Constraints on the K_{S}^{0}→μ^{+}μ^{-} Branching Fraction

Area of Science:

• Particle Physics
• High Energy Physics
• Standard Model Physics

Background:

• The decay of neutral kaons (K_{S}^{0}) into muon pairs (μ^{+}μ^{-}) is a rare process predicted by the Standard Model.
• Understanding such rare decays provides crucial tests of the Standard Model and probes for new physics beyond it.

Purpose of the Study:

• To search for the decay K_{S}^{0}→μ^{+}μ^{-} with unprecedented sensitivity.
• To set stringent upper limits on the branching fraction of this decay, constraining theoretical models.

Main Methods:

• Analysis of large datasets from proton-proton collisions at 13 TeV center-of-mass energy, collected by the LHCb experiment.
• Utilizing an integrated luminosity of 5.6 fb^{-1} from 2016-2018 data, combined with earlier data from 2011-2012.

Main Results:

• The observed signal yield for K_{S}^{0}→μ^{+}μ^{-} is consistent with zero.
• An upper limit of B(K_{S}^{0}→μ^{+}μ^{-})<2.2×10^{-10} at 90% confidence level (C.L.) was established.
• Combining with previous data, the limit improved to B(K_{S}^{0}→μ^{+}μ^{-})<2.1×10^{-10} at 90% C.L.

Conclusions:

• The stringent upper limit significantly constrains the branching fraction of K_{S}^{0}→μ^{+}μ^{-}.
• This result challenges certain theoretical extensions to the Standard Model that predict a larger branching ratio for this decay.