Special colloquium: Measurement of the Z boson mass with the LHCb detector

In the Standard Model, the Z boson acquires its mass of around 91 GeV through its gauge interaction with the vacuum energy of the Higgs field. Including higher order corrections the Z mass is predicted with a precision of better than 10 MeV, which corresponds to one part in 10,000. The Z mass was measured to 2 MeV by the four LEP experiments in a beam-energy scan of the LEP e+e- collider. Meanwhile, hadron collider experiments have dominated the measurements of the W boson mass, but their studies of the Z boson mass have been primarily in the context of calibrations for the W mass analyses.

Here we present the first dedicated Z boson mass measurement in proton-proton collisions. The data were recorded in 2016 by the LHCb experiment and correspond to an integrated luminosity of 1.7 fb-1. The Z mass is determined from a fit to the dimuon mass distribution in Z → μ⁺μ⁻  decays. Templates from simulation are used in the fit, with a momentum calibration based on quarkonium resonance decays. The result is compared with previous measurements of the Z mass and with the predicted value from the global electroweak fit.

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