Simultaneous Observation of the 1S–2S Hyperfine Components in Antihydrogen and Implications for CPT Tests
Invited-In-person · Invited · Withdrawn
Abstract
Antihydrogen, the antimatter counterpart of atomic hydrogen, provides an exceptional platform for precision tests of matter–antimatter symmetry. The ALPHA experiment now routinely synthesizes large samples of cold antihydrogen from positrons and antiprotons. This capability is enabled by a series of techniques including sympathetic cooling of positrons with laser-cooled beryllium ions, allowing the accumulation of over 15,000 atoms in less than seven hours1, laser cooling2 and adiabatic expansion3. Combined with improved experimental procedures, key spectral lines can now be characterized within a single day, representing a 70-fold enhancement in data-collection efficiency.
Using this suite of techniques, we have simultaneously observed both accessible hyperfine components of the 1S–2S transition in magnetically trapped antihydrogen. From these data, we determine the 2S hyperfine splitting and, through comparison with hydrogen, place constraints on charge–parity–time-reversal–violating coefficients within the Standard Model Extension framework4. The precision of the 2S hyperfine splitting was limited by our measurement of the ground-state hyperfine splitting, which is sensitive to the internal structure of the antiproton and has therefore been the focus of our most recent efforts.
These advances demonstrate that it is now possible to interrogate trapped antihydrogen atoms at velocities lower than those achieved in the hydrogen samples used for the most precise matter-based measurements. The ability to characterize a spectral line in antihydrogen within a single day provides unprecedented opportunities for controlling systematic effects and for searching for sidereal variations. Together, these capabilities pave the way for studies that were previously out of reach.
[1] The ALPHA Collaboration, Be+ assisted, simultaneous confinement of more than 15000 antihydrogen atoms. Nat Commun 16, 10106 (2025)
[2] The ALPHA Collaboration, Laser cooling of antihydrogen atoms. Nature 592, 35–42 (2021)
[3] The ALPHA Collaboration, Adiabatic expansion cooling of antihydrogen. Phys. Rev. Research 6, L032065 (2024)
[4] The ALPHA Collaboration, Precision spectroscopy of the hyperfine components of the 1S–2S transition in antihydrogen. Nat. Phys. 21, 201–207 (2025)
Using this suite of techniques, we have simultaneously observed both accessible hyperfine components of the 1S–2S transition in magnetically trapped antihydrogen. From these data, we determine the 2S hyperfine splitting and, through comparison with hydrogen, place constraints on charge–parity–time-reversal–violating coefficients within the Standard Model Extension framework4. The precision of the 2S hyperfine splitting was limited by our measurement of the ground-state hyperfine splitting, which is sensitive to the internal structure of the antiproton and has therefore been the focus of our most recent efforts.
These advances demonstrate that it is now possible to interrogate trapped antihydrogen atoms at velocities lower than those achieved in the hydrogen samples used for the most precise matter-based measurements. The ability to characterize a spectral line in antihydrogen within a single day provides unprecedented opportunities for controlling systematic effects and for searching for sidereal variations. Together, these capabilities pave the way for studies that were previously out of reach.
[1] The ALPHA Collaboration, Be+ assisted, simultaneous confinement of more than 15000 antihydrogen atoms. Nat Commun 16, 10106 (2025)
[2] The ALPHA Collaboration, Laser cooling of antihydrogen atoms. Nature 592, 35–42 (2021)
[3] The ALPHA Collaboration, Adiabatic expansion cooling of antihydrogen. Phys. Rev. Research 6, L032065 (2024)
[4] The ALPHA Collaboration, Precision spectroscopy of the hyperfine components of the 1S–2S transition in antihydrogen. Nat. Phys. 21, 201–207 (2025)
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Publication: The ALPHA Collaboration, Precision spectroscopy of the hyperfine components of the 1S–2S transition in antihydrogen. Nat. Phys. 21, 201–207 (2025)
Presenters
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April Cridland Mathad
- CERN