Direct Dark Matter Searches With ZEPLIN-III and Beyond

Henrique AraújoImperial College London

On behalf of the ZEPLIN-III Collaboration:Edinburgh University (UK), Imperial College London (UK),

ITEP-Moscow (Russia), LIP-Coimbra (Portugal)STFC Rutherford Appleton Laboratory (UK)

RHUL, 7th October 2009

RHUL (2009) 2

Outline

1. The old dark matter problem

Some compelling evidence

Weakly Interacting Massive Particles

2. How to catch a WIMP

3. The ZEPLIN-III experiment at Boulby

Workings of a two-phase xenon TPC

Results from the First Science Run

Status of Second Science Run

4. Tonne-scale xenon

Discovery potential of tonne-scale targets

LUX-ZEPLIN at SUSEL

LUX-ZEPLIN at DUSEL

5. Conclusions

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Astrophysical evidence

Virial theorem applied to gravitationally bound systems relates the mean potential and kinetic energies. In 1930s (!) Zwicky noted that galaxies in the Coma cluster were moving far too rapidly to be held together by the amount of luminous matter.

Rotation curves of spiral galaxies (including our own) show that the luminous matter is not sufficient to prevent it from flying apart! Rotation is too fast to originate only from the visible component.

X-ray emission from galaxy clusters shows more mass in intergalactic gas than in galaxies themselves – but not enough! It can also predict the cluster mass independently – but again, a large fraction is missing.

Gravitational lensing allows mapping of the mass distribution by measuring how object bends light from a background object. Lensing confirms that galaxy clusters are more massive than suggested by their luminosity.

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Total density: 0 = 1.02±0.02

Dark Energy:

= 0.73±0.04

Matter density: m = 0.27±0.04

Baryon density: b = 0.044±0.004

Neutrinos (HDM):

< ~0.015

Non-baryonic Cold Dark Matter (CDM)d = m - b = 0.22

96% of what makes up the Universe is unknown!!!

Baryons are missing on all scales (and times): galaxies, clusters, large scale structure, cosmological distances

Standard Cosmology made DM a problem of physics, not just astrophysics

Dark Matter,

22%

Dark Energy,

73%Atoms,

4.4%

The -CDM Concordance Cosmology

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Click to add meaning

“As we know,There are known knowns.There are things we know we know.We also knowThere are known unknownsThat is to sayWe know there are some thingsWe do not know.But there are also unknown

unknowns,The ones we don’t knowWe don’t know.”

Donald Rumsfeld, former US Secretary of DefenceDepartment of Defence news briefing, February 12, 2002

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Weakly Interacting Massive Particles

WIMPs remain the best motivated candidate

and attract most experimental effort

What do we know about these hypothetical particles?

Cold, non-baryonic, stable, no em/strong forces, GeV–TeV mass

WIMPs start out in thermal equilibrium in the early universe

As universe expands and cools they freeze-out with a well-defined relic abundance – of order required to close the universe: CDM ~ 0 !

There is no a-priori relation between the weak interaction and the closure parameter of the universe (cosmology); the energy scales involved are staggeringly different:TCMB ~ 10-13 GeV, H ~ 10-42 GeV, mPl ~ 1019 GeV

SUSY neutralinos are great WIMPs

SUSY models predict a new, stable, elementary particle with M<1 TeV which interacts weakly with ordinary matter

Neutralinos were not proposed to solve CDM problem!

