In the first phase of the LEP program (LEP1), the mass and width of the Z boson were measured to be mZ = 91.1867 0.0021 GeV and = 2.4939 0.0024 GeV , via measurements of cross-sections around the Z-peak.
In the second phase of the LEP program (LEP2), pairs of W bosons are produced. The new experimental challenge is to measure mW by direct reconstruction of the W decay products with a precision that matches that of the indirect measurements of mW ( 30 MeV). Significant disagreement between the direct and indirect determinations of mW might indicate the breakdown of the Standard Model. As the e+e- centre-of-mass energy continues to increase, we also search for a Higgs boson signal.