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CRESST: A Dark Matter Search Experiment

The dark matter puzzle is one of the most challenging questions of modern physics. Groups from the L.N.G.S, Max-Planck-Institute for Physics in Munich, the Technical University Munich, and the University of Oxford have initiated an experiment called CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) to attack this question.

 What is the dark matter puzzle?

From the observation of the rotation of galaxies resembling our own Milky Way, astronomers have found that there must be about ten times more mass in them than is visible as light emitting stars. This puzzling discrepancy indicates that there exists some matter which is not emitting light and is therefore called dark matter. The fact that this invisible matter makes up more than 90% of the mass of galaxies and also larger structures and therefore most of our universe, leads to one of the presently most challenging and intriguing questions of physics: What does most of our universe consist of?

 What is this dark matter?

Dark matter could be made of MACHOs, huge gas balls which are too small to ignite and to shine as stars, or faint stars overlooked by our telescopes, or black holes left over from the big bang. However searches for all these dark matter candidates have failed, re-emphasising the significance of the search for elementary particle dark matter undertaken by CRESST. This experiment looks for hypothetical massive elementary particles called WIMPs (Weakly Interacting Massive Particles). It is assumed that these WIMPs are present everywhere in our galaxy and in a vast halo around it. In spite of having a mass similar to that of a nucleus, like iron for example, their extremely weak interaction with normal matter enables them to travel easily through the earth or even our galaxy without a single interaction. This extremely weak interaction, combined with important role of these particles in our universe, makes the direct search for WIMP dark matter one of the most challenging and difficult tasks of modern astroparticle physics.

 How to search for WIMPs?

WIMPs cannot be explained in the ‘standard model’ of particle physics. The existence of WIMPs would imply new physics beyond the ‘standard model’. These particles are therefore of interest to two different groups of scientists, each attacking this task in a complementary way. At huge particle accelerators like at CERN, physicists are trying to produce and to analyse new particles. Finding new physics in the future, they will obtain insight into the structure and interactions of these new particles but will not be able to prove that these particles are the dark matter. This has to be shown by direct dark matter searches as CRESST at the L.N.G.S. Finding a dark matter candidate, they can determine the mass and abundance of this particle, can determine if it solves the dark matter problem but cannot provide detailed insight into the physics beyond the ‘standard model’. Direct dark matter searches and accelerator experiments are therefore working together to push particle and astrophysics beyond their present borders.

 How does CRESST work?

Searches like CRESST look for the collision of WIMPs with ordinary matter. In such a collision a WIMP transfers part of its energy to a nucleus. Such nuclear recoils induced by WIMPs are the signals dark matter searches look for. Since these events are very rare, experiments have to fight against the background of natural radioactivity and cosmic rays.

This is the reason why dark matter searches are usually in laboratories deep underground, as the L.N.G.S., and heavily shielded against radioactivity. These searches are conducted by two classes of detectors, conventional and cryogenic. Conventional detectors measure the ionization or scintillation caused by a particle. But this reveals only a small fraction of the energy of a nuclear recoil. The cryogenic calorimeters used in CRESST are fully sensitive to nuclear recoils and additionally relatively simple devices. They consist of an absorber crystal and a thermometer on it. A particle interacting in the absorber deposits some energy which will, after a thermalization process, be converted into heat and result in a temperature rise. The temperature rise of the thermometer can be read-out electronically and is proportional to the energy deposited. The detectors used in the first stage of CRESST have sapphire crystals (each 262g) as absorbers. To achieve the highest possible sensitivity these detectors are operated at a temperature of only a hundreth of a degree above absolute zero, at 0.012K. The minimum detectable energy is 500 eV. This high energy sensitivity makes these CRESST detectors unique in the search for low mass WIMPs. The maximum achievable sensitivity depends strongly on the amount of radioactive background. Most of this background is caused by the absorption of photons or electrons. Both cause electron recoils instead of the nuclear recoil caused by WIMPs. A detector able to discriminate between electron and nuclear recoils will thus be limited only by the much smaller neutron background and therefore have a strongly enhanced sensitivity to WIMPs. The next generation CRESST detectors will use such discriminating devices based on the simultaneous detection of scintillation light and heat. The main part of this detector is a cryogenic calorimeter with a scintillating crystal as absorber. A particle interacting in this absorber produces heat and scintillation light. While the heat is measured directly in this calorimeter, the scintillation light escapes from the absorber and is detected by a nearby second calorimeter especially optimised for this purpose. Since in this scintillation crystal a nuclear recoil produces about 8 times less scintillation light than an electron recoil, a comparison of heat and scintillation signal can be used to distinguish between nuclear and electron recoils. The strong discrimination of backgrounds obtained with these detectors is a unique feature of cryodetectors, making them the natural choice for a dark matter search.

 The CRESST Set-up at the L.N.G.S.

An experiment looking for rare events like WIMP interactions must be strongly shielded from cosmic rays and natural radioactivity. The laboratories of the L.N.G.S., deeply buried under the rock of the Gran Sasso, supply an excellent shielding against cosmic rays. Additionally the CRESST experiment itself is heavily shielded against radioactivity from the surrounding by layers 20 cm of lead and 14 cm of copper, adding up to a mass of 24 t of lead and 10 t of copper. The detectors are in a so- called ‘cold box’ which is surrounded by this shielding. The extremely low temperatures needed are produced by a dilution refrigerator and transported by a long ‘cold finger’ protected by thermal radiation shields into this cold box. The cold box is designed to house more than 100 kg of detectors. Special care has been taken for radioactive purity and cleanliness of the surfaces of the cold box and shielding. As an example, in these experiments a person’s fingerprint has to be considered as a possible radioactive contamination. Cold box and shielding are therefore in a special high quality clean room. To minimize interferences, the cryostat is placed in an electromagnetically shielded room and mounted on vibration dampers. This cryostat is the first set-up of this type in a deep underground location and is so far still unique in the world. The combination of this installation, its location in the L.N.G.S. and the next generation of CRESST detectors give CRESST the potential to become the leading dark matter search world-wide.

(Author: W. Seidel, MPI München)


Mounting the first detectors

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