The BESIII collaboration observed baryon polarization in baryon-antibaryon (matter-antimatter particle) events from J/ψ particles produced at the BEPCII collider. The paper was published in Nature Physics on May 6.

The reaction e⁺e^？→J/ψ→ΛΛ-bar , is illustrated in Fig. 1.  The baryons used in the study are Λ hyperons. Their weak decay into proton and negative pion, Λ→pπ^-, makes it possible to determine the hyperon polarization, providing a convenient hyperon polarimeter. An example of the e⁺e^？→J/ψ→ΛΛ-bar event with the subsequent decays Λ→pπ^- and Λ-bar→p-bar π⁺ observed in the BESIII detector is shown in Fig. 2. With BESIII data from 2009 and 2012 (corresponding to 1.3 J/ψ billionevents), the number of such reconstructed events is 420K.

The Λ hyperons in the reaction were polarized in the y direction (defined in Fig. 1) and BESIII has determined the precise value of the polarization, which depends on the production angle θ_Λ as shown in Fig. 3, with the maximum reaching 25%. The observed polarization makes it possible to determine new values for the parameters of hyperon decays. The BESIII result corrects by 17% the value used in many experiments for decades. This calls for reinterpretation of all Λ hyperon polarization results.

Finally, BESIII made a direct comparison of the decay parameters of the Λ hyperon to those of its antiparticle – Λ-bar antihyperon. This is the most sensitive test of matter-antimatter symmetry (CP symmetry) involving Lambda particles. This offers prospects for the further search for new sources of CP symmetry violation in baryon decay, using the large J/ψ data sample accumulated at BESIII and possibly even larger data samples at future experiments.

Fig. 1 Graphical illustration of the e⁺e^-→J/ψ→ΛΛ-bar reaction (Image by BESIII Collaboration)

Fig. 2. An e⁺e^-→J/ψ→ΛΛ-bar event as seen in the BESIII detector (Image by BESIII Collaboration)

Fig. 3. Polarization of Λ particle as a function of the production angle θ_Λ (Image by BESIII Collaboration)