Measurement of the Z-boson using e+e- -> Z gamma events at centre-of-mass energies above the Z pole

TL;DR

This paper measures the Z boson mass using e+e- -> Z gamma events at energies above the Z pole, confirming previous measurements and providing an alternative method to determine beam energy.

Contribution

It introduces a novel approach to measure the Z boson mass and LEP beam energy using initial state radiation events at higher energies.

Findings

Z boson mass measured as 91.272 GeV with uncertainties

LEP beam energy found to be slightly lower than nominal

Results agree with previous Z resonance measurements

Abstract

Events from the e+e- -> Zgamma process with hard initial state radiation collected with the L3 detector at centre-of-mass energies between 183 GeV and 209 GeV are used to measure the mass of the Z boson. Decays of the Z boson into hadrons or muon pairs are considered and the Z mass is determined to be 91.272 +- 0.032 (stat.) +- 0.033 (syst.) GeV, in agreement with the value measured at the Z resonance. Alternatively, assuming this measured value of the Z mass, the method determines the LEP beam energy, found to be 87 +- 34 (stat.) \pm 34 (syst.) MeV lower than the nominal value.