Study of the elusive $5s-4f$ level crossing in highly charged osmium with optical transitions suitable for physics beyond the Standard Model searches

Nils-Holger Rehbehn; Lakshmi Priya Kozhiparambil Sajith; Michael K. Rosner; Charles Cheung; Sergey G. Porsev; Marianna S. Safronova; Steven Worm; Dmitry Budker; Thomas Pfeifer; José R. Crespo López-Urrutia; Hendrik Bekker

arXiv · 2509.06710

Study of the elusive $5s-4f$ level crossing in highly charged osmium with optical transitions suitable for physics beyond the Standard Model searches

Nils-Holger Rehbehn·Lakshmi Priya Kozhiparambil Sajith·Michael K. Rosner·Charles Cheung·Sergey G. Porsev·Marianna S. Safronova·Steven Worm·Dmitry Budker·Thomas Pfeifer·José R. Crespo López-Urrutia·Hendrik Bekker

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Abstract

Optical transitions of highly charged ions can be very sensitive to hypothetical beyond-the-Standard-Model phenomena. Those near the $5s-4f$ level crossing, where the $5s$ and $4f$ are degenerate are especially promising. We present predictions from atomic theory and measurements of Os$^{15,16,17+}$ at an electron beam ion trap for identification of several transitions suitable for searches for a hypothetical fifth force and possible violations of local Lorentz invariance. The electric quadrupole (E2) transitions of Os$^{16+}$ that were found are especially suitable for frequency metrology due to their small linewidth of 44 ${\mu}$Hz. Our calculations show the need for including enough inner-shell excitations to predict transition rates between configurations, which can otherwise be overestimated. Ultimately, the predicted interconfiguration transitions were too weak to be detected.

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Nils-Holger Rehbehn, Lakshmi Priya Kozhiparambil Sajith, Michael K. Rosner, Charles Cheung, Sergey G. Porsev, Marianna S. Safronova, Steven Worm, Dmitry Budker, Thomas Pfeifer, José R. Crespo López-Urrutia, Hendrik Bekker. 2025-09-08. Study of the elusive $5s-4f$ level crossing in highly charged osmium with optical transitions suitable for physics beyond the Standard Model searches. https://arxiv.org/abs/2509.06710

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