The LEP Accelerator

The Large Electron-Positron storage ring (LEP) collider [10] is based at CERN (La Centre Europ $\acute{\it e}$ene pour la Recherche Nucl $\acute{\it e}$aire) beneath the border between Switzerland and France, near Geneva. It is the largest synchrotron accelerator in the world, with the main ring tunnel having a circumference of 26.67 km. Although studies and plans for LEP machines started as early as 1976, the first fill wasn't until 13th July 1989, with actual physics runs starting a month later, on the 13th August.

As the name suggests, LEP was designed to bring extremely high energy electrons and positrons into collision with one another. Initially, the energies of the positron and electron beams were such that the centre of mass energy $(\sqrt{s})$ at the point of collision was 91 GeV, the rest mass of the ${\rm Z}^{0}$ boson, thereby opening a whole new world of investigation into the neutral current interactions in the electroweak force. LEP was a huge success, with over 900,000 ${\rm Z}^{0}$ bosons being produced in the first year alone. LEP ran at this energy until 1995. Then the machine was upgraded [11,12], so that it could run at above the ${\rm W}^{\pm}$ pair threshold, allowing the investigation of the charged current sector of the electroweak force [13]. However, unlike for the ${\rm Z}^{0}$ boson, where it is most profitable to have collisions close to the ${\rm Z}^{0}$ boson mass, the higher the energy, the better for ${\rm W}^{\pm}$ bosons as its production cross-section increases with energy, up to a centre-of-mass energy of approximately 200 GeV. So each year the centre-of-mass energy was increased.

The complete LEP collider does not consist of just the very large LEP ring, although this is easily the greatest engineering feat of the project. There are a number of other, smaller, older, CERN accelerators around which the particles are accelerated, before injection into the main ring. Figure 2.1 shows the lay out of the complete system. The electrons are produced by thermionic emission, these are then accelerated along an electron linear collider, the Lep Injector Linac (LIL). Some of the electrons are collided with a tungsten target to produce the positrons, the remaining electrons, along with the positrons, are then passed into the Electron Positron Accumulator ring (EPA), where they are stored and accumulated before injection. The particles are then passed into the Proton Synchrotron (PS) where they are initially accelerated to a few GeV. They are then transfered to the Super Proton Synchrotron (SPS) where further acceleration takes place and finally they are injected into the LEP ring.

The LEP ring is 26.67 km in circumference and lies between 40 and 150 m below the surface. The plane of the ring is inclined by 1.4%. This is purely due to engineering reasons, ensuring that no shaft had to be deeper than 150 m, but also that the underground caverns and tunnel would be located in solid rock. The LEP ring consists of eight arcs and four straight sections. The arcs contain magnetic cells to guide the beams around the ring. Each magnetic cell is comprised of a defocusing quadrupole, a vertical orbit corrector, a group of six bending dipoles, a focusing sextuplet, a focusing quadrupole, a horizontal orbit corrector, a second group of six bending dipoles, and finally a defocusing sextuple. The total length of a cell being 79.11 m and each arc contains 31 of these cells. Acceleration of the beams occurs in the straight sections. Also on these straight sections are the four experiments, OPAL, ALEPH, DELPHI and L3, where the beams are brought into collision.

The energy and number of particles in a bunch is limited by the synchrotron radiation, causing an upper limit on both current and energy. The ring is designed with the maximum radius of curvature to minimise the energy loss through synchrotron radiation. The main loss of particles is through beam-gas interactions, so a high vacuum has to be maintained in the tunnel. Without beams the pressure in the tunnel is 10$^{-12}$ Torr, and with beams circulating this is degraded to 10$^{-9}$ Torr. The main problem with maintaining this vacuum is out-gassing caused by synchrotron radiation interacting with the beam pipe walls. Synchrotron radiation can also cause heating of the vacuum chambers, and so the chamber walls are made of aluminium which is cooled by surrounding water channels.

Figure 2.1: Diagram representing all the accelerators at CERN and how they are used in unison to produce a final high energy circulating ring around LEP.