First evidence of $CP$ violation in beauty baryon to charmonium decays

A study of the difference in the $CP$ asymmetries between $Λ^0_b \rightarrow J / ψp π^-$ and $Λ^0_b \rightarrow J / ψp K^-$ decays, $Δ{\cal A}{CP}$, is performed using proton-proton collision data collected by the LHCb experiment in the years 2015–2018, corresponding to an integrated luminosity of $6 ,{\rm fb}^{-1}$. This quantity is measured to be $ Δ{\cal A}{CP}=(4.03\pm 1.18\pm 0.23)%$, where the first uncertainty is statistical and the second is systematic. When combined with the previous LHCb result, a value of $Δ{\cal A}_{CP} = (4.31 \pm 1.06 \pm 0.28)%$ is obtained, corresponding to a significance of $3.9σ$ against the $CP$ symmetry hypothesis. Studies of triple-product asymmetries, which provide an additional probe of $CP$ violation, show no significant deviation from $CP$ symmetry.

💡 Research Summary

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The LHCb collaboration presents a precision measurement of the CP‑asymmetry difference between the decays Λ_b⁰ → J/ψ p π⁻ and Λ_b⁰ → J/ψ p K⁻ using the full Run 2 data set collected from 2015 to 2018, corresponding to an integrated luminosity of 6 fb⁻¹. These two channels are ideal for probing direct CP violation in b‑hadron decays because they are governed by the quark‑level transitions b → c c d (π⁻ mode) and b → c c s (K⁻ mode). The latter is expected to have negligible penguin contributions and therefore serves as a control channel with essentially zero CP asymmetry, while the former can receive sizable penguin‑induced weak phases, potentially leading to a measurable CP asymmetry.

The analysis begins with a trigger selection that isolates J/ψ → μ⁺μ⁻ candidates, followed by offline reconstruction of the Λ_b⁰ candidates by combining the J/ψ with a proton and a negatively charged hadron (π⁻ or K⁻). Particle‑identification (PID) information from the two Ring‑Imaging Cherenkov detectors is processed through a neural‑network classifier to distinguish pions from kaons. A multivariate Boosted‑Decision‑Tree (BDTG) classifier, trained on simulated signal and side‑band data, suppresses combinatorial background while retaining about 73 % of the signal. Additional vetoes remove specific backgrounds such as mis‑identified B⁰ → J/ψ K⁺π⁻ and Λ → p π⁻ decays.

Signal yields are extracted with an extended unbinned maximum‑likelihood fit performed simultaneously on the invariant‑mass spectra of Λ_b⁰ and its antiparticle. The signal shape is modeled with a Hypatia function whose peak position and width are allowed to float, while the combinatorial background follows an exponential distribution. Residual backgrounds from mis‑identified Λ_b⁰ → J/ψ p K⁻ events in the π⁻ sample are modeled using simulation. The fit returns 10 853 ± 134 Λ_b⁰ → J/ψ p π⁻ candidates and 125 380 ± 372 Λ_b⁰ → J/ψ p K⁻ candidates.

Raw asymmetries are defined as the difference in yields between Λ_b⁰ and Λ̄_b⁰ for each channel: A_raw(π) = (5.94 ± 1.14) % and A_raw(K) = (0.98 ± 0.30) %. Production asymmetries of Λ_b⁰ versus Λ̄_b⁰ and detection asymmetries of the final‑state proton are mitigated by re‑weighting the kinematic distributions (p, p_T, η) of the two samples to be identical, using the sPlot technique and Kish’s effective sample‑size correction.

A critical correction concerns the detection asymmetry between K⁻ and π⁻, which does not cancel in the difference of raw asymmetries. This is measured with calibration data from D⁺ → K_S⁰π⁺ and D⁺ → K⁻π⁺π⁺ decays, where the known CP‑conserving nature of these modes allows extraction of A_D(K⁻) − A_D(π⁻) = (−0.94 ± 0.04) %. Incorporating this correction yields the final CP‑asymmetry difference:

ΔA_CP = A_raw(π) − A_raw(K) + A_D(K⁻) − A_D(π⁻) = (4.03 ± 1.18 (stat) ± 0.23 (syst)) %.

Systematic uncertainties are evaluated by varying the peak parameters, signal and background shape models, the treatment of the B⁰ → J/ψ K⁺π⁻ background, the weighting procedure, and the PID and detection asymmetry corrections. The dominant contributions arise from PID asymmetry (0.074 %) and detection asymmetry (0.040 %), leading to a total systematic uncertainty of 0.23 %, well below the statistical error.

In addition to the rate asymmetry, the analysis investigates T‑odd triple‑product asymmetries (A_T‑odd) in Λ_b⁰ → J/ψ p π⁻, which are sensitive to CP‑violating phases independent of strong‑phase differences. No significant deviation from zero is observed, indicating that the observed ΔA_CP is driven primarily by the rate asymmetry.

When combined with the previous LHCb measurement based on Run 1 data (ΔA_CP = (5.7 ± 2.4 ± 1.2) %), the updated average becomes ΔA_CP = (4.31 ± 1.06 ± 0.28) %, corresponding to a 3.9σ exclusion of the CP‑conserving hypothesis. This constitutes the first evidence of direct CP violation in a beauty‑baryon decay to a charmonium final state. The result validates the expectation that b → c c d transitions can exhibit sizable penguin‑induced CP asymmetries, analogous to the well‑studied B‑meson sector, and opens a new avenue for testing the Standard Model’s description of CP violation in the baryon sector. Future data from LHC Run 3 and refined amplitude analyses will allow more precise determinations of the underlying weak phases and may reveal possible contributions from physics beyond the Standard Model.