Optical Clocks Narrow Down the Search for Dark Matter

Researchers have used the accuracy of optical clocks to close in on the mysterious components of dark matter, as well as the coupling between parts - particles and fields - postulated by the standard model of physics.

The existence of dark matter remains to be proven. Its presence is indirectly observed through its effects on visible objects such as galaxies and stars. One of the effects supposedly caused by dark matter is an oscillation of fundamental physics constants.

On the other hand, optical clocks are extremely precise and accurate timekeeping equipment. They are so accurate in fact, that scientists estimate 20 billion years - longer than the known age of the Universe - before it leads or lags by a second.

A team of researchers, led by Jun Ye from the University of Colorado and the National Institute of Science and Technology (NIST), worked on a new attempt to detect dark matter, submitted in the online repository arXiv. Through this precision of optical clocks, researchers propose that if the optical clocks still won't detect the dark matter oscillations, it would suggest that the interaction of dark matter with observable particles in the standard model is lower than the constraints available.

Determining Values for Fundamental Constants

Previous works aimed at detecting dark matter have involved large-scale studies, such as those conducted at the CERN Large Hadron Collider (LHC). Most recently, members of the ATLAS collaboration at the LHC inquired into dark matter using the Higgs boson - using the elementary particles, transforming it into particles that are "invisible."

Other related efforts include detecting interactions with weakly interacting massive particles (WIMPs), particles whose masses are close to that of a silver atom around 100 gigaelectronvolts (GeV).

Ye's team, however, used a state-of-the-art strontium optical lattice clock, a hydrogen maser, and its own cryogenic crystalline silicon cavity to try and capture possible interactions between dark matter and particles at the lower end of the mass spectrum, in the range below eV. In comparison, the mass of an electron at rest is close to 500,000 times larger than the limit used in the study.

Setting New Constraints For Future Studies

The optical clock allows researchers to observe variations in alpha (α), known as the fine structure constant and is used to characterize the strength of interactions between photons and charged particles. Researchers compared the frequency of the strontium atoms in the optical clock to those in the silicon cavity, which allows electromagnetic waves to bounce inside its chambers. This phenomenon creates a standing wave whose characteristic frequency can be controlled based on its cavity length. The frequency of the optical clock and the cavity is defined in terms of α and me, or the mass of an electron. Furthermore, data from these two pieces of equipment were also compared to the frequency of a hydrogen maser, a frequency standard using a hydrogen atom as reference.

Researchers were not able to observe the oscillations in fundamental constants caused by dark matter interactions. This result, however, establishes a new set of constraints - narrowing down the possible values for these interactions. Dark matter particles having masses from 4.5 × 10-16 down to 1 × 10-19 electronvolts, the strength of dark matter interactions - in terms of α - is theorized to be up by a factor of five. On the other hand, interactions in terms of me could have constraints by as much as 100 times, for masses 2 × 10-19 and 2 × 10-21 eV.

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