Most of our understanding of physics today can be neatly summarized in the Standard Model of particle physics (SM), a vastly successful model that accurately predicts – to our highest measurement precision – the interactions between subatomic particles. However, we know that the SM is incomplete since it cannot explain phenomena such as the existence of dark matter or the surplus of matter (over antimatter) in the universe. These are some of the most fundamental and challenging puzzles in physics today; dark matter comprises 85% of the matter in our universe and, despite concerted efforts, we still have not identified its origin or constituents.
This project applied atomic spectroscopy on trapped ions of ytterbium and calcium to search for a fifth fundamental force of nature that may unveil physics beyond the SM. We are searching for a new force between electrons and neutrons, which could be carried, or mediated, by a hypothetical dark-matter particle – a dark boson. If present, this force would cause small shifts in atomic energy levels [1]. Crucially, the magnitude of these shifts would depend on the number of neutrons in the atomic nucleus. Hence, this force can in principle be detected by looking for minute shifts in atomic transition frequencies between isotopes of the same element (i.e. atoms that are identical but have different numbers of neutrons in their nucleus).
However, there are already well-known SM effects that cause atomic-transition-frequency differences between isotopes, aptly named isotope shifts. To differentiate such shifts from any potential new-physics effects, we utilize a King Plot [2]. In a King plot, one measures frequency shifts on two transitions between at least 3 distinct pairs of isotopes x. Data points for each pair are then added to the King plot, with their x and y coordinates corresponding to the measured shifts on the first and second transitions respectively. If only first-order SM shifts are present, the data will lie along a straight line fit. A nonlinearity indicates that we are observing physics beyond first-order SM effects.
During the outgoing phase of this project, I performed isotope-shift spectroscopy of trapped singly charged ytterbium ions, alongside the Vuletić group at MIT. This work led to the first observation of a King nonlinearity in a search for new physics [3,4]. Apart from new physics, certain higher-order SM (nuclear) effects can also contribute to King-plot nonlinearity. However, we found that, by looking at the pattern of the scatter of the data points on the King plot, we could distinguish the possible physical effects contributing to the deviations – each effect produces a fixed pattern of scatter of the data points (i.e. a pattern of residuals from the fitted line), which is determined by that effect’s dependence on the isotope’s nuclear structure [3,4]. We know a priori what pattern will be produced by the presence of a new boson and, with the help of nuclear theory collaborators, we were also able to predict the patterns expected from higher-order nuclear effects [4]. From our analysis of the data, we concluded, with 4σ certainty (>99.99% certainty), that at least two distinct physical effects contributed to the nonlinearity we observed. The first was well-modelled by a higher-order nuclear effect, but the second remains unexplained.
To determine the source of the second effect, it was necessary to increase the precision with which we were measuring the transition frequency shifts. This increase in measurement precision was achieved during the incoming phase of this project, which took place at the Home group at ETH Zürich. By co-trapping calcium isotopes and using a technique called a decoherence-free subspace (DFS) to suppress any measurement noise common to both isotopes [6], we increased measurement precision by 4 orders of magnitude, to 0.1Hz in one transition in singly-charged calcium. We now plan to combine this data with measurements made by the group of Piet Schmidt on a transition in highly-charged calcium ions, to produce the first sub-Hz King plot. This King plot should breach our current sensitivities to new physics by an order of magnitude.
Aside from searching for new physics, the precise spectroscopic measurements in this project probed the nuclear structure of ytterbium and calcium with unprecedented precision, and comparison of our measurements with nuclear theory allowed the benchmarking of nuclear models in ytterbium [4]. This in turn can enable exciting progress in both nuclear and astrophysics, including the development of better models of the physics of neutron stars (which are astrophysical objects with physics that is analogous to that of heavy nuclei) and the identification of new hypothesized superheavy stable elements from astrophysical spectral lines [5].
[1] J. C. Berengut et al, PRL 120, 091801 (2018)
[2] W. H. King, Plenum Press, New York, 1984.
[3] I. Counts*, J. Hur*, D. P. L. Aude Craik, et al, Phys. Rev. Lett. 125, 123002 (2020)
[4] J. Hur*, D. P. L. Aude Craik*, I. Counts* et al, Phys. Rev. Lett. 128, 163201 (2022)
[5] V. A. Dzuba et al, PRA 95, 062515 (2017)
[6] Manovitz et al, PRL 123, 203001 (2019)
