Astronomical observations suggest that dark matter, comprising over 80% of all matter, only interacts gravitationally with visible matter. Researchers at PTB used sensitive atomic clocks to seek evidence of ultralight dark matter affecting the fine-structure constant, but found no significant changes, refining our understanding of its potential interactions and the stability of the constant over time.
Comparisons between optical clocks at PTB bolster the quest to detect potential interactions between ultralight dark matter and photons.
Observations in astronomy suggest the presence of “dark matter,” accounting for over 80% of all matter. To our current understanding, it primarily interacts with visible matter through gravitational forces. Notably, there’s no established evidence that it interacts with photons, the fundamental particles that also form light. This lack of interaction is why it’s termed “dark.” The composition of dark matter and any potential unknown interactions with regular matter continue to be intriguing puzzles.
A particularly promising theoretical approach implies that dark matter could consist of particles that are extremely light and behave more like waves than individual particles: so-called “ultralight” dark matter. In this case, previously undiscovered, weak interactions of dark matter with photons would lead to minuscule oscillations of the fine-structure constant.
The fine-structure constant is the natural constant that describes the strength of the electromagnetic interaction. It determines the atomic energy scales and thus influences the transition frequencies that are used as references in atomic clocks. Since different transitions are sensitive to possible changes of the constant to varying degrees, comparisons of atomic clocks can be used to search for ultralight dark matter. For this purpose, researchers at PTB have now used an atomic clock that is particularly sensitive to possible changes of the fine-structure constant in such a search.
For this purpose, this sensitive atomic clock was compared with two other atomic clocks with lower sensitivities in months-long measurements. The resulting measurement data were investigated for oscillations, the signature of ultralight dark matter. Since no significant oscillations were found, the dark matter remained “dark“, even under closer examination.
Detection of the mysterious dark matter was therefore not achieved. The absence of a signal allowed for the determination of new experimental upper limits on the strength of a possible coupling of ultralight matter to photons. Previous limits were improved by more than one order of magnitude over a wide range.
At the same time, the researchers also studied whether the fine-structure constant might change over time, for example by increasing or decreasing very slowly. Such a variation was not detected in the data. Here, existing limits were also tightened, indicating that the constant remains constant even over long periods of time.
In contrast to previous clock comparisons, where each atomic clock required its own experimental system, two of the three atomic clocks were realized in a single experimental setup in this work. For this purpose, two different transition frequencies of a single trapped ion were used: The ion was interrogated alternately on both optical transitions. This is an important step towards making optical frequency comparisons even more compact and robust – for example, for a future search for dark matter in space.
Reference: “Improved Limits on the Coupling of Ultralight Bosonic Dark Matter to Photons from Optical Atomic Clock Comparisons” by M. Filzinger, S. Dörscher, R. Lange, J. Klose, M. Steinel, E. Benkler, E. Peik, C. Lisdat and N. Huntemann, 22 June 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.253001
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