02

01

~~~~

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Main neutralino interactions

Annihilation

into fermion pairs, photons, …

Indirect searches

Elastic interaction

Spin-independent, ~ A2

Spin-dependent, ~ J/(J+1)

Direct searches

+ N + NEnr, keV

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xxxWhat does ‘our’ dark matter look like?

You are looking at a snapshot of n-body simulations of the Milky Way’s DM halo (Aquarius Project http://www.mpa-garching.mpg.de/aquarius/)

Approximate as isothermal sphere (no lumps) with Maxwellian velocity dispersion

local density 0 ~0.3 GeV/c2/cm3 (~1/pint at 100 GeV)

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How to catch a WIMP

Direct detection signatures

Nuclear recoils from elastic scattering

Annual modulation in Earth-bound detector

Directionality in Earth-bound detector

Indirect detection

HE -rays, neutrinos, cosmic-rays, etc, from annihilation mainly in galactic centres (

n2)

Physics BSM at accelerators

Looking for missing energy, SUSY, etc

Cannot establish that new particle is the dark matter,

Or that it is the only dark matter component…

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Building a WIMP detector

Consider 1 kg targetSensitive to Edep >1 keV

Expected WIMP rates

10-6−10-1 evt/day

However…

Cosmic rays, -rays

~(1−10)x106 evt/day

Neutrons are THE background!

many evt/day

1 kg

n

WIMP

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Building a WIMP detector

Move underground

Use radio-pure materials

Shield external -rays

Shield external neutrons

Actively veto neutrons

Discriminate e-recoils (, ) from n-recoils (WIMPs, n)

WIMP

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Discrimination: single channels

Hph

onon

s

ionisation

Q

L

scintillation

Exploring single detection channels: statistical or no discrimination

ScintillatorsTargets: NaI, Xe, Ar

() Energy per photon ~15 eVNR energy collection eff. 1-3%

Light gain 2-8 phe/keVSensitivity (PMTs) ~1 keV

ZEPLIN I (2 keV), NAIAD (4 keV) (PSD) DAMA (2 keV), XMASS (10 keV)

Ionisation DetectorsTargets: Ge, Si, CdTe

() Energy per e/h pair 1-5 eVNR energy collection eff. 10-30%

Sensitivity (HEMT JFET, TES) < 1 keVIGEX (4 keV), HDMS,

GENIUS (3.5 keV)

BolometersTargets: Ge, Si, Al2 O3 , TeO2

() Energy per (n.e.) phonon ~meVNR energy col. eff. (th.) ~100%

Sensitivity (TES) << 1 keVCRESST-I (0.6 keV),

CUORICINO, CUORE (5 keV)

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Discrimination: hybrid detectors

Heat & Ionisation BolometersZIP/NTD for Q & H channels

Targets: Ge,SiCDMS, EDELWEISScryogenic (<50 mK)

Light & Heat BolometersTES/NTD for L & H channelsTargets: CaWO4, BGO, Al2 O3

CRESST, ROSEBUDcryogenic (~10 mK)

Light & Ionisation DetectorsPMT for L & Q channels

Targets: Xe, ArZEPLIN-II, -III, XENON, LUX

WARP, ArDMmildly cryogenic (LN2 )

Combining response channels: real event-by-event discrimination

All 3 hybrid technologies> 99.9% discrimination @ 10-20 keV NR energy

Hph

onon

s

ionisation

Q

L

scintillation

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ZEPLIN-III Operation PrincipleReadout of scintillation light

and ionisation charge with

array of 31 photomultipliers

Liquid xenon

Gaseous xenon

Primary Scintillation

photomultipliers

S1Sensitivity to single ionisation electrons!

Edwards et al., arXiv:0708.0768Secondary Scintillation (electroluminescence)

S2

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ZEPLIN-II

Single Electron Sensitivity in S2!

Edwards et al., Astroparticle Phys. 30 (2008) 54

Sensitivity: Quantum of ionisation measured in >kg LXe targets!

Calibration: S2 electron yield calibrates absolute ionisation yields, electroluminescence, trigger thresholds

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ZEPLIN-III design features

Good light collection for scintillation

Photomultipliers immersed in the liquid

Slab geometry (35 mm drift height, D/h~10)

Better discrimination

‘Open plan’ target, no extraction grids

High field operation (4 kV/cm)

Precision 3D position reconstruction

Low background construction

Copper construction, low background Xe

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ZEPLIN-III: Entrails

