CERN’s LEP electron-positron collider stubbornly refused to lie down quietly in 2000. The world’s largest synchrotron storage ring was scheduled to be closed forever at the end of September, and dismantled to make way for the LHC proton collider to be built in the same 27 km tunnel.
However, with tantalizing glimpses of the long-awaited Higgs particle appearing at the last gasp, LEP was accorded a six-week stay of “Higgs execution”. The machine duly finished its 2000 run on 2 November. In a specially convened meeting of the LEP Experiments Committee on 3 November, LEP physicists revealed the fruit of these extra few weeks of autumn running.
The Higgs particle, which breaks electroweak symmetry and endows particles with mass, is the missing link in the Standard Model of particle physics, and a major objective at LEP. As LEP’s energy was increased over the years, more and more Higgs territory has been covered without finding any signs of the elusive particle – until this year.
To boost the energy of LEP’s particles, from 1996 the machine was equipped with superconducting radiofrequency accelerating cavities. The remarkable success of this scheme, together with astute planning and skilled machine operations, have enabled LEP to reach collision energies of up to 209 GeV, beyond its planned energy horizon.
For the past several years, LEP has been running in exactly the energy band where the Higgs had been most expected. Each time the energy was increased, physicists held their breath. As data started to accumulate above 206 GeV late this summer, a few electron-positron events suggested Higgs production with a mass of around 114 -115 GeV.
In these events, a LEP electron-positron pair could produce a Higgs back-to-back with another particle. However, the Higgs signals are right at the extreme edge of LEP’s kinematic reach, and are difficult to disentangle from more common processes, notably the production of Z and W particle pairs.
The particles can decay in a number of ways. The initial candidates saw four confined sprays (“jets”) of particles, two from the Higgs. However, other decay patterns are possible, and the recent run has also revealed events with two slices of “missing mass”, indicating the production of two otherwise invisible neutrinos, and other signals.
In the combined results of the four LEP experiments – ALEPH, DELPHI, L3 and OPAL – confidence in the candidate Higgs signal therefore slightly increased as a result of the autumn run, but still fell short of the level needed to claim a physics discovery. The experiments therefore requested a further extension of LEP running in 2001.
However, with the LHC knocking loudly on the door, this has been ruled out. LEP has run for the last time, and its ultimate findings point the way to future physics at the LHC.
At the 3 November meeting where the latest LEP results were disclosed, there was an ovation for the LEP operations team which had delivered the high-energy goods and provided such a cliffhanger finish to the machine’s 11 year career (see LEPilogue: marking the end of an era).