to
ZEPLIN-III
XENON100 WARP 140
LUX350
25 LUX-ZEPLIN – tonne-scale targets
{ US LUX team and European ZEPLIN team join forces { LZ 1.2 -tonne fiducial xenon at SUSEL (4850 ft level at Homestake, USA) { LZ20 20-tonne xenon at DUSEL (7200 ft level) – WIMP astrophysics? { Coordinated proposals to NSF/DOE/STFC/… Summary
{ Second science run under way z High duty cycle, operation largely automated z Post-upgrade background exactly as predicted z Anti-coincidence veto fullyyp operational
{ Nuclear recoil scintillation efficiency in liquid xenon z New measurement soon, with reduced systematics z Especially important for light WIMP scenarios
{ Sensitivity ~1x10-8 pb/n with one year of running
{ LUX-ZEPLIN z Very strong collaboration established z Searching full SUSY space for detection and measurement z But plans delayed (need to reward UK bankers first…)
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