New insights into CP violation via penguin decays

24 April 2023

A report from the LHCb experiment.

LHCb figure 1

At the recent Moriond Electroweak conference, the LHCb collaboration presented a new, high-precision measurement of charge–parity (CP) violation using a large sample of B0s→ ϕϕ decays, where the ϕ mesons are reconstructed in the K+K final state. Proceeding via a loop transition (b → sss, such “penguin” decays are highly sensitive to possible contributions from unknown particles and therefore provide excellent probes for new sources of CP violation. To date, the only known source of CP violation, which is governed by the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, is insufficient to account for the huge excess of matter over antimatter in the universe; extra sources of CP violation are required.

A B0s or B0s meson can change its flavour and oscillate into its antiparticle at a frequency Δms/2π, which has been precisely determined by the LHCb experiment. Thus a B0s meson can decay either directly to the ϕϕ state or via changing its flavour to the B0s state. The phase difference between the two interfering amplitudes changes sign under CP transformations, denoted ϕs for B0s or –ϕs for B0s decays. A time-dependent CP asymmetry can arise if the phase difference ϕs is nonzero. The asymmetry between the decay rates of initial B0s and B0s mesons to the ϕϕ state as a function of the decay time follows a sine wave with amplitude sin(ϕs) and frequency Δms/2π. In the Standard Model (SM) the phase difference is predicted to be consistent with zero, ϕSMs  = 0.00 ± 0.02 rad.

This is the most precise single measurement to date

The observed asymmetry as a function of the B0sϕϕ decay time and the projection of the best fit are shown in figure 1 for the Run 2 data sample. The measured asymmetry is diluted by the finite decay-time resolution and the nonzero flavour mis-identification rate of the initial B0s or B0s state, and averaged over two types of linear polarisation states of the ϕϕ system that have CP asymmetries with opposite signs. Taking these effects into account, LHCb measured the CP-violating phase using the full Run 2 data sample. The result, when combined with the Run 1 measurement, is ϕs = –0.074 ± 0.069 rad, which agrees with the SM prediction and improves significantly upon the previous LHCb measurement. In addition to the increased data sample size, the new analysis benefits from improvements in the algorithms for vertex reconstruction and determination of the initial flavour of the B0s or B0s mesons.

This is the most precise single measurement to date of time-dependent CP asymmetry in any b → s transition. With no evidence for CP violation, the result can be used to derive stringent constraints on the parameter space of physics beyond the SM. Looking to the future, the upgraded LHCb experiment and a planned future phase II upgrade will offer unique opportunities to further explore new-physics effects in b → s decays, which could potentially provide insights into the fundamental origin of the puzzling matter–antimatter asymmetry.

Further reading

LHCb Collab. 2023 LHCB-PAPER-2023–001.


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