ArDM
Encyclopedia
ArDM is a particle physics
experiment based on a liquid argon
detector, aiming at measuring signals from WIMPs (Weakly Interacting Massive Particles), which probably constitute the Dark Matter
in the universe. Elastic scattering
of WIMPs from argon nuclei is measurable by observing free electrons from ionization
and photons from scintillation
, which are produced by the recoiling nucleus interacting with neighbouring atoms. The ionization and scintillation signals can be measured with dedicated readout techniques, which constitute a fundamental part of the detector.
In order to get a high enough target mass the noble gas argon is used in the liquid phase as target material. Since the boiling point of argon is at 87 K at normal pressure, the operation of the detector requires a cryogenic system
.
It is not known how frequently a signal from WIMP-argon interaction can be expected. This rate depends on the underlying model describing the properties of the WIMP. One of the most popular candidates for a WIMP is the Lightest Supersymmetric Particle
(LSP) or neutralino from supersymmetric theories
. Its cross section
with nucleons presumably lies between 10−12 pb
and 10−6 pb, making WIMP-nucleon interactions a rare event. The total event rate can be increased by optimizing the target properties, for example by increasing the target mass. The ArDM detector is planned to contain approximately one ton of liquid argon. This target mass corresponds to an event rate of approximately 100 events per day at a cross section of 10−6 pb or 0.01 events per day at 10−10 pb.
Small event rates require a powerful background rejection. An important background comes from the presence of the unstable 39Ar isotope in natural argon liquefied from the atmosphere. 39Ar decays via beta disintegration
with a halflife of 269 years and an endpoint of the beta spectrum at 565 keV. The ratio of ionization over scintillation produced by electron and gamma interactions with argon is different from the ratio produced by WIMP elastic scattering. The 39Ar background is therefore well distinguishable, if a precise determination of the ionization/scintillation ratio is achieved. As an alternative, the use of 39Ar-depleted argon procured from underground well gases is considered.
Neutrons emitted by detector components and by materials surrounding the detector interact with argon in the same way as WIMPs. The neutron background is therefore indistinguishable and has to be reduced as well as possible, as for example by carefully choosing the detector materials. Furthermore, an estimation or measurement of the remaining neutron flux is necessary.
The detector is planned to be run underground in order to avoid backgrounds induced by cosmic rays.
; Geneva, Switzerland. Separate tests have been carried out for the light readout, the charge readout and the high voltage supply devices. The mounting of the detector above ground is planned to take place in December 2006. In a first phase of running the detector above ground, the readout devices and the high voltage supply will be tested and tuned in order to assess the detector performance characteristics as its energy threshold, its resolution and its background rejection power.
In a second phase (end of 2007), the detector is planned to be moved underground. An EoI (Expression of Interest) has been submitted to the Canfranc Underground Laboratory
in Spain. During the run underground it will hopefully be possible either to detect the WIMP or to confirm/place limits on the WIMP-nucleon cross section.
Beyond the one-ton version, the detector size can be increased without fundamentally changing its technology. A ten ton liquid argon detector is a thinkable expansion possibility for ArDM. Current experiments for Dark Matter detection at a mass scale of 1 kg to 100 kg with negative results demonstrate the necessity of larger-mass experiments, if one wants to know what Dark Matter really is.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
experiment based on a liquid argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
detector, aiming at measuring signals from WIMPs (Weakly Interacting Massive Particles), which probably constitute the Dark Matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
in the universe. Elastic scattering
Elastic scattering
In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. In this process, the kinetic energy of the incident particles is conserved, only their direction of propagation is modified .-Electron elastic scattering:When an alpha particle...
of WIMPs from argon nuclei is measurable by observing free electrons from ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
and photons from scintillation
Scintillation (physics)
Scintillation is a flash of light produced in a transparent material by an ionization event. See scintillator and scintillation counter for practical applications.-Overview:...
, which are produced by the recoiling nucleus interacting with neighbouring atoms. The ionization and scintillation signals can be measured with dedicated readout techniques, which constitute a fundamental part of the detector.
In order to get a high enough target mass the noble gas argon is used in the liquid phase as target material. Since the boiling point of argon is at 87 K at normal pressure, the operation of the detector requires a cryogenic system
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
.
Experimental goals
The ArDM detector aims at directly detecting signals from WIMPs via elastic scattering from argon nuclei. During the scattering, a certain recoil energy - typically lying between 1 keV and 100 keV - is transferred from the WIMP to the argon nucleus.It is not known how frequently a signal from WIMP-argon interaction can be expected. This rate depends on the underlying model describing the properties of the WIMP. One of the most popular candidates for a WIMP is the Lightest Supersymmetric Particle
Lightest Supersymmetric Particle
In particle physics, the Lightest Supersymmetric Particle is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models. In models with R-parity conservation, the LSP is stable. There is extensive observational evidence for an additional...
(LSP) or neutralino from supersymmetric theories
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...
. Its cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
with nucleons presumably lies between 10−12 pb
Barn (unit)
A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
and 10−6 pb, making WIMP-nucleon interactions a rare event. The total event rate can be increased by optimizing the target properties, for example by increasing the target mass. The ArDM detector is planned to contain approximately one ton of liquid argon. This target mass corresponds to an event rate of approximately 100 events per day at a cross section of 10−6 pb or 0.01 events per day at 10−10 pb.
Small event rates require a powerful background rejection. An important background comes from the presence of the unstable 39Ar isotope in natural argon liquefied from the atmosphere. 39Ar decays via beta disintegration
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
with a halflife of 269 years and an endpoint of the beta spectrum at 565 keV. The ratio of ionization over scintillation produced by electron and gamma interactions with argon is different from the ratio produced by WIMP elastic scattering. The 39Ar background is therefore well distinguishable, if a precise determination of the ionization/scintillation ratio is achieved. As an alternative, the use of 39Ar-depleted argon procured from underground well gases is considered.
Neutrons emitted by detector components and by materials surrounding the detector interact with argon in the same way as WIMPs. The neutron background is therefore indistinguishable and has to be reduced as well as possible, as for example by carefully choosing the detector materials. Furthermore, an estimation or measurement of the remaining neutron flux is necessary.
The detector is planned to be run underground in order to avoid backgrounds induced by cosmic rays.
Construction status
The ArDM detector is currently (August 2006) assembled at CERNCERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
; Geneva, Switzerland. Separate tests have been carried out for the light readout, the charge readout and the high voltage supply devices. The mounting of the detector above ground is planned to take place in December 2006. In a first phase of running the detector above ground, the readout devices and the high voltage supply will be tested and tuned in order to assess the detector performance characteristics as its energy threshold, its resolution and its background rejection power.
In a second phase (end of 2007), the detector is planned to be moved underground. An EoI (Expression of Interest) has been submitted to the Canfranc Underground Laboratory
Canfranc Underground Laboratory
The Canfranc Underground Laboratory is an underground scientific facility situated in a former railway tunnel in the Spanish Pyrenees under Monte Tobazo in Spain....
in Spain. During the run underground it will hopefully be possible either to detect the WIMP or to confirm/place limits on the WIMP-nucleon cross section.
Beyond the one-ton version, the detector size can be increased without fundamentally changing its technology. A ten ton liquid argon detector is a thinkable expansion possibility for ArDM. Current experiments for Dark Matter detection at a mass scale of 1 kg to 100 kg with negative results demonstrate the necessity of larger-mass experiments, if one wants to know what Dark Matter really is.