The current status of LEP-2 is that the Summer 1996 run at 161 GeV is now completed and the Autumn 1996 run at 172 GeV is currently underway. The following items have been reported relating to the measurement of the W mass at LEP-2 from these runs.
The first (summer 1996) LEP-2 run took results at 161 GeV, a CMS energy which is well above the W-pair cross section. This means that the W-pair cross section is only about 3 pb (about a factor of about 10000 down on the Z0 cross section at LEP-1). Nevertheless, the first W+W- production in e+e- collisions has now been observed, and this is significant in marking the real start of LEP-2.
The nicest thing about running at 161 GeV is that the sensitivity of the data to the W mass is just as big as at higher energy. This is a bit of a coincidence, since at 161 GeV the W mass sensitivity comes about because the cross section expected varies rather strongly with Mw - enabling the measurement of Mw from the WW cross section to be made. At higher energies, further above the WW threshold, the W mass is instead derived by directly reconstructing the decay products and inferring the mass from kinematic fits the the jet-jet opening angles, etc. This is a qualitatively different method of measurement to the threshold measurement, which has not yet been published.
On a world scale of W mass production, the new result which has been reported by the OPAL collaboration is hardly competitive: the error is around 430 MeV whilst the best Tevatron average is around 125 MeV. Nevertheless, it does indicate that LEP-2 is now in business, and that this type of measurement can be made. Also, when all four LEP-2 experiments have threshold measurements with 400-450 MeV precision, the LEP average will be 200-250 MeV, which is not so far from the Tevatron just from this small first run. When the new 171 GeV data is analysed the error should be down to 150 MeV or so, and the result should be available next spring.
After the 172 GeV run, LEP-2 will next take data at 186 GeV in 1997, and then at 194 GeV in 1998 and 1999
[November 1996]