MESON2026

Testing charge conjugation invariance: From pions to positronium

27 Jun 2026, 12:10

20m

Medium lecture hall (B) (Auditorium Maximum)

Parallel Tests of fundamental symmetries and precision experiments Parallel session B5

Speaker

Pooja Tanty (Jagiellonian University)

Description

Bound systems composed of matter-antimatter are known to provide substantial testing ground for fundamental symmetries and decay dynamics. For example the lightest quark-antiquark system, the neutral pion $\pi_0$, which decays predominantly into two photons ($\sim$ 98.82$\%$). Due to Charge conjugation (C) symmetry $\pi_0$ cannot decay into three photons. However, if calculated the branching ratio (BR) of $\pi_0 \rightarrow 3 \gamma$ with respect to $\pi_0 \rightarrow 2 \gamma$, with amplitude consistent with gauge invariance and Bose symmetry intact, the BR is extremely suppressed ($\sim10^{-31}$). Such decays are a straightforward test for the invariance of C-symmetry. The present experimental upper limit on this decay channel is $3 \times 10^{-8}$ at 90 $\%$ C.L.

Since the $\pi_0$ production typically requires high energy proton beams or photons of GeV scale, a low-energy alternative is the electron-positron bound state called Positronium (Ps). Ps is produced in the interaction of positron with electron in a porous material medium into two states - singlet, para-Positronium (p-Ps) and triplet ortho-Positronium (o-Ps). Due to C-symmetry, o-Ps (p-Ps) decays into even (odd) number of photons. The decay dynamics of p-Ps is additionally constrained by the bosonic nature of the photons, which forbids its decay into a configuration of 4 photons flying off in the direction of a regular tetrahedron vertices. Hence, observation of o-Ps decaying into this particular configuration could be used to test the violation of C-symmetry while mitigating the major background from p-Ps. Utilizing the triggerless data acquisition in the modular J-PET detector, we obtain a non-zero detection efficiency for such rare events. In this presentation, the ongoing study of this forbidden decay with the J-PET detector shall be discussed.

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