Precise Electroweak Results from LEP2

The high luminosity delivered by LEP after the doubling of the e+e− collision energy means that LEP2 is now providing substantial samples of W bosons with which tomake complementary tests of the Standard Model to those of LEP1. This, together with collision energies in excess of 200 GeV, is ensuring that the three central physics goals of the LEP2 program are properly explored: the precise measurement of the W mass; the measurement of vector-boson self-interactions; and the search for new particles. This review discusses the first two of these objectives: the third is addressed elsewhere in these proceedings [1]. After a hesitant start of LEP above the W pair production threshold in 1996, subsequent years have seen increasingly large data samples accumulated by the experiments (Table 1). A total of around 480 pb−1 has now been recorded by each experiment at LEP2. This integrated exposure can be expected to pass 650 pb−1 by the time data-taking is completed in the second half of 2000. During 1999, the collision energy has reached, and surpassed1, its design value of √ s = 200 GeV. As illustrated in Fig. 1, the machine performance has been better than ever in 1999, in spite of the increased load on the RF system imposed by the higher energies. This report describes various Standard Model tests made with the first parts of the LEP2 data. Because analyses are in varying stages of completion, essentially all results quoted are preliminary. In addition to the measurements made with W pairs, studies of fermion-pair production, QED tests, and Z pair production are reviewed.