Dark Matter: The Invisible Part of the Universe

Detecting Dark Matter with the XENON Experiment

The properties of so-called dark matter and dark energy are among the biggest mysteries of the universe. Both concepts were introduced, because many astronomic observations cannot be explained without them. According to our current understanding, nearly three quarters of the universe consist of dark energy. While dark matter makes up 80 percent of the remaining quarter, the visible universe accounts for only about 20 percent: Atoms, of which we, our environment, planets, and stars are made. The nature of dark matter and dark energy is still poorly understood. KIT researchers want to shed light on darkness.

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Our Universe

Issue 2023/4 of the research magazine lookKIT deals with the main topic of the Science Year 2023.

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Astrophysical and cosmological observations provide strong indications of the existence of dark matter, from the rotation behavior of spiral galaxies to the cohesion of galaxy clusters to the development of cosmic structures. “Without dark matter, our universe would look very different today,” says Professor Kathrin Valerius, who heads the Low-energy Universe Group at KIT’s Institute for Astroparticle Physics (IAP). She adds: “The stable galaxies we observe would not exist. Their outer stars have speeds that are too high and would have left the galaxies long ago.”

Dr. Klaus Eitel from the Institute for Astroparticle Physics (IAP) at KIT talks about the history of dark matter and the search for it in the Campus Report (in German only).

Researchers worldwide are trying to unveil the properties of dark matter. Dark matter is not subject to electromagnetic interaction, it emits no light. Moreover, it is not subject to strong interaction that holds together atomic nuclei. Hence, it is fundamentally different from the known “visible” matter. It is mainly evident through gravitation. “The standard model of elementary particles does not provide any explanation – but we can exclude that dark matter consists of a particle already known to us,” Kathrin Valerius says. “We are now looking for new, so far undiscovered particles that may lead us to a new understanding of particle physics.”

Many Potential Building Blocks

There is a large range of candidates for the building blocks of dark matter, such as superheavy neutrinos or so-called axions that might be a billion times lighter than electrons. Other candidates are WIMPs (Weakly Interacting Massive Particles), whose mass might be a thousand times bigger than that of protons, or primordial black holes that may have formed by the Big Bang. Each of these hypotheses is associated with different scenarios regarding the formation, particle mass, and potential interactions of dark matter. Another hypothesis discussed is that no dark matter is required, but laws of gravity change on large cosmic scales.

Dr. Klaus Eitel, Head of the Dark Matter and Neutrinos Group of IAP, explains why WIMPs are among the most probable candidates of dark matter. “The WIMP has been such an attractive and long searched for candidate, because theoretical models predicting its existence combine two important properties: The expected very low interaction rate and the existence of exactly the right amount of these particles in the early universe to explain the lacking contribution to the energy density of the universe.”

The XENON Experiment

This promising candidate might be detected underground: Shielded by the 1400 m thick rocks of the Apennine Mountains, the Laboratori Nazionali del Gran Sasso (LNGS) are located deep below the Gran Sasso massif. The LNGS are among the world’s biggest underground experimental labs. The rocks shield cosmic rays – particle showers in our atmosphere that are triggered by high-energy particles from the universe and hit the Earth’s surface. And shielding is necessary. The LNGS accommodate some of the world’s most sensitive experiments to study elementary particles and their interactions.

One of these experiments is XENONnT, the biggest and most sensitive experiment in the LNGS so far. With it, about 180 scientists from more than 20 institutions worldwide are searching for WIMPs. The research teams from Germany come from KIT, the universities of Freiburg, Mainz, and Münster, and the Max Planck Institute for Nuclear Physics in Heidelberg.

Recoiling Nuclei and Light Flashes

When dark matter fills the whole universe and in particular galaxies, it continuously passes the Earth and the Sun circulating around the galactic center. In some cases, dark matter might collide with atomic nuclei and release part of its energy. These reactions would be extremely rare and presumably take place in an energy range, in which they would be superposed by many other events, such as natural radioactivity or cosmic rays. The experiment must be accordingly selective and sensitive.

