Rarest strange decay shrinks from sight

For every trillion K⁰_S, only five are expected to decay to two muons. Like the better known B_s → μ⁺ μ^– decay, which was first observed jointly by LHCb and CMS in 2013, the decay rate is very sensitive to possible contributions from yet-to-be discovered particles that are too heavy to be observed directly at the LHC, such as leptoquarks or supersymmetric partners. These particles could significantly enhance the decay rate, up to existing experimental limits, but could also suppress it via quantum interference with the Standard Model (SM) amplitude.

Despite the unprecedented K⁰_S production rate at the LHC, searching for K⁰_S → μ⁺μ^– is challenging due to the low transverse momentum of the two muons, typically of a few hundred MeV/c. Though primarily designed for the study of heavy-flavour particles, LHCb’s unique ability to select low transverse-momentum muons in real time makes the search feasible. According to SM predictions, just two signal events are expected in the Run-2 data, potentially making this the rarest decay ever recorded.

The analysis uses two machine-learning tools: one to discriminate muons from pions, and another to discriminate signal candidates from the so-called combinatorial background that arises from coincidental decays. Additionally, a detailed and data-driven map of the detector material around the interaction point helps to reduce the “fixed-target” background caused by particles interacting with the detector material. A background of K⁰_S → π⁺π^– decays dominates the selection, and in the absence of a compelling signal, an upper limit to the branching fraction of 2.1 × 10^–10 has been set at 90% confidence. This is approximately four times more stringent than the previous world-best limit, set by LHCb with Run-1 data. This result has implications for physics models with leptoquarks and some fine-tuned regions of the Minimal Supersym­metric SM.

The upgraded LHCb detector, scheduled to begin operating in 2021 after the present long shutdown of the LHC, will offer excellent opportunities to improve the precision of this search and eventually find a signal. In addition to the increased luminosity, the LHCb upgrade will have a full software trigger, which is expected to significantly improve the signal efficiency for K⁰_S → μ⁺μ^– and other decays with very soft final-state particles.