Particle Physics
Illuminating the Nature of Dark Matter Through Precision
“Intracluster light": light emitted by stars that have been torn away from their parent galaxies – in an intensified white-grey coloration. Image: ESA / Euclid space mission
Without dark matter, the universe would look entirely different; there might not even be stars or galaxies. Yet, despite intensive direct searches, the particles that make up dark matter remain unknown. They evade electromagnetic interaction but could leave subtle traces in another fundamental force of physics, the weak force. The interaction of this force with ordinary matter is the focus of the “Zeptometry” project, which includes two precision experiments, one of which is the P2 experiment at the Mainz Electron Accelerator MESA.
In physics, the largest mysteries are often linked to the smallest particles. This is certainly true for one of the greatest unsolved puzzles: Dark matter. "Astronomy tells us that matter is missing everywhere in the universe," explains Frank Maas. “For example, the arms of galaxies rotate too quickly for the visible matter there.” Over the past few decades, more and more astronomical observations have revealed gravitational phenomena that can only be explained by the presence of an enormous amount of invisible matter: About five-sixths of the matter in the universe neither reacts to light nor to microwaves or X-rays. It escapes all electromagnetic interaction.
Frank Maas (Institute for Nuclear Physics, JGU Mainz, GSI/FAIR and Helmholtz Institute Mainz) Photo: privat/ Sabrina Hopp
But what is dark matter made of? This remains completely unclear. What is certain, however, is that it cannot be particles explained by the so-called Standard Model of particle physics. The search for the building blocks of dark matter is, therefore, a search for “new physics” beyond this model, which today explains the microscopic world of particles that form our visible universe with great precision. Many research teams around the world are engaged in this crucial search, and the “Zeptometry” project has now joined them. An international consortium is leading the project as part of an ERC Advanced Grant, with one of the three principal scientists being Prof. Dr. Frank Maas, section leader at the Helmholtz Institute Mainz and senior scientist at the GSI Helmholtzzentrum für Schwerionenforschung. A zeptometer is one trillionth of a meter, one millionth smaller than the diameter of a proton. On this unimaginably small scale, the particle physics project aims to search for possible traces of dark matter particles.
A Weakness for the Weak Force?
The idea is that if dark matter evades electromagnetic forces and only reveals itself through gravity, it might still interact with the “weak force”, just like ordinary matter. This force, one of the four fundamental forces of physics, is responsible for radioactive decay. In the world of particle physics, fundamental forces are mediated by tiny “virtual” particles. For the electromagnetic force, these are virtual photons. Since photons have no rest mass, they can, in principle, travel indefinitely to mediate forces between matter particles, giving the electromagnetic force a long range. The weak force, on the other hand, is mediated by three particles with relatively high rest masses, including the Z boson. This gives the weak force a very short range of about one-thousandth the diameter of a proton. According to a special form of Heisenberg’s uncertainty principle, the lifetime of virtual particles is inversely related to their rest mass: The heavier they are, the shorter their range.
Der Teilchenbeschleuniger MESA
The new Mainz experiment, known as P2, aims to investigate the range of the weak force with extreme precision. For this, the new Mainz electron accelerator MESA, developed and built at the University of Mainz as part of the Excellence Initiative, will shoot a dense stream of electrons into liquid hydrogen, cooled to about -250 degrees Celsius. The electrons must be prepared in a specific way to be sensitive to the weak force. The experiment is designed so that the electrons will be able to sense subtle effects of potentially new particles down to a zeptometer, one-thousandth of the weak force's range. This is where the projects name, “Zeptometry”, comes from. Like a space probe performing a “swing-by maneuver” around a planet, the electrons will be deflected and eventually detected by a device that registers them.
Attractive Deviations for New Physics
The precise flight path of the electrons toward the detector contains information about the effect of the Z boson, which mediates the weak force and has influenced the electron’s trajectory. According to the Standard Model of particle physics, one electron out of 25 million arriving on a specific flight path should deviate slightly. If the number of outliers significantly deviates from this prediction, it would be a sign of “new physics” beyond the Standard Model. “That would be the exciting case,” says Maas. The weak force could then provide the critical clue as to where particle physics should focus its search for dark matter particles.
The small number of predicted outliers among the electrons also highlights how precisely the Mainz experiment must operate. Out of the 25 million electrons mentioned, the error rate must not exceed 0.01 electrons. “The entire MESA accelerator is, therefore, part of the experiment and must be precisely controlled and stabilized,” Maas emphasizes. Only if it delivers electrons with extreme stability will the measurement achieve the required precision. Moreover, an enormous amount of data must be collected over about five years to gather enough of these rare outlier events for solid statistics.
The consortium aims to exploit another important property of the weak force: It grows stronger at higher collision energies. For this reason, a team led by Maarten Bonekamp from the Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) in France will study it at the highest energies at CERN in Geneva, while the Mainz team will focus on lower energies that are inaccessible to the powerful LHC accelerator in Geneva. Together, both measurements should provide a more accurate picture of the weak force on the zeptometer scale. Theoretical physics will also play a crucial role in interpreting the data, with a team led by Prof. Jens Erler from the Institute of Nuclear Physics at Johannes Gutenberg University leading this effort.
Readers comments