Italian entrance to the Fréjus tunnel, which houses the Modane Underground Laboratory -
© LSM (Modane Underground Laboratory)
"The EDELWEISS experiment" aims to detect dark matter which would represent nearly 80% of the matter in the universe, according to our partner The Conversation.
To achieve this, researchers are working with detectors operating at very low temperatures called bolometers.
The analysis of this phenomenon was carried out by physicists Laurent Bergé (Paris-Saclay University) and Antoine Cazes (University of Lyon).
In the Universe, everything is a matter of gravity.
It is gravity that makes the Moon revolve around the Earth.
It is gravity that makes the Earth revolve around the Sun, and it is again gravity that drives the Sun in the motion of our galaxy.
There is therefore a very strong relationship between mass and the movement of celestial bodies.
It was from this observation that Fritz Zwicky, in the early 1930s, observed the Coma cluster.
It is a local concentration of several galaxies, close enough to each other to feel their mutual gravity.
The result of his observations is surprising: their speeds are too high compared to their measured masses.
The mass of galaxies can in fact be evaluated thanks to their luminosity (the more light it emits, the more massive it is), and corrected according to their distance.
Zwicky's explanation is that a large part of the mass of these galaxies comes from non-light-producing matter, "dark" or "black" matter.
In the 1970s, Vera Rubin measured the rotation curve of galaxies, i.e. the speed of rotation of stars in a spiral galaxy, according to their distance from the center of the galaxy.
She arrives at the same conclusion: there is a lack of mass to explain these rotational speeds.
The quest for dark matter is therefore launched!
Distribution of the energy density of the Universe after use of the first data from the Planck satellite. Dark matter is one of its main components © ESA + S. Villeneuve CC BY-SA 4.0
Since then, many other observations can only be explained by the presence of this unknown material.
Observation of the diffuse cosmological background predicts that it should represent 80% of the matter in the Universe, and its presence is useful in explaining the formation of matter since the Big Bang.
But all these observations being indirect proofs, it is necessary to succeed in directly detecting this material to prove its existence.
This is the project of the EDELWEISS experiment.
From suspicion to direct evidence
The idea is therefore the following: this dark matter, present in our galaxy, is considered as a kind of gas of new particles in which we would bathe.
As the Earth moves in the galaxy, it directly encounters these particles, so it is not necessary to look very far for it.
But to be able to observe it directly, it is necessary that it interacts with the "standard" matter.
For that, you have to make an assumption.
We know that dark matter is not sensitive to electromagnetic interaction, since it does not produce light.
It is not a priori sensitive either to the "strong" interaction (which ensures the cohesion of atomic nuclei by "sticking" the protons and the neutrons together), because this interaction being very intense, its effects would probably already have been. detected.
While we know that dark matter is sensitive to gravitational interaction, it is impossible to detect it in this way at this time (and for a long time to come), because gravity is an extremely weak force compared to others at the scale of elementary particles, and poorly understood theoretically at this scale.
The EDELWESS cryostat, surrounded by its lead and polyethylene shielding, in white © Collaboration EDELWEISS + Authors (via The Conversation)
There remains the weak interaction (that of neutrinos or beta radioactivity): nothing forces dark matter to be sensitive to it, but nothing prohibits it either.
In this case, we speak of WIMP (the equivalent of "massive weakly interacting particle" in English).
Indeed, if we want to explain the balance between dark matter and known matter, from the Big Bang to the present day, they must interact with each other, with an intensity comparable to that of the weak interaction.
The hypothesis put forward is therefore very reasonable.
A detector for WIMPs
It remains to design a detector that would be sensitive enough to measure these weak interactions of WIMPs with ordinary matter.
Many experiments have been tried, with different technologies.
The EDELWEISS collaboration has chosen to work with detectors operating at very low temperatures called bolometers.
The principle is as follows: a WIMP interacts by tapping on one of the germanium atoms that make up the bolometer.
This atom moves back by absorbing energy, then resumes its place by making vibrations.