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Boulby Underground LaboratoryWe’re all in the gutter, but some of us are looking underground…

Depth 1100 m (2.8 km w.e.)

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Shielding

Shielding against rock radioactivity

30 cm hydrocarbon, 20 cm boxed lead

105 attenuation for both neutrons and gamma-rays

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Drifting electrons over cm lengths requires <ppb impurity levels

UHV gas system with special getters, pump recirculation, safety mechanisms

Portable electron lifetime chamber for external lifetime measurements

Cu cold finger

View port

Ionisation chamber

Cooling fingers

Xenon vessel

Vacuum jacket

Cu cold finger

View port

Ionisation chamber

Cooling fingers

Xenon vessel

Vacuum jacket

Free electron lifetime in liquid xenon

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First Science Run

Transport from lab at IC and recommissioning u/g during 2007

Commissioning stage completed mid-Feb 2008

All systems ready: gas handling, cooling, shielding, external levelling, DAQ, slow control, calibration delivery, data pipeline, …

Required electron lifetime achieved (>20 us)

Purification in gas phase through external getters

No degradation once in target (construction with xenon-friendly materials)

Operational parameters defined

4 kV/cm in LXe, 8 kV/cm in GXe (17 kV between electrodes)

4 mm gas gap, 1.6 bar operating pressure

S1 light yield 1.8 phe/keV @4 kV/cm (5.0 phe/keV @0 kV/cm)

S2 light yield ~20 phe/electron @4 kV/cm

Very stable operation for nearly 5 months!

LN2 consumption: <20 litres/day as per thermal design

Stable pressure (temperature) throughout

Occasional (Poissonian) trips of PMT power supply

No anode/cathode trips during entire science run (many months)!

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Free electron lifetime in liquid xenon

First purification achieved ~20 us at high field (no degradation in target)

(note that there is a strong field dependence: low-field value >100 us!)

This increased slowly during the run – with NO external recirculation!

run day

lifet

ime,

us

Full depth drift time: 14 µs

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First Science Run –

datasets

Early calibrations (Am-Be, Cs-137, Co-57, Co-60, …)

Confirm performance, optimise operation parameters

First Science Run (shielded) begins 27 Feb 2008

Daily calibration with Co-57 gamma-rays

S1 & S2 stability, electron lifetime, horizontal tilt correction

Daily data dip-test (10%)

Quality monitoring, electron-recoil background, analysis tuning

Science Run ends 20 May 2008

83 days at 84% duty cycle, 27 TB raw “dark” data

850 kg*days raw exposure (12 kg LXe)

450 kg*days fiducial exposure (6.6 kg LXe)

128 kg*days efficiency-corrected exposure in WIMP-acceptance region

Final calibration runs

Extended neutron (Am-Be) calibration (5 hours)

Extended gamma (Cs-137) calibration (122 hours, volume ~ dark data)

Engineering & Physics runs (a couple of weeks)

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Data Processing

Pulse finding and event display: ZE3RA (ZEplin 3 Reduction & Analysis)

High gain

S2S1

Low gain

31 channels

x-yposition

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Co-57 daily calibration

Calibration is well understood – including low-rate, low-energy Compton feature

S1 calibration defines energy scale for electron (and nuclear) recoils

Excellent energy resolution by exploiting S1-S2 anti-correlation (=5.4@122keV)

Ionisation e- contribute either to S1 (recombination) or to S2 (charge extraction)

S1 S2

E*

S2 fraction

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Electron & nuclear recoil backgrounds

10 events/kg/day/keV in PMT -rays – excellent agreement with MC prediction

1.2±0.6 nuclear recoils predicted for FSR acceptance box from PMT neutrons

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Nuclear recoil calibration (Am-Be neutrons)

5 keVee NR from neutron elastic scatter

2-20 keVee NRx-y distributionbiased towards source location

WIMP BOX:2-16 keVee

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Electron recoil calibration (Cs-137 gammas)

14 bins in 2-16 keVee fitted with Skew-Gaussian functions:

