LPC Physics Forum - Yihui Lai on Observation of the η → μ+μ-π+π- Decays

Thursday 13 Aug 2026, 13:00 → 14:00 US/Central

Sunrise (WH11NE) (FNAL/Zoom)

https://cern.zoom.us/j/96898909340
contact lpc-pf@fnal.gov for the passcode

lpc_physics_forum_13Aug2026.pdf

13:00 → 14:00 Observation of the η → μ+μ-π+π- Decays

The η meson's rare decays with muon-pair final states are among the least explored in the pseudoscalar sector because of their tiny branching fractions and the challenge of reconstructing highly collimated decay products at the LHC. Measuring these tiny probabilities offers one of the most sensitive tests of the SM at low energies, probing both the strong interactions and possible contributions from new light particles beyond the SM.

Over the past three years, CMS has steadily expanded the experimental study of the η meson's rarest decay modes. Following the first observations of η → μ⁺μ⁻μ⁺μ⁻ in 2023 and η → μ⁺μ⁻e⁺e⁻ earlier this year, CMS has now achieved another milestone with the first observation of η → μ⁺μ⁻π⁺π⁻. Using proton-proton collision data collected at √s = 13.6 TeV, this decay is observed with a significance of 6.2 standard deviations, together with an improved measurement of the η → μ⁺μ⁻μ⁺μ⁻ branching fraction, both measured relative to the normalization channel η → μ⁺μ⁻.

Speaker: Yihui Lai (Princeton University (US))

13August2026_LPC_Physics_Forum_Yihui_Lai_Recording.mp4

PhysicsForum_eta_2mu2pi.pdf