A study that takes a novel approach to the search for dark matter has been performed by the BASE Collaboration at CERN working together with a team at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU). For the first time the researchers are exploring how dark matter influences antimatter instead of standard matter. Their findings are now published in the latest edition of eminent scientific journal Nature. They are the results of research undertaken by scientists at Japan's RIKEN research center, the Max Planck Institute of Nuclear Physics in Heidelberg (MPIK) and the National Metrology Institute Braunschweig (PTB), working jointly in the Max Planck-RIKEN-PTB Center for Time, Constants and Fundamental Symmetries, as well as scientists from CERN, the Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), the University of Tokyo, the GSI Helmholtz Center for Heavy Ion Research in Darmstadt and the Leibniz University Hannover.
"To date, scientists have always conducted high-precision experiments at low energies using matter-based samples in the hope of finding a link to dark matter," explains Dr. Christian Smorra, the lead author of the study. Currently working at Japan's RIKEN research institute, he intends to use an ERC Starting Grant to establish a work group at JGU's Institute of Physics. "Now we've decided to search explicitly for interactions between dark matter and antimatter. It is generally assumed that interactions of dark matter will be symmetric for particles and antiparticles. Our study seeks to determine whether this is really the case."
The project's participants in fact see a double benefit in this approach: Little is known at this point about the microscopic characteristics of dark matter. At present one much-discussed possible component of dark matter is what is known as ALPs (axion-like particles). Moreover, the standard model of particle physics offers no explanation of why there is apparently so much more matter than antimatter in our universe. "Through our experiments, we hope to find clues that could provide a link between these two aspects," notes Dr. Yevgeny Stadnik, who participated in the study as part of a Humboldt Fellowship at HIM. "Possible asymmetrical interactions of this kind have not yet been explored, neither at the theoretical nor at the experimental level. Our current research work is taking a first real step in that direction."
Captured antiprotons could deliver insights into dark matter
The scientists are focusing their attention on one single antiproton that has been captured in a special device known as a Penning trap. The particle was produced by scientists using the Antiproton Decelerator (AD) at CERN, the world's only research institution capable of generating low-energy antiprotons. The scientists then stored and experimented with the antiprotons created there using the BASE Collaboration's trap system.
An antiproton has both a charge and a spin. Within a magnetic field, the spin precesses around the magnetic field lines at a constant, highly specific rate - known as the Larmor or spin precession frequency. "This means we can detect the presence of dark matter as it influences this frequency," says Christian Smorra. "For this purpose, we assume that potential dark matter particles act in the same way as a classical field with a specific wavelength. The waves produced by dark matter pass continuously through our experiment and thus have a periodic effect on the spin precession frequency of the antiproton that would otherwise be expected to remain constant."
Using their experimental set-up, the researchers have already explored a specific frequency range but without success - no evidence pointing to the influence of dark matter has come to light to date. "We've not yet been able to identify any significant and periodic changes to the antiproton's spin precession frequency using our current measurement concept," explains Stefan Ulmer, spokesperson of the BASE Collaboration at CERN. "But we have managed to achieve levels of sensitivity as much as five orders of magnitude greater than those employed for observations related to astrophysics. As a result, we can now redefine the upper limit for the strength of any potential interactions between dark matter and antimatter based on the levels of sensitivity we've managed to accomplish."
Merging of two research groups
The current project in effect merged the efforts of two research groups. The BASE Collaboration at CERN has a long and successful history of research into the fundamental properties of antiprotons, while the group led by Prof. Dmitry Budker, a researcher at the PRISMA+ Cluster of Excellence at JGU and HIM, is very active in the search for dark matter and provided important interpretive input to the study. "We determined that there is a great deal of overlap in our research and this resulted in the idea for this new approach in the search for dark matter," points out Dmitry Budker.
Going forward, the scientists hope to further enhance the precision of measurement of antiproton spin precession frequency - an essential requirement if the antimatter-based search for dark matter is to prove successful. In this connection, a team headed by Prof. Jochen Walz at the Institute of Physics at JGU, working in collaboration with MPIK and RIKEN, is developing new methods for cooling protons and antiprotons, while a group of scientists from PTB Braunschweig, the Leibniz University Hannover, and RIKEN is implementing methods for quantum logic based antiproton-spin-state readout. A variety of other promising and similar antiparticle-related studies also beckon, for example, using positrons and antimuons.
Stefan Ulmer working at the BASE experiment at CERNs Antiproton Decelerator (AD).
Foto/©: Maximilien Brice / CERN
Penning trap system of the BASE Collaboration
Foto/©: Stefan Sellner, Fundamental Symmetries Laboratory, RIKEN, Japan
C. Smorra, Y. V. Stadnik, P. E. Blessing, M. Bohman, M. J. Borchert, J. A. Devlin, S. Erlewein, J. A. Harrington, T. Higuchi, A. Mooser, G. Schneider, M. Wiesinger, E. Wursten, K. Blaum, Y. Matsuda, C. Ospelkaus, W. Quint, J. Walz, Y. Yamazaki, D. Budker& S. Ulmer, „Direct limits on the interaction of antiprotons with axion-like dark matter"