Antihydrogen Measurement Sharpens Antimatter Symmetry Test
Matter and antimatter remain stubbornly alike. A new measurement of a tiny energy splitting in antihydrogen has subjected one of physics’ most fundamental symmetries to yet another test, finding no evidence of cracking. The measurement, performed by CERN’s Antihydrogen Laser Physics Apparatus (ALPHA) Collaboration, determined, with a precision 100 times better than previous measurements, the energy difference between two “hyperfine” ground states of antihydrogen. This hyperfine splitting turned out to be, within the experimental uncertainty, the same as that seen in hydrogen [1].
The result is a stringent test of charge–parity–time-reversal (CPT) symmetry in a regime sensitive to magnetic and spin-dependent interactions. CPT symmetry predicts that matter and antimatter should be perfect mirror images, obeying the same laws of physics while having opposite charges. The observation of a CPT violation would point to a breakdown of some of the deepest assumptions of the standard model of particle physics.
Antihydrogen is made up of an antiproton and an antielectron, or positron. It is the antimatter counterpart of hydrogen (a proton plus an electron). Since hydrogen is one of the most precisely studied systems in science—theoretically and experimentally—its comparison with antihydrogen could reveal subtle matter–antimatter differences. The challenge is producing enough antihydrogen atoms, keeping them “alive,” and controlling them precisely enough to perform spectroscopy.
The ALPHA experiment was built at CERN to tackle this challenge. The collaboration developed techniques for producing antihydrogens by bringing together antiprotons (created in particle collisions) and positrons (produced in radioactive decays). The antiatoms are then held by magnetic traps in vacuum, so they don’t annihilate by hitting regular atoms.
After the first demonstration of antihydrogen trapping and storing in 2010 [2], ALPHA turned the antiatom into a precision laboratory. The collaboration first confirmed that hydrogen and antihydrogen share the same 1S–2S transition frequency [3] and later demonstrated that antihydrogen falls downward under gravity, just like ordinary hydrogen (See Research News: Antimatter Feels Gravity Just like Matter) [4].
The team’s latest target is the hyperfine splitting transition of antihydrogen. In hydrogen, this transition is responsible for the 21-cm radio emission that astronomers use to map hydrogen clouds in the Milky Way and in other galaxies. While the 1S–2S antihydrogen transition had been previously measured with parts-per-trillion precision [3], the hyperfine transition can deliver complementary information about possible violations of CPT symmetry.
In particular, hyperfine splitting depends on details of the internal structure of antihydrogen’s nucleus—the antiproton. The antiproton isn’t a point particle, but a bundle of antiquarks and gluons that determine the spatial distribution of charge and magnetism. This inner architecture—according to theory—leaves a subtle imprint on the hyperfine splitting, modifying it by about 40 parts per million (ppm).
ALPHA first measured antihydrogen’s hyperfine splitting in 2017 [5], but the new result improves the precision by a factor of 100. The improvement was the result of “years of very hard efforts” on several fronts, says ALPHA Collaboration member Timothy Friesen of the University of Calgary in Canada. Specifically, increased trapping rates allowed the team to accumulate samples of roughly 1500 antiatoms at a time, analyzing a total of 24,000 antihydrogen atoms across several experimental runs.
Another factor was greater control of the strong magnetic field that confines and brings together the antiprotons and positrons for antihydrogen formation. Making a high-precision hyperfine-splitting measurement in this challenging magnetic environment is an “impressive achievement,” says Eberhard Widmann of the Austrian Academy of Science, who wasn’t involved in the work.
The researchers measured the ground-state hyperfine splitting to be 1,420,404.8 kHz with an uncertainty of about 6 kHz (4 ppm). With a precision 10 times smaller than the expected 40-ppm structure contribution, the experiment is entering a regime where effects related to the internal antiproton structure become relevant to interpreting the observations, Friesen says. “[This measurement] is the first one that’s really this sensitive to the structure of the antiproton.”
Despite the progress, there is ample room for future gains: Because of the challenges of working with antimatter, hyperfine measurements of antihydrogen are still a million times less precise than those of ordinary hydrogen. Friesen says laser cooling and new techniques for reducing magnetic-field-induced broadening could yield another 100-fold improvement within a few years. “Who knows what we’ll find,” he says.
In the meanwhile, the Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) Collaboration, also based at CERN, is following a complementary measurement approach based on a beam of antihydrogens rather than on trapped antiatoms. The beam approach could drastically reduce systematic uncertainties associated with the magnetic trapping environment, says Widmann, who serves as cospokesperson for ASACUSA. “Pursuing more precise hyperfine measurements is very promising” for the search of CPT violations, he says.
–Matteo Rini
Matteo Rini is the Editor of Physics Magazine.
References
- R. Akbari et al., “Four ppm measurement of the antihydrogen ground-state hyperfine splitting,” Nature 653, 1022 (2026).
- G.B. Andresen et al., “Trapped antihydrogen,” Nature 468, 673 (2010).
- C. J. Baker et al., “Precision spectroscopy of the hyperfine components of the 1S–2S transition in antihydrogen,” Nat. Phys. 21, 201 (2025).
- E. K. Anderson et al., “Observation of the effect of gravity on the motion of antimatter,” Nature 621, 716 (2023).
- M. Ahmadi et al., “Observation of the hyperfine spectrum of antihydrogen,” Nature 548, 66 (2017).