1. Progress with cryogenic dark matter searches
CRESST to EURECA –
Progress with cryogenic dark matter searches
Hello my name’s Sam Henry, I’m from Oxford, and
I’m going to be talking about progress with cryogenic dark matter searches.
This will be a short status report on the CRESST experiment, and a summary of EURECA – which if you haven’t heard of it yet – is the proposed future European dark matter search using cryogenic detectors.
I begin with the usual pictures of mountain scenery – on the left is the Gran Sasso tunnel, home of CRESST; and on the right, the Modane tunnel where we plan to build EURECA.
Sam Henry University of Oxford

2. Dark matter - Evidence CMBR WMAP + HST: Galactic dark matter
Rotation curves of spiral galaxies
Galaxy velocities within clusters
Since I believe mine is the only talk on dark matter, I should say a few words about what it is.
In the past I would have had to explain at length all the evidence that the universe is flat and so on.
But now we just quote the figures from WMAP, which are: matter density, 0.27; baryonic matter density, So looking all across the universe, there’s a lot missing.
But there’s also evidence it exists on galactic scales, namely the rotation curves of spiral galaxies, and that is really more relevant from an experimental point of view as it’s in the galaxy that we’re trying to find it.

3. Dark matter - candidates
Neutrinos
Axions
Gravitinos, axinos
WIMPs - Weakly Interacting Massive Particles
Supersymmetric neutralinos
Kaluza Klein particles
Alternative gravity
MOND - TeVeS
Here’s the list of dark matter candidates.
We are looking for any sort of Weakly Interacting Massive Particle, and the most likely candidate is the supersymmetric neutralino.
The plot on the left shows the region of parameter space where they are expected to exist. That’s plotting the scattering cross section against WIMP mass.
But there are other WIMP candidates – Kaluza Klein particles seems to be very fashionable among some theorists at the moment.
And another thing to remember is that there are other explanations which do away with dark matter altogether, namely new theories of gravity.
So it’s by no means a forgone conclusions that neutralinos exist, and hence we need experimental dark matter searches to test the theory.
And even if neutralinos are discovered at the LHC or wherever, we still need such as experiment to show they do (or do not) account for the missing mass in our galaxy.
Ruiz de Austri, Trotta & Roszkowski

4. CRESST – Detectors Temperature pulse (~6keV)
heat bath
Resistance [m]
normal- conducting
super- conducting T R
thermal link
thermometer (W-film)
absorber crystal
Width of transition: ~1mK Signals: few  K Stablity: ~  K
Particle interaction in absorber creates a temperature rise in thermometer which is proportional to energy deposition in absorber
So how do we do that?
Well CRESST (which by the way means Cryogenic Rare Event Search with Superconducting Thermometers) uses cryogenic detectors consisting of a crystal absorber, with a thermometer deposited on the surface.
This is cooled to milli-kelvin temperature. A particle interaction in the absorber deposits most of its energy as phonons, which become trapped in the thermometer, and thermalise producing a small temperature rise.
The thermometer is a film of tungsten, biased in the centre of it’s superconducting transition between its superconducting and normal conducting phases. At this point a very small change in the temperature produces a large change in the resistance. This is then read out using a SQUID.
At least that’s how our first generation detectors worked. But a limitation of this approach is that we don’t know what sort of particle hit the detector. To achieve that we developed a new type of particle detector.
Temperature pulse (~6keV)

5. Energy in light channel keVee] Energy in phonon channel
Phonon – Scintillation
Discrimination of nuclear recoils from radioactive backgrounds (electron recoils) by simultaneous measurement of phonons and scintillation light
separate calorimeter as light detector
light reflector
W-SPT
300 g CaWO4
high rejection: 99.7% > 15 keV 99.9% > 20 keV
Energy in light channel keVee]
This is our new design of Cryogenic Phonon Scintillation Detector. In this case the crystal is a scintillator – usually CaWO4, so in addition to phonons, a particle interaction also produces a lot of scintillation photons, which are detected by a separate cryodetector above the crystal.
The benefit of this is that with two signals – phonon and light, we can distinguish electron recoil events from nuclear recoil events, as the electron recoils produce a lot more scintillation light.
This lets us cut out most of the background from alphas, betas, and gammas. There are still some remaining events due to neutrons. But we can in fact go further, as it turns out the Quenching Factor – the ratio of the phonon to light signal – or the gradient of this line – depends on the recoiling nucleus. It’s highest for tungsten the heaviest nucleus, less for Oxygen. This is very useful as we know most of the neutrons scatter off oxygen nuclei; but WIMPs will prefer the heavier tungsten.
Energy in phonon channel

6. Energy in Phonon Channel / keV
CRESST II – Results 2004
Run28, 2004, 10.5 kg d
90%
O-Recoils 50
100
150
-0.5
0.0
0.5
1.0
1.5
Light/Phonon energy
This plot shows the first significant run we did with the new detectors. It gives ratio of the light signal to the phonon signal, against energy
This band is the electron recoils. And these event – which all produced very little light – are nuclear recoils – which could be WIMPs, but are more likely to be neutrons
So we do a cut. The exclusion line is here for O recoils. And here for Ws
So the events between these lines are almost certainly neutrons, but below the lower line they are potential dark matter candidates. Therefore we get the best limit on dark matter by just considering the tungsten recoils.
Since this run we have significantly upgraded the experiment, installing a neutron shield, muon veto, and a readout system and detector holder to accommodate up to 33 detector modules.
90%
W-Recoils
Energy in Phonon Channel / keV

