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# Polarization and entanglement in baryon–antibaryon pair production in electron–positron annihilation

## Abstract

Particles directly produced at electron–positron colliders, such as the J/ψ meson, decay with relatively high probability into a baryon–antibaryon pair1. For spin-1/2 baryons, the pair can have the same or opposite helicites. A non-vanishing phase ΔΦ between the transition amplitudes to these helicity states results in a transverse polarization of the baryons2,3,4. From the joint angular distribution of the decay products of the baryons, this phase as well as the parameters characterizing the baryon and the antibaryon decays can be determined. Here, we report the measurement of ΔΦ = 42.4 ± 0.6 ± 0.5° using Λ →  and $$\bar \Lambda \to \bar p\pi ^ + ,\bar n\pi ^0$$ decays at BESIII. We find a value for the Λ →  decay parameter of α = 0.750 ± 0.009 ± 0.004, 17 ± 3% higher than the current world average, which has been used as input for all Λ polarization measurements since 19785,6. For $$\bar \Lambda \to \bar p\pi ^ +$$ we find α+ = −0.758 ± 0.010 ± 0.007, giving ACP = (α + α+)/(α − α+) = −0.006 ± 0.012 ± 0.007, a precise direct test of charge–parity symmetry (CP) violation in Λ decays.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

The BESIII collaboration thanks the staff of BEPCII and the IHEP computing centre for their support. This work is supported in part by the National Key Basic Research Program of China under contract no. 2015CB856700; the National Natural Science Foundation of China (NSFC) under contract nos. 11335008, 11375205, 11425524, 11625523, 11635010, 11735014, 11835012 and 11875054; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; the CAS Center for Excellence in Particle Physics (CCEPP); Joint Large-Scale Scientific Facility Funds of the NSFC and CAS under contract nos. U1532257, U1532258, U1732102, U1732263 and U1832207; CAS Key Research Program of Frontier Sciences under contract nos. QYZDJ-SSW-SLH003 and QYZDJ-SSW-SLH040; 100 Talents Program of CAS; the CAS President’s International Fellowship Initiative; INPAC and Shanghai Key Laboratory for Particle Physics and Cosmology; German Research Foundation DFG under the contracts Collaborative Research Center CRC 1044 and FOR 2359; Istituto Nazionale di Fisica Nucleare, Italy; Koninklijke Nederlandse Akademie van Wetenschappen (KNAW) under contract no. 530-4CDP03; Ministry of Development of Turkey under contract no. DPT2006K-120470; National Science and Technology fund; The Swedish Research Council; the Knut and Alice Wallenberg Foundation; US Department of Energy under contract nos. DE-FG02-05ER41374, DE-SC-0010118, DE-SC-0010504 and DE-SC-0012069; University of Groningen (RuG); Helmholtzzentrum fuer Schwerionenforschung GmbH (GSI), Darmstadt. All consortium work was carried out at affiliations 1–67.

## Author information

### Contributions

All authors have contributed to this publication, being variously involved in the design and construction of the detectors, writing software, calibrating sub-systems, operating the detectors, acquiring data and analysing the processed data.

### Corresponding author

Correspondence to A. Kupsc.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Journal peer review information: Nature Physics thanks Anna Zuzana Dubnickova, Ulrik Egede and Ilya Selyuzhenkov for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary figures and tables.

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The BESIII Collaboration. Polarization and entanglement in baryon–antibaryon pair production in electron–positron annihilation. Nat. Phys. 15, 631–634 (2019). https://doi.org/10.1038/s41567-019-0494-8

• Accepted:

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• DOI: https://doi.org/10.1038/s41567-019-0494-8

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