First results from LEP2

CERN1's Large Electron-Positron collider, LEP, produced its first pair of fundamental particles known as W+ and W- today, taking particle physics research into new and unexplored territory. This follows a busy winter of upgrades which have transformed LEP into a new accelerator, earning it the name LEP2. Hundreds of physicists from all over the world come to CERN to do their research at LEP2, which will be further upgraded over the coming years, bringing the possibility of new discoveries and extending our understanding of the Universe.

LEP was designed to study one of nature's fundamental forces, the weak force which fuels the sun and is responsible for some forms of natural radioactivity. The weak force is carried by W+, W- and Zo particles. For seven years, LEP has produced collisions with the right energy to make the Zo, which we now know weighs 91.1884 ± 0.0022 GeV, or about the same as 97 hydrogen atoms. LEP2 will run at around twice this energy, sufficient to produce W+ and W- particles in pairs – a big step forward in CERN's precision studies of the weak force.

Accelerating radiofrequency cavities are LEP's engines, and the transformation to LEP2 was accomplished by adding new superconducting cavities to the machine. Superconducting cavities generate much more acceleration than ordinary copper ones, and use less electricity into the bargain. Last autumn, with 60 superconducting cavities installed, LEP took its first step up in energy, from around 90 GeV to 140 GeV. A further 84 superconducting cavities installed over the winter allowed LEP2 to start up earlier this month with a beam-collision energy of 161 GeV -- enough to make W+ W- particle pairs. By 1998, 128 more superconducting cavities will have been added to LEP, boosting the energy to 192 GeV and maximising the discovery potential of the accelerator.

Every increase in energy brings the possibility of new discoveries or surprises, and physicists are eagerly waiting to see what the revamped LEP will reveal. Perhaps the long sought-after Higgs boson will make its first appearance over the coming years at LEP2? Or maybe evidence for a new and better theory of the Universe will be revealed. Many theoreticians predict that such a theory, called supersymmetry, is just around the corner, and that supersymmetric particles are just waiting to be found. If supersymmetry is correct, one of the lightest of these, the chargino, could be within LEP's reach. Whatever LEP2 may bring, physicists at CERN are keeping their eyes wide open for tell-tale signs of new physics.

LEP in numbers

Circumference of the accelerator 26.659 kilometres
Depth of the tunnel housing LEP 50 to 175 metres below the Jura foothills

Magnets

Dipole bending magnets 3368
Focusing quadrupoles 816
Focusing sextupoles 504
Correcting magnets 700

All of these LEP components are aligned to an accuracy of 0.1 mm, and so precise is the machine's energy measured that LEP can detect the orbit of the moon, heavy rainfall, and changing water levels in Lake Geneva. Even the departure of the TGV from Geneva bound for Paris does not escape LEP's attention.

The vacuum inside the LEP beam pipe, 10-9 torr, is a billionth of atmospheric pressure. High vacuum is needed to make sure that electrons and positrons circulating in LEP do not collide with particles of air inside the beam pipe.

Electrons and positrons travel around LEP at a shade (about 0.035 km/h) below the speed of light, lapping LEP over 11 200 times a second.

LEP's accelerating cavities through the years:

Date Copper cavities Superconducting cavities Accelerating voltage per lap (MV) Energy per beam (GeV)
1990 128 0 300 45
Nov 1995 120 60 750 70
June 1996 120 144 1600 81
Oct 1996 120 172 1900 88
1997 86 240 2500 94
1998 52 272 2700 96

 

Footnote(s)

1. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, Turkey, the European Commission and Unesco have observer status.

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