Actinium Monofluoride: A New Frontier in the Search for CP Violation

In the quest to understand why our universe contains matter but almost no antimatter, physicists have long searched for violations of charge-parity (CP) symmetry. The Standard Model of particle physics contains some CP violation, but not enough to explain the observed cosmic imbalance. Now, a groundbreaking study published in Nature reveals that actinium monofluoride (AcF), a radioactive molecule, offers unprecedented sensitivity to CP-violating effects, potentially improving current constraints by three orders of magnitude.

The CP Violation Problem

CP violation refers to the phenomenon where the laws of physics are not symmetric under the combined operations of charge conjugation (changing particles to antiparticles) and parity (mirror reflection). According to Andrei Sakharov's conditions for baryogenesis, CP violation is essential for explaining why the universe contains more matter than antimatter. While CP violation has been observed in the weak nuclear force, the strong nuclear force appears to be CP-invariant—a puzzle known as the strong CP problem.

The search for new sources of CP violation has led physicists to investigate exotic systems where tiny effects might be amplified. Heavy polar molecules like AcF are particularly promising because they experience enormous internal electric fields that can enhance sensitivity to CP-violating nuclear moments. As reported in the Nature study, radioactive molecules containing deformed nuclei like actinium-227 offer exceptional potential for these searches.

Producing Actinium Monofluoride

The experimental breakthrough came at CERN's ISOLDE facility, where researchers successfully produced intense, chemically pure beams of 227Ac19F+. Actinium nuclides were created through nuclear reactions induced by 1.4-GeV protons bombarding a uranium carbide target. After irradiation, radiogenic actinium was extracted by heating the target to over 1,300°C, with carbon tetrafluoride supplied to form fluoride molecules.

A forced-electron-beam-induced arc-discharge ion source created ions through electron impact and plasma ionization. The resulting 227Ac19F+ beam was accelerated to 40 keV and purified using magnetic dipole separators, achieving intensities between 6×10⁶ and 2×10⁷ ions per second. Multi-reflection time-of-flight mass spectrometry confirmed the beam contained only 227Ac19F+ with no identifiable contaminants.

Spectroscopic Breakthrough

The research team performed the first spectroscopic study of AcF using collinear resonance ionization spectroscopy (CRIS). They observed the predicted strongest electronic transition from the ground state at approximately 387 nm, corresponding to the (8)¹Π ← X¹Σ+ transition. This transition is crucial for efficient readout in precision experiments searching for symmetry-violating interactions.

The observed spectrum showed a diagonal vibrational progression with visible changes in contour shape as vibrational quantum numbers increased. While the radiative lifetime of the upper state was calculated to be 6.65 nanoseconds, experimental measurements could only establish an upper limit of 38 nanoseconds due to laser pulse width limitations. Future rotationally resolved spectroscopy will provide additional information about the vibrational potential anharmonicity.

Exceptional Sensitivity to CP Violation

The most significant finding concerns AcF's sensitivity to CP-violating parameters. Relativistic coupled cluster calculations determined the molecule's sensitivity to the nuclear Schiff moment as Wₛ = -7,748(545) e/4πε₀a₀⁴. Nuclear density functional theory calculations revealed that the 227Ac nucleus has the largest laboratory-frame Schiff moment among all investigated nuclei, with sensitivity coefficients significantly higher than other promising systems like radium-225.

When combined with the molecular sensitivity, these nuclear properties make AcF exceptionally powerful for constraining CP-violating parameters. The researchers performed a global analysis incorporating existing experiments and found that a precision experiment with AcF achieving 1 mHz uncertainty would reduce the volume of the seven-dimensional CP-violation parameter space by a factor of 6×10³. At 0.1 mHz precision, this improvement increases to 6×10⁴.

Practical Implications and Future Directions

The demonstrated production rates at CERN-ISOLDE suggest that realistic precision experiments are feasible. Using Ramsey interferometry with conservative estimates for efficiency and fringe contrast, an uncertainty of 1 mHz could be achieved in 100 days of measurements with current technology. Further improvements in production rates, kinetic energy reduction, or detection efficiency could reach 0.1 mHz precision.

Future developments will focus on adapting gas-jet techniques used in radioactive atom studies to molecular systems. These approaches could provide lower rotational temperatures and improved signal-to-noise ratios for both spectroscopy and precision measurements. The techniques developed for AcF will also enable studies of other radioactive molecules with complex electronic structures, advancing fundamental, nuclear, and chemical physics research.

Conclusion

The successful production and spectroscopic characterization of actinium monofluoride represents a major advancement in the search for CP violation. With its exceptional sensitivity to nuclear Schiff moments and other CP-violating parameters, AcF offers a promising path toward discovering new physics beyond the Standard Model. As researchers develop the necessary experimental techniques, this radioactive molecule may help answer one of cosmology's most fundamental questions: why our universe is made of matter rather than antimatter.