The BESIII collaboration has reported the ratio of ϕ meson decays into K_S⁰ K_L⁰ and K⁺ K^-  in charm meson decays for the first time. The results have been published in Physics Review Letters on 15 October 2025 [Phys. Rev. Lett. 135, 161902 (2025)].

The ϕ meson holds significant importance for studying both the strong and electroweak interactions. Its primary decay channels are the charged channel ϕ→K⁺ K^- and the neutral channel ϕ→K_S⁰ K_L⁰, with their combined branching fractions exceeding 80%, making them the most crucial decays for investigating the properties of the ϕ meson. Thus, precise measurement of these branching fractions and their ratio R_ϕ=B(ϕ→K_S⁰ K_L⁰)/B(ϕ→K⁺ K^-) would have significant theoretical and experimental implications. However, there are certain inconsistencies between theoretical predictions and experimental measurements of R_ϕ, and it has not been updated for decades. Therefore, there is an urgent need to measure R_ϕ using a novel approach independent of previous methods, thereby providing experimental basis for relevant theories.

Based on e⁺ e^- collision data at center-of-mass energies between 4.178–4.226 GeV, the BESIII collaboration measured the branching fraction of D_S⁺→ϕπ⁺, ϕ→K_S⁰ K_L⁰ through an amplitude analysis of D_S⁺→K_S⁰ K_L⁰ π⁺. Combining this result with the world average branching fraction of D_S⁺→ϕπ⁺, ϕ→K⁺ K^-, a precise value of R_ϕ was determined to be 0.593 ± 0.023_stat ±0.014_syst ±0.016_ϕπ, which deviates from the world average value by more than three standard deviations. This result will have a significant impact on the foundational inputs for studies related to non-perturbative QCD, the structures of hadronic matters, and CP violation. The figure below shows the invariant mass projections from the amplitude analysis and comparison of R_ϕ result.
   

Figure 1: (Left) The projections on invariant mass of the amplitude analysis results. (Right) Comparison of R_ϕ measured in this Letter with the existed experimental and theoretical results.

The measurement of the ϕ meson's decay branching fraction has prompted a series of reflections. For instance, given the currently measured decay proportion of ϕ decay, which are significantly less than 100%, does this suggest the existence of other undiscovered decay channels? Or does it indicate that the current absolute branching fractions of ϕ meson decays need to be remeasured? Furthermore, should existing relevant theories be revised? These questions necessitate further investigation.

Journal Link:https://journals.aps.org/prl/abstract/10.1103/6py9-h8qv