These vibrations correspond to an increase in temperature of the order of a microdegree, proportional to the energy deposited during the collision.
This temperature rise can be measured by placing the bolometers at an extremely low operating temperature, of the order of 10 millikelvins, close to absolute zero.
Avoid pollution of measurements by natural radioactivity and cosmic rays
The main difficulty of the experiment is to distinguish the collision of a WIMP - of an extremely low probability (less than one collision per year and per kilogram of germanium) - from other much more frequent events, generally produced by the natural radioactivity of detector materials, or by cosmic rays.
To do this, a second measurement of the interaction of the WIMP with the Germanium of the bolometers is carried out, this time electric.
Doubling the measurement makes it possible to reject measurements linked to radioactivity which do not correspond to what we are really looking for.
Part of EDELWEISS III detectors: the circular electrodes are evaporated on the 800 gram germanium crystal, with the temperature sensor in the center © Collaboration EDELWEISS + Authors (via The Conversation)
In addition, the materials that make up the experimental set-up are carefully chosen for their very low radioactivity and polyethylene and lead shields surround the cryostat.
Lead is very effective in protecting against external radioactivity, but unfortunately it itself contains a weakly radioactive isotope (lead 210, which has a half-life of 26 years).
This armor is therefore itself covered with a layer of archaeological lead, coming from a sunken Roman boat discovered by archaeologists: the radioactivity of this lead, which is several hundred years old, has completely disappeared.
In addition, it remained protected by seawater from cosmic rays, which induce this radioactivity.
Due to this constraint on the radioactivity external to the detectors, the experiment was installed in the Underground Laboratory of Modane, right in the middle of the Fréjus tunnel, which links France and Italy.
Under 1,700 meters of rock, cosmic rays are a million times less numerous than on the surface.
Their interactions will therefore be much weaker at the level of the underground laboratory and they will therefore pollute the detector much less.
The extreme sensitivity of EDELWEISS bolometers will make it possible to search for low-mass WIMPs, a field that has not yet been explored very much.
Conversely, other detectors, less sensitive, but more massive, will be optimized to discover the heavier WIMPs.
EDELWEISS III contains a series of 36 detectors © Collaboration EDELWEISS + Authors (via The Conversation)
Where is the quest for dark matter?
With all of this effort, have we found dark matter?
Well not yet!
So goes the research.
EDELWEISS began in the 1990s, and has continued to advance its technology to probe deep into nature ever since.
At each step, failing to discover a WIMP, we gradually set limit curves that express the sensitivity of the experiment.
These curves represent the probability of interaction of WIMPs with matter, as a function of the mass of WIMPs: above this limit, the detector is able to detect WIMPs.
If it does not detect them, it is because they would be below the line in terms of mass and low probability of interaction.
This graph represents the probability of interaction of WIMPs as a function of their mass. The curves are the sensitivity limits of the EDELWEISS experiments (EDELWEISS-III LT, the latest results), and Xenon in its 100Kg version (XENON100 LT) and 1 ton (XENON1T (Standard)). EDELWEISS-Surf is a prototype allowing measurements at lower mass. The results of a week of surface measurements are presented with a standard analysis, and with an analysis using the Migdal effect (an effect which, if it exists, predicts a different measurement of the recoils of the nuclei) © Modified by Elsa Couderc to from Collaboration EDELWEISS, CC BY SA-3.0
Competition in particle physics is fierce!
After having given the best limits a few years ago, EDELWEISS has been overtaken by other experiments in terms of sensitivity, the best results currently being held by the Xenon experiment.
We are currently in an intense period of R & D to improve our detectors.
Our results on the test bench (EDELWEISS-surf on the graph) are already the most efficient.
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We are therefore confident to reach the best limit again, and we will continue to lower this curve little by little, until one day we discover dark matter… if it really exists!
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This analysis was written by Laurent Bergé, physicist at the University of Paris-Saclay and Antoine Cazes, physicist, lecturer at the Institute of Nuclear Physics of Lyon (University of Lyon).
The original article was published on The Conversation website.
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