7. CRESST – Results 2007 Next step – install and run 17 detector modules
…and this is the results of a commissioning run we did last year with the new setup and a small number of detectors.
The big improvement compared to the previous slide is there are now far fewer neutrons.
During this run we also sorted out a number of technical problems, and the next step is to install a lot more detectors.
So the cryostat is now open and this week our colleagues are installing a total of 17 detector modules.
Next step – install and run 17 detector modules

8. CRESST / EDELWEISS – Results
Upper Limit for Spin-independent WIMP Nucleon Cross section for cryogenic detector experiments
(Preliminary)
From those data we have been able to slightly improve our dark matter limit. This plot shows the latest results from cresst and several other experiments.
We are here. The current leaders are CDMS – another experiment using cryogenic detectors over in America.
EDELWEISS are a mainly French collaboration, who also use cryogenic detectors.
XENON is using a different approach using liquid xenon as a scintillator, it is sometimes called a cryogenic experiment, but a few hundred Kelvin is hot.
There are many other projects, but I could only fit so many lines on the slide.
While CDMS and EDELWEISS both use cryodetectors, they take a different approach to CRESST, as while we measure the phonon and scintillation light signals; they use germanium detectors to measure the phonon and ionization signals; this allows them to achieve the same background discrimination.
The shape in the background is the theoretical prediction.
With the current setup CRESST aims to reach a sensitivity down the 10-8pb, the bottom setup, but the theoretic plot extends well below this plot down to 10-10pb or so, getting last low will require a new experiment.

9. European Underground Rare Event Calorimeter Array
Search to σ~10-10pb
(~1 event/tonne/year)
CRESST and EDELWEISS,
with additional groups joining.
Target: Ge, CaWO4, ZnWO4….
(A dependence)
Mass: above 100 kg towards 1 tonne
Modane Laboratory
Which will be EURECA or the European Underground Rare Event Calorimeter Array.
This is a proposed 1 tonne cryogenic experiment which will continue CRESST and EDELWEISS
(Unfortunately I don’t have time to talk about EDELWEISS in any detail. Don’t get the impression that EURECA is just the next CRESST, it’s as much their project as ours)
EURECA is taking a multi-target aproach, so we intend to use Ge (as in EDELWEISS), CaWO4 as in CRESST, and other scintillators.
This will be very useful in the event of a positive signal, as we known the scattering cross-section for WIMPs off nuclei scales with the atomic mass squared, so by comparing the event rate in multiple detectors made from different materials we can see if it’s due to WIMPs or neutrons.

10. EURECA challenges Detectors Large volume – 1000kg
Multiple target materials – Ge + scintillators
Radiopurity
Maintain background discrimination
Cryostat
Cool ~1000kg to ~10mK
Allow easy removal / insertion of detectors
Readout
1000+ channels
Radiation shielding
Neutrons
EURECA introduces a whole list of new challenges, which we need to start thinking about now.
Such as how to assembly 1 tonne of detectors; where to find a supply of high purity crystals; and how to cool them to milli-kelvin temperature.
We have an active research programme aiming to identify further suitable scintillator materials; working with crystal growers in Ukraine and Russia
As the last slide showed, 10-10pb corresponds to just one event per year in 1 tonne of material; so good radiopurity and background discrimination isn’t good enough, it will have to be perfect, if one neutron sneaks through then we’re stuffed.

11. CRESST + EDELWEISS + ROSEBUD + …
EURECA Collaboration
CRESST + EDELWEISS + ROSEBUD + …
United Kingdom
Oxford (H Kraus, coordinator)
Germany
MPI für Physik, Munich
Technische Universität München
Universität Tübingen
Universität Karlsruhe
Forschungszentrum Karlsruhe
Russia
DNLP Dubna
Ukraine
INR Kiev
France
CEA/DAPNIA Saclay
CEA/DRECAM Saclay
CNRS/CRTBT Grenoble
CNRS/CSNSM Orsay
CNRS/IPNL Lyon
CNRS/IAP Paris
Spain
Zaragoza
CERN
Here’s the current collaboration list. Every time I use this slide I have to look up another flag.
This includes all the cresst groups (Oxford, MPI, Munich, TUM and Tubingen) and the EDELWEISS groups, and some new groups, most recently Kiev, who bring a lot of useful expertise with scintillators

12. Finally I want to show the advert for the meeting of the astroparticle physics group, which we’re hosting in Oxford in June.
I hope some of you can come. Thank you.

14. Dark matter detection

15. Why use cryodetectors? Energy to extract electron in PM tube: ~300eV
Energy to ionize gas molecule: ~30eV
Energy to produce electronhole pair: 3.6eV
Energy to break superconducting electron pair: ~meV !
Initial recoil energy
Displace-
ments,
Vibrations
Athermal
phonons
Ionization
(~10 %)
Thermal phonons
(Heat)
Scintillation
(~1 %)

16. EDELWEISS – Detectors Target: Cyl. Ge crystal, 320 g
Ø 70 mm, h = 20 mm
Phonon - signal:
NTD-Ge (~ 20 mK)
Ionisation - signal:
Inner disc / outer guard ring