Dark matter is still ‘dark’: scientists are using atomic clocks to shed new light

Astronomical observations indicate that dark matter, which makes up more than 80% of all matter, interacts only gravitationally with visible matter. The PTB researchers used sensitive atomic clocks to search for evidence of very light dark matter affecting the fine structure constant, but found no significant changes, improving our understanding of their possible interactions and the stability of the constant over time. time.

Comparisons of light clocks at the PTB advance the search to discover possible interactions between very light dark matter and photons. Observations in astronomy indicate that there is “dark matter”, which is more than 80% of all matter. To the best of our current knowledge, it primarily interacts with visible matter through gravitational forces. In particular, there is no consistent evidence that it interacts with photons, the fundamental particles that also make up light. This lack of interactivity is why it’s called “dark.” The composition of dark matter and any possible unknown interactions with ordinary matter remain intriguing mysteries. One particularly promising theoretical approach suggests that dark matter could be made up of very light particles and behave more like waves than individual particles: so-called “very light” dark matter. In this previously undetected case, weak dark matter interactions with photons can give rise to small oscillations in the fine structure constant.

The fine structure constant is the natural constant that describes the strength of electromagnetic interaction. It defines atomic energy scales and therefore influences the transition frequencies used as references in atomic clocks. Because different transitions are sensitive to potential changes in the constant to varying degrees, atomic clock comparisons can be used to search for extremely dark matter. For this purpose, the PTB researchers have now used an atomic clock that is particularly sensitive to potential changes in the fine structure constant in such research.

For this purpose, this sensitive atomic clock was compared to two other atomic clocks with lower sensitivities in month measurements. The resulting measurement data was examined for oscillations, the signature of light dark matter. Since there were no large oscillations, dark matter remained “dark,” even under close examination. So the detection of the mysterious dark matter did not materialize. The lack of a signal allowed the determination of new experimental upper limits for the potential coupling strength of light matter with photons. The above limits are improved by more than an order of magnitude on a large scale.

At the same time, the researchers also studied whether the fine structure constant could change over time, for example by increasing or decreasing very slowly. No such difference was detected in the data. Here, the current limits have also been adjusted, indicating that the constant remains constant even for long periods of time. Unlike previous clock comparisons, where each atomic clock required its own experimental setup, two of the three atomic clocks were achieved in a single experimental setup in this work. For this purpose, two different transition frequencies were used for a single trapped ion: the ion was alternately interrogated over both phototransitions. This is an important step to make optical frequency comparisons more compact and robust, for example, for future searches for dark matter in space.

Reference: “Improved limits on the coupling of light bosonic dark matter to photons from optical atomic clock comparisons” by M. Felzinger, S. Lysedat N. HUNTIMAN, 22 Jun 2023, available here. Physical Review Letters.DOI: 10.1103/PhysRevLett.130.253001