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How elementary particles obtain the property of mass is one of the greatest unanswered questions in physics. Measuring as precisely as possible the masses of elementary particles, particularly gauge bosons, will be vital data for the theoretical physicist wanting to tackle the mass mechanism problem. Then when a candidate theory exists, such as the Higgs mechanism, it has to be tested by experiment.

In the first phase of the LEP program (LEP1), the mass and width of the Z boson were measured to be mZ = 91.1867$ \pm$ 0.0021 GeV and $ \Gamma_{\rm Z}^{}$ = 2.4939$ \pm$ 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 ($ \sim$ 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.

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