This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The BESIII Collaboration, led by the Institute of High Energy Physics of the Chinese Academy of Sciences, has achieved the world's most precise measurement of the electric dipole moment (EDM) of the Lambda (Λ) hyperon using quantum-entangled Λ–anti-Λ pairs produced in J/ψ decays. The result improves the experimental sensitivity by about three orders of magnitude compared with the previous measurement, providing a new way to probe charge-parity (CP) violation in particles containing strange quarks.
One of the most profound mysteries in modern physics is why the observable universe is dominated by matter rather than antimatter. Although the Standard Model has been remarkably successful in describing elementary particles and their interactions, its known sources of CP violation are insufficient to account for this cosmic imbalance. Searching for additional sources of CP violation is therefore an important path toward physics beyond the Standard Model.
Electric dipole moments provide exceptionally sensitive probes in this search. For a particle with spin, a permanent nonzero EDM would signal a preferred orientation of its electric charge distribution relative to its spin. Such an effect would violate time-reversal symmetry and, assuming CPT symmetry, would therefore also imply CP violation.
EDM searches can reveal tiny traces of new CP-violating interactions even when no new particle is directly observed. So far, stringent EDM constraints have been obtained in systems such as electrons, neutrons, atoms and molecules. Hyperons—unstable baryons containing strange quarks—offer a complementary window on CP violation but are considerably more difficult to study.
They decay extremely rapidly, and EDM measurements based on observing spin precession in an external electromagnetic field are challenging. The previous direct limit on the Λ EDM was established more than four decades ago in a fixed-target experiment at Fermilab. In this study, the BESIII Collaboration adopted a fundamentally different strategy.
Rather than relying on spin precession in an external field, the researchers exploited the quantum entanglement naturally present in Λ–anti-Λ pairs produced through the process J/ψ → Λ–anti-Λ. The Λ and anti-Λ subsequently decayed into proton–pion and antiproton–pion final states. As these weak decays preserved information about the spin directions of their parent particles, angular distributions of the final-state particles provided a way to analyze the correlated spins of the Λ and anti-Λ.
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