Spotting kaon decays into four muons

The LHCb experiment is designed to study heavy-flavour particles containing beauty and charm quarks. Nevertheless, thanks to the large strangeness production cross-sections at the LHC as well as the excellent reconstruction performance of LHCb at low momenta, the experiment is also able to produce precise results in strange decays, complementary to those from dedicated experiments such as NA62 and KOTO. The collaboration has recently released a “trillio-scale” upper limit on the branching fraction of the decay K⁰_S → μ⁺μ^–μ⁺μ^–, being the first at this scale at the LHC. The same dataset was used to search for K⁰_L → μ⁺μ^–μ⁺μ^–, yielding the world best upper limit f and the first LHC result on a K⁰_L decay.

According to the Standard Model (SM), K⁰_S (K⁰_L) mesons decay into four muons at a very small rate of a few 10^–14 (10^–13). The decay rates of these processes are very sensitive to possible contributions from new, yet-to-be discovered particles such as dark photons, which could significantly enhance or suppress the decay rate via quantum interference with the SM amplitude. Despite the unprecedented K⁰-meson production rate at the LHC, performing this search is challenging due to the low transverse momentum (typically a few hundred MeV) of the muons. LHCb exploits its unique capability to select, in real time, low transverse-momentum muons – a capability that has improved in recent years thanks to the versatility of its online trigger system. The analysis used machine learning to discriminate long-lived particles from combinatorial background, as well as a data-driven and detailed map of the detector material around the interaction point. The invariant mass of the four-muon system is used as a control variable to statistically separate the potential signal from the remaining combinatorial background.

No selected event consistent with the decay of K⁰_S into four muons, which should appear in the region around the K⁰_S mass of 498 MeV, was observed (see figure 1). In the absence of a signal, upper limits on the respective branching fractions are set to 5.1 × 10^–12 for the K⁰_S decay mode and 2.3 × 10^–9 for the K⁰_L mode at 90% CL. These results represent the world’s most precise searches for these decays, and the branching fraction for K⁰_S → μ⁺μ^–μ⁺μ^– is the most stringent upper limit on a K⁰_S decay mode. 

The upgraded LHCb detector, which started data-taking this year, offers excellent opportunities to further improve the search precision and eventually find evidence of this decay. In addition to the increased luminosity, the LHCb upgrade has a fully software trigger, which is expected to significantly improve the efficiency for K⁰ decays into four muons and other decays with very soft final-state particles.