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.
The study was published in Science on September 3.
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 therefore 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. Tiny differences in these distributions could therefore be used to search for an EDM-induced CP-violating effect.
Using the enormous J/ψ data sample collected with the BESIII detector at the Beijing Electron Positron Collider II (BEPCII), the researchers selected about three million high-purity J/ψ → Λ–anti-Λ events and performed a multidimensional full angular analysis of the entangled system.
No evidence for a nonzero Λ EDM was observed. The measurement nevertheless improved the experimental sensitivity by about three orders of magnitude compared with the previous direct result, reaching the 10-19 e·cm level for the first time.
Besides setting a much stronger constraint on the Λ EDM, the researchers demonstrated how quantum entanglement can be used as a precision tool to investigate fundamental symmetries in short-lived particles that are difficult to study with conventional techniques.
The Λ hyperon is particularly valuable in this context because it contains a strange quark. If new interactions beyond the Standard Model have a flavor-dependent structure, their effects on strange quarks could differ from those on lighter up and down quarks. Hyperon EDM measurements therefore provide information complementary to EDM searches involving neutrons, atoms, and molecules, helping to constrain a broader range of possible sources of new CP violation.
The strategy developed in this study can be extended to other members of the hyperon family, including Σ and Ξ baryons. Such measurements could establish a systematic program of EDM searches across the strange-baryon sector and further test physics beyond the Standard Model.
