Double Beta Potential of Zeplin Can we do HDM search with CDM detectors? Very, very preliminary assessment Roland Lüscher Zeplin: brief summary of DM programme Doing bb-search with a DM detector?

Direct Detection Kinematics: WIMP Ro dR = e dE Eor 10 8 6 4 2 -2 5 15 20 25 Event Energy (keV) Event Rate (events/kg/day) Raw spectrum Noise corrected Nuclear Recoil Limits Predicted DM spectrum WIMP MD MT dR dE R = Ro Eor e -ER/Eor recoil energy incident kinematic factor = 4MDMT/(MD + MT)2 event rate per unit mass total event rate (point like nucleus) Background rejection tools: (1) Nuclear recoil discrimination (2) Directional signal

Background Discrimination Nuclear recoil discrimination Signal Channel Differences Ge, Si 40keV Nuclear Recoil M≈ 200GeV v = 230km/s WIMP Scintillation Ionisation Phonon + - Pulse Shape Differences NaI, CsI, Xe Xe Gamma ray (or beta decay) 40keV Electron Recoil Different dE/dX, Range, etc.

LXe: Why using it? Heavy nuclei (A~130), sensitive to spin-independent interactions (Isi~A2). Isotopes with large spin-dependent factors, Isd~0.3 for 129Xe (23Na in NaI or 73Ge in Ge ~0.1) ... … as well as easier enrichement. Scintillation in UV (l=175 nm), low visible energy threshold. Large quenching factor for Xe recoils g(A)>0.2 (I in NaI: g(A)=0.09, Na: g(A)~0.3), low recoil energy threshold. Low background Scintillation properties enables Pulse-Shape Discrimination (PSD) between background (electron recoil) and nuclear recoil events. Interesting ionisation properties … Scale up possibilities (up to 1 ton ?)

ZEPLIN I Xe+ +Xe Xe2+ +e- Xe* Xe** + Xe +Xe Xe2* 2Xe Ionisation Electron/nuclear recoil Xe2+ Excitation +e- (recombination) Xe* Xe** + Xe +Xe Xe2* 175nm 175nm Triplet 27ns Singlet 3ns 2Xe

ZEPLIN I Installation Counts vs. time Xe temp vs. time room temp vs. time Pb shielding Top of veto Boulby stub 2 laboratory xenon purification

ZEPLIN I Energy Resolution 57 Co Calibration 122 keV peak 136 keV peak (10%) Linear response 1.5 - 2.5 p.e./keV (57Co calibration is effective point source) Resolution 30 keV X-ray 92 keV recoil e- 106 keV recoil e-

Z1 Energy Calibration Multi-source calibration Variation in yield commensurate with volume response matrix Geant4 Simulation of 57Co calibration With/without energy resolution

Uniformity, light collection Part of detector response matrix 17% best, 14% bulk Falls to 4% below Xe delivery pipe Light collection simulations allow S3 calculations S3 cut < 0.408 (66% on 1: 0.5 + 0.5/3) Fiducial volume 3.1kg (excluding turrets + ~1cm below) Scan through detector girth Single PMT event Trigger condition is set to ‘free-run’ Noise trigger Bulk events Turret events Double PMT event Fiducial volume cut Scan through one PMT plane

cPCI DAQ system Dual range DAQ: Acqiris cPCI digitizers, 1ns sampling, 8 bits Mean data rate: 2Hz Max rate: 800 Hz (cal) Deadtime ~1 ms Dual range DAQ: one covers ‘high’ energy up to 150 keV (daily E cal) 2nd is the dark matter interval, up to 40 keV. (less digital noise at low E)

ZEPLIN I Discrimination 10-20keV Using different fitting techniques Exp, Mean, mean to 90%, median Fitted ‘gamma’ density function in 1/t Surface calibrations data to 7keV Surface ambient neutrons to 4 keV Gamma source Neutron source

Statistical methods 91 day livetime, 280kg.days data  calibration from veto-ed events -density fit (actually in 1/t) as guide: smooth slope Analysis: 2 over signal region, poisson stat on tail c2 Poisson

ZEPLIN I limit ‘Standard’ DM model Nuclear physics: Quenching = 0.2, Form factor

Dual Phase Xenon e- g Applying an electric field Prevents (part of) recombination Measure both scintillation and ionisation Electro-luminescence Gas phase Active, liquid phase Primary-Scintillation e- or nucl. recoil g

Dual phase discrimination .. but still keeping some interesting pulse shape properties on primary Active volume: 30kg LXe

Zeplin 3 Gas phase Active volume 6kg Inverse field region … 31 PMTs

Projected limits

Zeplin Max - towards 1t ? Modular approach ? Zeplin-2 type

LXe: Why using it? - - ++ Low background Scintillation properties enables Pulse-Shape Discrimination (PSD) between background (alphas) and electron recoil events. Interesting ionisation properties … Scale up possibilities (up to 1 ton ?) 3 candidates to look at: 136Xe, with E0 = 2.48 MeV, 8.9% natural abundance 134Xe, with E0 = 0.85 MeV, 10.4% natural abundance 124Xe, with E0 = 0.82 MeV, 0.1% natural abundance Phase-space factor, hence rate, is going with E05 World yearly of enriched 124Xe is … 5 liters! - - ++

bb in 136Xe Gotthard experiment: 5.3kg Xe TPC 62.5% 136Xe

Limit on 136Xe bb? T 1/2 = (NA/0.136) A e (M/(sB))1/2 A: Isotopic abundance 8.9% or 65% e: Detector efficiency nearly 1 M: Target mass 3.1 kg, 30kg, 250kg s: Energy resolution 40 keV (p.e. statistics) 60 keV (extrapol. from low E, 1.2√E) B: Background rate ? ? ? What is it in Z1? Can I extrapolate to Z2? To Zmax?

(from different locations) Z1: Background at high E Pulse Shape analysis at high Energy gammas Alphas (from different locations)

Z1: possible limit on bb ‘Gamma-like’ background: ~ exponential shape Assuming a spectroscopic smearing due to the varying light collection efficiency through the active volume - a quick ‘back of the envelope’ estimate: 0.05 counts/kev/kg/year T1/2 > 1.9 x 1024 yr (90%CL, assuming enrichment of 65%, 1yr data) => 0.9 - 2.5 eV Dama, 6.5kg LXe DM detector, 68.8% 136Xe: T1/2 > 1.2 x 1024 yr PLB, 546 (2002) 23 Gotthard, 5.3kg Gxe TPC, 62.5% 136Xe: T1/2 > 4.4 x 1023 yr PLB, 434 (1998) 407

Extrapolation to Z2 and ZMax Zeplin 2: 30kg, same background rate (?) > 5.9 x 1024 yr, < 0.5 - 1.4 eV Zeplin Max: 250kg module, same background rate (?) > 1.7 x 1025 yr, < 0.3 - 0.8 eV To be compared with: < 0.1eV (Cuore), < 0.02 eV (Genius, EXO) (for 1 ton, 1yr) 0.35 eV (Heidelberg-Moscow claim) Competitive with large scale bb expt. … … but only if we can B (factor 10)

Conclusion Can we do HDM search with Zeplin? May be, yes! Slight problem: isotopic enrichment would be needed … ……… buy one get, one free?!? Well, what about a free cookie? WIMP spin in/dependant cross section studies - - would require some isotopic separation! 128Xe 129Xe 130Xe 131Xe 132Xe 134Xe 136Xe 1.9 26.4 4.1 21.2 26.9 10.4 8.9 Odd Even