1. 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?

2. Direct Detection Kinematics: WIMP Ro dR = e dE Eor10 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

3. 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.

4. 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 ?)

5. 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

6. 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

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

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

9. 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 < (66% on 1: /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

10. 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)

11. 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

12. 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

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

14. 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

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

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

17. Projected limits

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

19. 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!
- -
++

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

21. 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?

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

23. 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)
=> 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

24. Extrapolation to Z2 and ZMax
Zeplin 2: 30kg, same background rate (?)
> 5.9 x 1024 yr, < eV
Zeplin Max: 250kg module, same background rate (?)
> 1.7 x 1025 yr, < 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)

25. 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
Odd Even