Direct limits on the interaction of antiprotons with axion-like dark matter


Astrophysical observations indicate that there is roughly five times more dark matter in the Universe than ordinary baryonic matter1, and an even larger amount of the Universe’s energy content is attributed to dark energy2. However, the microscopic properties of these dark components remain unknown. Moreover, even ordinary matter—which accounts for five per cent of the energy density of the Universe—has yet to be understood, given that the standard model of particle physics lacks any consistent explanation for the predominance of matter over antimatter3. Here we present a direct search for interactions of antimatter with dark matter and place direct constraints on the interaction of ultralight axion-like particles (dark-matter candidates) with antiprotons. If antiprotons have a stronger coupling to these particles than protons do, such a matter–antimatter asymmetric coupling could provide a link between dark matter and the baryon asymmetry in the Universe. We analyse spin-flip resonance data in the frequency domain acquired with a single antiproton in a Penning trap4 to search for spin-precession effects from ultralight axions, which have a characteristic frequency governed by the mass of the underlying particle. Our analysis constrains the axion–antiproton interaction parameter to values greater than 0.1 to 0.6 gigaelectronvolts in the mass range from 2 × 10−23 to 4 × 10−17 electronvolts, improving the sensitivity by up to five orders of magnitude compared with astrophysical antiproton bounds. In addition, we derive limits on six combinations of previously unconstrained Lorentz- and CPT-violating terms of the non-minimal standard model extension5.

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Fig. 1: Measurement of the magnetic moment of the antiproton.
Fig. 2: Results of the signal detection.
Fig. 3: Exclusion limits for the axion–antiproton interaction.

Data availability

The datasets analysed for this study will be made available on reasonable request.

Code availability

The analysis codes will be made available on reasonable request.


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We acknowledge technical support by the Antiproton Decelerator group, CERN’s cryolab team and all other CERN groups providing support to Antiproton Decelerator experiments. We acknowledge discussions with Y. Ding about the SME limits, and we thank A. Schwenk and K. Hebeler for sharing computing equipment for the Monte Carlo studies. We acknowledge financial support by RIKEN, MEXT, the Max Planck Society, the Max Planck–RIKEN–PTB Center for Time, Constants and Fundamental Symmetries, the European Union (Marie Skłodowska-Curie grant agreement number 721559), the Humboldt Program, the CERN fellowship programme and Helmholtz Gemeinschaft. Y.V.S. was supported by a Humboldt Research Fellowship from the Alexander von Humboldt Foundation and by the World Premier International Research Center Initiative (WPI), MEXT, Japan. D.B. acknowledges support by the DFG Reinhart Koselleck project, the ERC Dark-OsT advanced grant (project 695405), the Simons Foundation and the Heising-Simons Foundation.

Author information

S.U., Y.V.S. and C.S. initiated the project. C.S. analysed the experimental data and Y.V.S. provided their theoretical interpretation and derived the constraints, which were discussed with D.B. The manuscript was written by S.U., C.S. and Y.V.S. and edited by D.B. All authors discussed and approved the manuscript.

Correspondence to C. Smorra or S. Ulmer.

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Peer review information Nature thanks Gianpaolo P. Carosi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Smorra, C., Stadnik, Y.V., Blessing, P.E. et al. Direct limits on the interaction of antiprotons with axion-like dark matter. Nature 575, 310–314 (2019).

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