Measurement of the Z boson invisible width at √s = 13 TeV with the ATLAS detector

A measurement of the invisible width of the Z boson using events with jets and missing transverse momentum is presented using 37 f^b-1 of 13 TeV proton–proton data collected by the ATLAS detector in 2015 and 2016. The ratio of Z → inv to Z ↔ ℓℓ events, where inv refers to non-detected particles and ℓ is either an electron or a muon, is measured and corrected for detector effects. Events with at least one energetic central jet with p_T ≥ 110 GeV are selected for both the Z → inv and Z → ℓℓ final states to obtain a similar phase space in the ratio. The invisible width is measured to be 506 ± 2 (stat.) ± 12 (syst.) MeV and is the single most precise recoil-based measurement. The result is in agreement with the most precise determination from LEP and the Standard Model prediction based on three neutrino generations.