BESIII Collaboration Sets New Limit on Lambda Hyperon EDM

The BESIII Collaboration has achieved the world's most precise measurement of the electric dipole moment of the Lambda hyperon. The result improves experimental sensitivity by about three orders of magnitude compared to the previous direct measurement from over four decades ago.

Led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the team used a novel method exploiting quantum entanglement. They analyzed about 3 million J/ψ particle decays into quantum-entangled Lambda-anti-Lambda pairs at the Beijing Electron Positron Collider II. No evidence for a nonzero electric dipole moment was found, setting a new limit at the 10^-19 e·cm level.

A New Probe for CP Violation

Electric dipole moments are considered exceptionally sensitive probes for charge-parity (CP) violation. A permanent, nonzero EDM for a spinning particle would indicate a preferred orientation of its electric charge relative to its spin, violating time-reversal symmetry and implying CP violation. Finding new sources of CP violation is important for explaining why the universe is made of matter and not antimatter, a mystery the Standard Model of particle physics cannot fully account for.

While stringent EDM limits exist for electrons, neutrons, atoms, and molecules, hyperons-unstable particles containing strange quarks-have been far harder to test. Their rapid decay makes traditional measurement methods, which rely on observing spin precession in external electromagnetic fields, extremely challenging.

Exploiting Quantum Entanglement

The BESIII researchers adopted a fundamentally different strategy. They use the natural quantum entanglement present in Lambda-anti-Lambda pairs produced from J/ψ decays. As these hyperons decayed into proton-pion and antiproton-pion pairs, the angular distributions of the final-state particles preserved information about the spins of their parent particles.

A full angular analysis of this entangled system allowed the team to search for tiny, CP-violating differences in these distributions that would signal an electric dipole moment. "The researchers demonstrated how quantum entanglement can be used as a precision tool to investigate fundamental symmetries in short-lived particles," the source report states.

Extending the Search to Strange Baryons

The Lambda hyperon is a valuable target because it contains a strange quark. If new physics beyond the Standard Model has a flavor-dependent structure, its effects on strange quarks could differ from those on the lighter up and down quarks probed in neutron or atomic EDM searches. Therefore, hyperon measurements provide complementary information.

The methodology pioneered in this study can be extended to other hyperons, such as Sigma and Xi baryons. This could establish a systematic program of EDM searches across the strange-baryon sector, further testing the limits of known physics. The work marks a significant advance in using quantum correlations to probe the fundamental symmetries of nature in regimes previously considered inaccessible.