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 [1], 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.