Being at CERN for a couple of weeks, I could not refrain from following yesterday's talks in the Main Auditorium, which celebrated the 90th birthday of Herwig Schopper, who directed CERN in the crucial years of the LEP construction.

A talk I found most enjoyable was John Ellis'. He gave an overview of the historical context preceding the decision to build LEP, and then a summary of the incredible bounty of knowledge that the machine produced in the 1990s.

Ellis noted that arguably the first step toward LEP was a paper written by Burton Richter in 1976, while the Nobel laureate was on sabbatical at CERN. In NIM 136 (1976), p.47, titled "Very high-energy electron-positron colliding beams for the study of weak interactions" he wrote that it was conceivable to reach up to 200 GeV with a circular machine, and laid out the general ideas of the project and the physics one could extract.

Richter convinced CERN to set up a working group, a machine study. In parallel with that there was the first physics study, with many notable contributors, such as Carlo Rubbia, Jack Steinberger, Mary Gaillard, and Ellis himself. This produced a CERN report, number 76-18, titled "Physics with high-energy e+e- colliding beams".

The next landmark in preparing for LEP was a Les Houches workshop held in 1978, organized by Maurice Jacob. As he was showing a few pictures of physicists relaxing in the mountain setting of Les Houches, Ellis unbuttoned his black shirt to show the t-shirt underneath, which was produced for that workshop. It says "LEPS DO IT - Les Houches 1978" - you can barely see it in the picture on the right, which I grabbed with my cellphone. Ellis noted that it is not the oldest one in his collection!

He then discussed the construction and operation of the machine, noting the incredible features of the beam energy measurement, which allowed the Z mass to be measured with 2 MeV precision. In order to reach that, it was necessary to account for tidal effects from the Moon, for rain, Lake Geneva levels, and departure of TGV trains from the Geneva station.

Ellis then mentioned some of the landmark measurements produced by LEP, such as the one of the number of light neutrinos, the one of the "θ_w" angle (theta-w), radiative corrections, asymmetries. Concerning theta_w, he mentioned that it is unclear whether the W is the last letter in Glashow's name or the first in Weinberg's. He also made a point that the top quark mass could be predicted by radiative corrections, quoting his own paper with Fogli of 1989, where they had M(top)=95+-66 GeV. I would say this is not so great a prediction, but still a valid one given the fact that in those times we only had a lower limit at 60 GeV or so.

All in all, it was a very enjoyable summary of 15 exciting years of electroweak physics.