Charmed Baryon Decays Could Reveal New CP Violation

Physicists are looking for small differences between how particles and antiparticles decay. A 2024 theoretical study by Xiao-Gang He and Chia-Wei Liu predicts that some charmed-baryon decays could show CP-violating asymmetries at the level of a few parts per thousand—large enough to be realistic targets for high-precision experiments.

The calculation does not solve the mystery of why the universe contains far more matter than antimatter. It does, however, identify specific decay channels that can test the Standard Model and improve scientists' understanding of CP violation in particles containing a charm quark.

What CP violation means

CP combines two transformations. Charge conjugation (C) replaces a particle with its antiparticle, while parity (P) reverses spatial coordinates as if viewed in a mirror. If CP symmetry were exact, a decay and its CP-mirrored counterpart would occur at identical rates. A measured difference is called CP violation.

CP violation is one of the ingredients needed to generate a matter-dominated universe, but the amount contained in the Standard Model is not considered sufficient to explain the observed cosmic imbalance on its own. Precise decay measurements are therefore useful both for testing the model and for searching for effects it does not predict.

Why charmed baryons are useful

Baryons are particles made from three quarks. Protons and neutrons are the familiar lightest examples; charmed baryons contain at least one charm quark and decay into lighter particles. Mesons, by contrast, contain a quark and an antiquark.

CP violation had already been measured in charm-meson decays, but predictions for charmed baryons were less developed. He and Liu used flavor SU(3) symmetry together with a model of final-state rescattering to relate suppressed decay amplitudes to better-known ones.

Rescattering can amplify the observable effect

After an initial weak decay, the outgoing particles can continue to interact through the strong force. This final-state rescattering changes the relative phases of decay amplitudes. When amplitudes with different weak and strong phases interfere, a measurable CP asymmetry can result.

In the researchers' calculation, rescattering generates previously undetermined suppressed amplitudes and raises some predicted asymmetries by roughly an order of magnitude. The difference between the CP asymmetries of the decays Ξ_c⁰ → pK⁻ and Ξ_c⁰ → Σ⁺π⁻ could reach about −3.68 × 10⁻³. That is a prediction from a theoretical framework, not an experimental measurement.

What experiments need to establish

Experiments such as BESIII, Belle II and LHCb can search for the proposed charm-baryon signals as their data sets and analyses improve. A useful test would compare several related decay channels: a consistent pattern could support the rescattering model, while a significant mismatch could expose missing hadronic effects or point toward new physics.

The field has already reached a separate milestone. In 2025, LHCb reported the first observation of CP violation in a baryon decay, involving a beauty baryon rather than the charmed baryons discussed here. That result establishes that baryonic matter and antimatter can decay differently, but it does not confirm the particular charm-baryon predictions in this study. CERN provides a summary of the LHCb result, while the He–Liu calculation is available in the research preprint.

Where the result fits in particle physics

The Standard Model describes electromagnetic, weak and strong interactions among elementary particles; it does not incorporate gravity. The charm-baryon calculation works within that framework, so observing an asymmetry of the predicted size would primarily validate a difficult Standard Model calculation rather than automatically demonstrate new physics.

For background, see TipsMake's overview of major scientific breakthroughs including the Higgs boson and its explainer on whether geometry could play a role normally associated with the Higgs field. The immediate next step for this research is experimental: measure the selected charmed-baryon decay rates precisely and determine whether the predicted matter–antimatter differences are present.