Electron-recoil populations in Cs-137 and WIMP-search datasets almost consistent,

but small discrepancies exist due to higher rate and different source distribution

had to use e-recoils in dark data instead!

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S2/S1 calibration (dark data and AmBe)

Prediction for ER leakage into WIMP acceptance box: 11.6±3.0 events

e-re

coils

in d

ata

n-r

ecoils

in n

-cal

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Not forgetting Eee

to Enr

conversion…

WIMP acceptance box: 2-16 keVee →

~10-30 keVnr

Recall size of 5 keVee NR!

2 keVee

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Looking for detection inefficiency

Monte Carlo (GEANT4) systematics?

Correct implementation of ENDF-VI –VII

Resilient to most experimental parameters

Confirmed with independent MC FAUST

Software cut efficiencies?

No significant effect at 10 keVee

Effect persists in uncut data

Data reduction problem?

Visually scanned hundreds/thousands of events

Verified pulse parameterisations ‘by hand’

Non-linear S1 response?

Very linear system over relevant response range

Confirmed by statistics-based calibration method

Trigger inefficiency?

Software simulations confirm trigger function

Hardware tests (pulser) confirm trigger function

Lower-threshold AmBe dataset (effect <4 keVee)

Varying scintillation yield?

Leff and/or Sn must vary with recoil energy!

resp

onse

seq

uen

ce

CH1 response below 5 phe (3 keVee)

Neves 2009, arXiv:0905.2523

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Science Data –

7 events in WIMP box

• 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

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Poisson analysis on a WIMP signal

• Expectation exceeds observation (may lead artificially to lower limit)

• Opted for simple, conservative Poissonanalysis, making fewest assumptionsabout data and background:

1. E-recoil background expected near upper boundary of acceptance box, so box divided into two regions with 20% and 80% acceptance fractions

2. No WIMPs observed in lower region

3. Up to 7 WIMPs in upper region

• Most probable WIMP signal is =0• 90% CL limit is 3.05 events (2-sided)

• Scalar cross-section of 8.1x10-8 pb/nucleon near 60 GeV/c2

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FSR Result –

SI cross-section

Lebedenko et al, PRD 80: 052010 (2009)

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FSR Result –

SD W-n cross-section

Lebedenko et al, PRL 103: 151302 (2009)

WIMP mass, GeV/c2

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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!

• 7 events turn to 5 – now with no significant spatial bias

• e/n-recoil discrimination improves to 1:7400 in 10-30 keVnr

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New scintillation quenching measurements

D. McKinsey (Yale) @ TAUP’09

Neutron beam measurements with prototype cell

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Phase II –

Integration with Veto

An important tool for both neutron rejection as well as diagnostics!

Inner Gd-loaded hydrocarbon surrounded by plastic scintillator veto

Delayed coincidence detection of capture gammas from Gd and H

Final stages of commissioning completed

n

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Phase II –

Phototube upgrade

PMTs limited sensitivity of first run (gammas-rays at least)

Custom design for ultra low-background tubes, pin-by-pin compatible

Factor ~30 improvement in gamma-ray activity expected

Assembly onto ZEPLIN-III array completed

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Beyond Z3: a 5-year forward look

Why 5 years?

High readiness level for several technologies (LAr, Ge, LXe)

Reasonable timeframe for LHC announcement on SUSY

Assumptions

Standard DM halo with truncated Maxwellian velocity distribution

v0 =220 km/s, vE =240 km/s, vesc =600 km/s, o =0.3 GeV/cm3

Later consider better parameters and triaxial (non-Maxwellian) halos

Demonstrated experimental performance

Heritage from smaller systems, energy threshold, energy resolution, etc

Effective fiducial exposures (mass*time)

Excluding operational duty cycle, fiducialisation, efficiencies, etc

WIMP-nucleon XS of 1x10-9 pb with 50 GeV/c2 and 500 GeV/c2 masses