“For XENONnT, we use about ten tons of highly pure xenon gas liquified at -100 degrees Celsius in a steel tank equipped with 500 highly sensitive light sensors,” Klaus Eitel says. When a WIMP hits a xenon nucleus, the xenon atoms are excited by the recoiling nucleus and emit a flash of light that is detected by the light sensors. At the same time, the recoiling nucleus releases electrons in the liquid, which are passed to the surface by an electric field. There, they generate a second flash of light in a thin xenon gas atmosphere.

“The combination of these two flashes of light with their intensities and time lag is characteristic of a recoiling nucleus and allows conclusions to be drawn with respect to the mass of the WIMP,” Klaus Eitel adds. “To shield interfering signals of the radioactivity and cosmic rays at the Earth’s surface, the detector is located deep underground and is additionally protected by a big water tank.”

Portraitfoto von Professorin Kathrin Valerius. Amadeus Bramsiepe, KIT
“Without dark matter, our universe would look very different today,” explains Professor Kathrin Valerius, Head of the Low-energy Universe Group at IAP
Portraitfoto von Dr. Klaus Eitel XENON Collaboration
The head of the "Dark Matter and Neutrinos" group at the IAP, Dr. Klaus Eitel, says that WIMPs are promising candidates to explain dark matter.
Das Innere eines mit silberfarbenem Material ausgekleideten Zylinders. XENON Collaboration
View into the water tank of the XENONnT detector, which is designed to help shield interference signals.
Eine Person steht im Schutzanzug auf einer Leiter neben einem etwa zwei Meter hohen, kupferfarbenen Zylinder. XENON Collaboration
The heart of the XENONnT detector: The copper structure forms an electric field inside. In this field, electrons released by collisions between WIMPs and xenon nuclei are guided upwards through the liquid xenon.

Construction under Most Difficult Conditions

XENONnT started operation in 2021. The data measured so far confirm that XENONnT reaches an extremely high purity of xenon and, hence, a very long “survival time” of the electrons to be measured thanks to its sophisticated and technically complex equipment. This is even more important when detectors are extended and drift distances of electrons become longer. XENONnT also reaches a so far unmatched low background level of electron scattering in the detector, which improves the sensitivity to signals of smallest energies close to the detection threshold. This means that XENONnT cannot only hunt for WIMPs, but also search for “new physics,” such as axion particles from the Sun, or for so-called “dark photons” or the magnetic properties of a neutrino.

KIT’s XENON group was established in 2020 and took part in the commissioning of the XENONnT detector, partly under difficult conditions due to the pandemic. “Since then, we have contributed to collecting data and evaluating measurements. A doctoral thesis of our group focuses on the question how the XENONnT detector could be used to detect the next galactic supernova via its neutrino signal and how XENON could contribute to the global supernova detection network SNEWS,” Klaus Eitel says.

The Next Generation Is Already Planned

While the present generation of XENON detectors is still using its scientific potential, work in the next decade is already being planned. The next DARWIN experiment will search for WIMPs and address a broad spectrum of current physical questions, e.g. whether neutrinos are their own antiparticles. For that, the detector with presently around 10 tons of xenon will be expanded by a factor of five at least.

“KIT will contribute its expertise gained from other large-scale experiments to the setup of DARWIN,” Kathrin Valerius says. “Many technologies established for the KATRIN experiment of KIT will be incorporated in DARWIN and the technical and scientific staff of three KIT institutes will provide valuable experience. Our team is being extended continuously. This year, we recruited Yanina Biondi from the University of Zurich. Within the framework of her YIG Prep Pro Fellowship, she is working on the development of a high-voltage system for DARWIN.”

Dr. Joachim Hoffmann, December 14, 2023

Translated by: Dipl.-Übs. Maike Schröder

The co-discoverer of the Higgs boson, Professor Markus Klute from the Institute of Experimental Particle Physics (ETP) at KIT, talks about different strategies in the search for dark matter in the Campus Report (in German only).