2.1 The LEP Machine
As shown in figure 2.1,
LEP is located on the Swiss-French border near Geneva.
It is roughly circular (actually eight straight sections
on either side of each cavern, interspersed with curved
sections), 26.7 km in circumference and between 50 and
170 m below ground level.
Location of the LEP collider at CERN.
The positions of the four experiments (DELPHI, OPAL,
ALEPH, and L3) and the SPS accelerator are also shown.
The injector system, which starts at the main CERN site,
is shown in figure 2.2.
The LEP Injector Linacs (LIL) produce electrons and
positrons (the positrons from the collision of 200 MeV
electrons with a tungsten converter), which are
separately accelerated to 600 MeV.
After storage in the Electron-Positron Accumulator (EPA),
they are injected into the
Proton Synchrotron (PS) and thence into
the Super Proton Synchrotron (SPS); these
accelerate the particles to 3.5 and 20 GeV respectively.
After injection into LEP, the counter-rotating
electrons and positrons are accelerated
to 45 GeV using a radio frequency (RF)
acceleration system powered by sixteen 1 MW klystrons,
operating in two of the straight sections of the ring.
The beams are bent into orbit by 3368 dipole magnets
and focused with 808 quadrupole and 504 sextupole magnets.
Superconducting quadrupoles provide additional focusing around
the four interaction regions, where the beams are squeezed
to a RMS width of about
The LEP injector complex.
The two closest detector caverns
on the LEP beamline (DELPHI and L3) are also shown.
The expected event rate is
where is the cross-section for the process in
question ( nb
In LEP, the luminosity, , defined by
equation 2.1, is approximately
The bunches in each beam circulate round the ring with
a frequency . Each bunch contains electrons or
positrons and has RMS dimensions
(given above) at the interaction point.
Typical 1994 values and kHz
(corresponding to a beam cross-over rate of 11
give a luminosity, ,
The LEP design and commissioning is described
in , while its subsequent operation and
performance is summarized in .
As well as higher currents and better operational
efficiency, the luminosity was improved using the
Pretzel  and bunch train 
schemes, which increased the number of bunches of
in the machine from
(1989-92) to (1992-4),
and thence to (1995).
Table 2.1 summarizes the luminosity
and number of events observed by DELPHI.
Integrated luminosity and total number of
events  recorded
by DELPHI in the period 1989-95.
is higher in 1992 and 1994
relative to the integrated luminosity because during the other years
a significant amount of data was taken off-peak, where
the cross-section is much lower.