Not ‘just around the corner’ SI cross-section

50 GeV is more detectable, 500 GeV is more ‘CMSSM-favoured’

Araujo, Strigari

& Trotta

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The two WIMPs

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Technology readinessTWO-PHASE ARGON

A~40, <ER >= 13 keV @50 GeV/c2, 35 keV @500 GeV/c2

very scalable (cheap, large LAr systems demonstrated)

poor energy threshold, low atomic weight, Ar-39 background

WARP, ArDM, (DEAP/CLEAN) working on 0.1—1 tonne targets

5-tonne system within 5 years is (optimistically) possible

CRYOGENIC GERMANIUM

A~73, <ER >= 13 keV @50 GeV/c2, 57 keV @500 GeV/c2

excellent energy resolution, excellent discrimination

difficult to scale (small detector modules, <50 mK cryostats)

CDMS, EDELWEISS, (CRESST) working on 10—20 kg targets

EURECA and SuperCDMS propose ~100 kg target in 5 year timescale

TWO-PHASE XENON

A~131, <ER >= 11 keV @50 GeV/c2, 85 keV @500 GeV/c2

scalable, low threshold

control of xenon purity to <ppb is demanding

ZEPLIN-III, XENON100, LUX350, (XMASS), working on 10-100 kg

XENON1T and LUX-ZEPLIN propose 1 tonne two-phase xenon targets

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Nominal case-study systems

• ARGON 5 tonne Eth =60 keV• GERMANIUM 0.1 tonne Eth =10 keV• XENON 1 tonne Eth =10 keV

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What would a detection look like?

Markov-Chain MC results for 50 GeV/c2 and 500 GeV/c2 WIMPs

• 1t Xe superior with light WIMP, best mass constraint for heavier WIMP• 5t Ar also very useful in heavier WIMP scenario• 0.1t Ge does not profit from excellent energy resolution

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The case for tonne-scale xenon

TWO-PHASE ARGON

Useful in heavier WIMP scenario

Interesting for annual modulation – but S/N~0.3 with 5 tonne*year…

(background-free SI sensitivity 1.2x10-10 pb*year at 500 GeV/c2)

CRYOGENIC GERMANIUM

Exploitation of energy resolution requires 1 tonne target

or SI cross-section just around the corner from present limits

By this time xenon/argon could be at multi-tonne…

(background-free SI sensitivity 7.5x10-10 pb*year at 500 GeV/c2)

TWO-PHASE XENON

Best mass and cross-section constraints

Best discovery potential

(background-free SI sensitivity 5.2x10-11 pb*year at 500 GeV/c2)

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The case for three target materials

DEPLOYMENT OF ALL THREE MATERIALS WOULD BE IDEAL

Cancel systematics introduced by galactic halo by exploring spectral ratios

Eventually explore energy-dependencies of annual modulation signals

Control some backgrounds by deploying in same underground laboratory?

ASTROPARTICLE PHYSICS OR ASTRO & PARTICLE PHYSICS?

Direct searches derive their result for W-n and MW

using astrophysical parameters (0 , f(v), …) that are

affected by significant uncertainty

Particle physics model uncertainties also propagate

to interaction cross-sections (e.g. hadronic scattering

matrices, see Ellis arXiv:0801.3656v2)

Both sets of uncertainties must be studied

and reduced if the dark matter problem is

to be truly solved (i.e. all theories but one

remain standing)as

trophys

ics

model

par

ticl

e phys

ics

model

WIM

P-nucl

eon inte

ract

ion X

S

WIMP mass

WIMP

RHUL (2009) 47

LUX-ZEPLIN – tonne-scale targets

Still a long way to go to exclude full SUSY parameter space!

US LUX team and European ZEPLIN team have joined forces

LZ 1.2-tonne xenon at SUSEL (4850 ft level at Homestake, SD, USA)

LZ20 20-tonne xenon at DUSEL (7200 ft level) – WIMP astrophysics?

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TONNE-SCALE XENON AT SUSEL

4-year programme for 1.2 tonne in Davis Cavern (NSF/DOE/STFC/…)

Design study for 20 tonne at DUSEL (S4 Solicitation in US, STFC)

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Summary

The nature of dark matter is an outstanding problem in Physics that should be keeping us awake at night

The impending second run of ZEPLIN-III promises to bite into a portion of favoured parameter space – discovery potential!

Liquid xenon is the strongest technology for next generation experiments in terms of discovery potential

The LUX-ZEPLIN team has a strong track record – and the UK plays a significant role