CMS observes rarest Z boson decay mode

The amazing performance of the LHC provides CMS with a large sample of Z bosons. With such high statistics, the CMS collaboration can now probe rare decay channels that were not accessible to experiments at the former Large Electron Positron (LEP) collider. One of these channels, first theoretically studied in the early 1990s, is the decay of the Z boson to a J/ψ meson and two additional leptons. Theoretical calculations of this process, illustrated in the top figure, predict a branching fraction of 6.7–7.7 × 10^–7.

The new analysis was performed using proton–proton collision data collected during 2016, corresponding to an integrated luminosity of 35.9 fb^–1. To separate signal and background events, a 2D unbinned maximum likelihood fit was used which exploits as discriminating variables the invariant masses of the reconstructed J/ψ and Z states. Due to the limited separation sensitivity of the prompt J/ψ decays from ψ(2S) → J/ψ X decays, the sum of the two modes is indicated with ψ. The decay modes Z → ψ μ⁺μ^– and Z → ψ e⁺e^– were searched for, resulting in a yield of 13 and 11 reconstructed candidates in the two channels, respectively. The significance of the Z → ψ ℓ⁺ℓ^– observation (where ℓ = μ, e) is greater than five standard deviations.

Using the Z → μ⁺μ^–μ⁺μ^– decay mode as a reference sample and after removing the (ψ2S) → J/ψ X contribution, the branching fraction ratio B(Z → J/ψ ℓ⁺ℓ^–)/B(Z → μ⁺μ^–μ⁺μ^–) in the fiducial phase space of the CMS detector is measured to be 0.70 ± 0.18 (stat) ± 0.05 (syst), assuming null J/ψ polarisation.

Extrapolating from the fiducial volume to the full space and assuming that the extrapolation uncertainties of the two channels cancel in the ratio, a qualitative estimate of B(Z → J/ψ ℓ⁺ℓ^–) can be extracted. The measured value of approximately 8 × 10^–7 is consistent with the prediction of the Standard Model.

This is the first observation of this decay mode, and is the rarest Z-decay channel observed to date. With this analysis, CMS has started a new era of rare Z decay measurements. Looking forward, the full Run 2 data can lead to a more precise measurement of this decay’s branching fraction. This is particularly important since this process is a background to the even rarer process whereby a Higgs boson decays into a J/ψ and lepton pair, and rare decays are a rich target in which to detect new physics.