CERN’s flagship accelerator, the 27 km Large Electron Positron collider (LEP), began its final year in fine style in April colliding beams at a record 104 GeV per beam, just three weeks after start-up. With the search for the elusive Higgs particle top of the LEP physics agenda for 2000, high-energy running is receiving maximum attention.
LEP’s full complement of superconducting accelerating cavities, all running at their maximum design gradient, gives the machine an energy reach of 96 GeV per beam. To reach the magic figure of 100 GeV in 1999, LEP’s engineers had to push most of the cavities to 7 MV/m, more than 16% beyond the design gradient.
To extract yet higher energies this year has required some ingenuity on the part of the LEP team. Eight of the old (normal conducting) copper cavities, which provided LEP’s energy for the years that it ran at around 50 GeV per beam, have been reinstalled. The superconducting cavities are being pushed still further. Magnets designed to provide small corrections to beam orbits will also be used to reduce the amount of energy lost by the beams as they travel round the ring.
All this adds up to a high-risk strategy – there is literally no spare capacity left in the machine – but the rewards could be high. Results from LEP have already tied down the mass of the Higgs particle to the 108-190 GeV range. With collisions at 104 GeV per beam, the experiments are sensitive to a Higgs mass of up to about 115 GeV. Before the machine is switched off to make room for the LHC proton collider, the least that can be expected from LEP is a smaller mass range for future physicists to aim for. The best might be a major discovery to crown LEP’s already illustrious career.
LEP is treading on fertile physics ground. According to orthodox physics, the Higgs should be lighter than two hundred giga electronvolts (see figure). The minimal supersymmetric model requires at least one Higgs particle lighter than 1.5 times the mass of the Z – less than about 135 GeV.