Probing Visible Matter with Polarised and Entangled Baryon-Antibaryon Pairs

by Prof. Karin Schönning (Dept. of Physics and Astronomy, Uppsala University)

Wednesday 9 Dec 2026, 11:00 → 12:00 Europe/Vienna

Besprechungsraum 2 (Marietta Blau Institute for Particle Physics, Dominikanerbastei 16 [PSK], 1010 Vienna)

How does the strong interaction generate the massive protons and neutrons from almost massless quarks? And why is there so much more matter than antimatter in the universe? These two questions both belong to the most intriguing puzzles of contemporary physics and have sparked numerous large-scale experimental and theoretical undertakings worldwide. At first glance, these puzzles may seem disparate. While the first illuminates the remarkable complexity of the strong interaction and its emergent phenomena within the Standard Model of particle physics, the second is intimately associated with searches for physics beyond it. However, the two questions are connected through a common diagnostic tool: spin-polarised and entangled baryon-antibaryon pairs.

Strange and charmed baryons are heavy and unstable siblings of the protons and neutrons that constitute the visible universe. Through their weak-self-analysing decays, unstable baryons reveal information that is difficult to extract from the insanely stable protons: their spin properties. The Beijing Spectrometer (BESIII) at the Beijing Electron-Positron Collider (BEPC-II) in China is uniquely suited for such studies by virtue of its clean and abundant baryon-antibaryon pair production. In this talk, I will present how the spin polarisation and entanglement of such pairs have been exploited to extract the femtometer structure of baryons, i.e. the scale where quarks form hadrons. The very same methodological framework provides a precision tool to test fundamental symmetries such as charge conjugation and parity (CP) conservation in baryon decays. Violation of CP symmetry is a necessary criterion for Baryogenesis – the dynamic generation of matter with respect to antimatter – to be possible. An overview of recent spin-dependent CP tests by BESIII will be presented, followed by a discussion of future opportunities at BESIII and beyond.

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