Observation of charge-parity symmetry breaking in baryon decays

Abstract

The Standard Model of particle physics-the theory of particles and interactions at the smallest scale-predicts that matter and antimatter interact differently due to violation of the combined symmetry of charge conjugation (C) and parity (P). Charge conjugation transforms particles into their antimatter particles, whereas the parity transformation inverts spatial coordinates. This prediction applies to both mesons, which consist of a quark and an antiquark, and baryons, which are composed of three quarks. However, despite having been discovered in various meson decays, CP violation has yet to be observed in baryons, the type of matter that makes up the observable Universe. Here we report a study of the decay of the beauty baryon ${\Lambda }_0^b$ to the pK^-π⁺π^- final state, which proceeds through b → u or b → s quark-level transitions, and its CP-conjugated process, using data collected by the Large Hadron Collider beauty experiment¹ at the European Organization for Nuclear Research (CERN). The results reveal significant asymmetries between the decay rates of the ${\Lambda }_0^b$ baryon and its CP-conjugated antibaryon, providing, to our knowledge, the first observation of CP violation in baryon decays and demonstrating the different behaviours of baryons and antibaryons. In the Standard Model, CP violation arises from the Cabibbo-Kobayashi-Maskawa mechanism², and new forces or particles beyond the Standard Model could provide further contributions. This discovery opens a new path in the search for physics beyond the Standard Model.

Fig. 1. Illustration of Λ0b production in a pp collision and decay into the pK⁻π⁺π⁻ final state.

The two inset diagrams on the left illustrate the fundamental tree-type and loop-type quark-level processes that mediate the ${\Lambda }_b^0\to p{K}^{-}{\pi }^{+}{\pi }^{-}$ decay. The quarks in these processes eventually form p, K⁻, π⁺ and π⁻ particles, combined with further $u\overline{u}$ and $d\overline{d}$ quark pairs created from the vacuum. The final states may also arise through intermediate hadronic resonances. The resulting hadrons were directly detected by the LHCb detector.

Fig. 2. Mass distributions together with the fitted projections.

a,b, Mass distributions for the signal channel: ${\Lambda }_b^0\to p{K}^{-}{\pi }^{+}{\pi }^{-}$ (a) and ${\overline{\Lambda }}_b^0\to \overline{p}{K}^{+}{\pi }^{-}{\pi }^{+}$ (b). The different components used in the fit are described in detail in Methods and listed in the legend. The area under a curve represents the yield of the corresponding component. Comb. bkg., combinatorial background.

Fig. 3. Mass distributions in the R(pπ⁺π⁻) resonance phase space.

a, Distribution of the pπ⁺π⁻ mass including both ${\Lambda }_b^0$ and ${\overline{\Lambda }}_b^0$ candidates. The low-mass structure corresponds to excited nucleon resonances decaying to the pπ⁺π⁻ final state, whereas the broad structure at higher masses arises from other decay processes of the ${\Lambda }_b^0$ baryon. b,c, Mass distributions of candidates within the region delimited by the red box in a are shown for ${\Lambda }_b^0\to p{K}^{-}{\pi }^{+}{\pi }^{-}$ (b) and ${\overline{\Lambda }}_b^0\to \overline{p}{K}^{+}{\pi }^{-}{\pi }^{+}$ (c) decays, together with the fitted projections and individual components.

Extended Data Fig. 1. Mass distributions of the control channel together with the fit projections.

Displayed are the mass distributions for the control channel: (a) ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$, (b) ${\overline{\mathrm{\Lambda }}}_b^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\pi }^{+}$.

Extended Data Fig. 2. Distributions of two-body and three-body masses of final-state particles.

The mass distributions of (a) pK⁻ and (b) π⁺π⁻, corresponding to the ${\mathrm{\Lambda }}_b^0\to R\left(p{K}^{-}\right)R\left({\pi }^{+}{\pi }^{-}\right)$ phase-space region; (c) pπ⁻ and (d) K⁻π⁺, corresponding to the ${\mathrm{\Lambda }}_b^0\to R\left(p{\pi }^{-}\right)R\left(K^{-}{\pi }^{+}\right)$ phase-space region; (e) pπ⁺π⁻, representing the ${\mathrm{\Lambda }}_b^0\to R\left(p{\pi }^{+}{\pi }^{-}\right)K^{-}$ phase-space region; and (f) K⁻π⁺π⁻, representing the ${\mathrm{\Lambda }}_b^0\to R\left(K^{-}{\pi }^{+}{\pi }^{-}\right)p$ phase-space region. The ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ samples are combined for the plots.

Extended Data Fig. 3. Mass distributions in regions of phase space with the fit projections also shown.

Mass distributions of ${\mathrm{\Lambda }}_b^0\to p{K}^{-}{\pi }^{+}{\pi }^{-}$ and ${\overline{\mathrm{\Lambda }}}_b^0\to \overline{p}{K}^{+}{\pi }^{-}{\pi }^{+}$ for (a, b) ${\mathrm{\Lambda }}_b^0\to R\left(p{K}^{-}\right)R\left({\pi }^{+}{\pi }^{-}\right)$, (c, d) ${\mathrm{\Lambda }}_b^0\to R\left(p{\pi }^{-}\right)R\left({\pi }^{+}{K}^{-}\right)$, and (e, f) ${\mathrm{\Lambda }}_b^0\to R\left(K^{-}{\pi }^{+}{\pi }^{-}\right)p$ decays.