Interaction Point (IP)

The electron and positron beams which circulate in the LEP ring collide at the Interaction Point at the center of the detector. The beams have a vertical size of about 10 microns, a width of about 120 microns and a length (along direction of motion of the beams) of about 1 cm. The smaller the transverse dimensions of the beams the more likely is an interaction between an electron and a positron. The likelihood of an interaction, of course, also depends on the number of electrons and positrons which are circulating.

The rate of interactions is proportional to the Luminosity and the luminosity is proportional to the product of the number of particles of each type in the beams (or the product of the currents in the beams) and is inversely proportional to the product of the horizontal and vertical beam sizes. The number of reactions of a particular type observed per second depends on the cross section or the intrinsic probability for that process.

Beam Pipe
The electron and positron beams circulate in a beam pipe under very high vacuum. Near the center of the detector the beam pipe is made of 1.1 mm beryllium. The choice of beryllium serves to reduce the amount of multiple Coulomb scattering of charged particles emerging from the IP. Reducing the amount of scattering permits more precise determination of the decay point of very short-lived particles like tau leptons and B mesons. These particles travel only a few millimeters and their decay point is observed by extrapolating charged particle tracks back inside the beam tube.

Electrons and positrons circulate in opposite directions in the beam vacuum tube. Under normal operation there are four equally spaced bunches of each type which collide at the four interaction points where the four LEP experiments are situated. The collisions take place approximately every 22 microseconds ( check that!--the circumference of LEP is 26.66 km).
The average current per beam is about 3 mA (how many particles per bunch is that?). Once the beams are injected into the LEP ring and accelerated up to about 45 GeV they circulate for about 12 hours with gradually decreasing intensity. The loss of beam particles is due first to beam beam collisions then to collisions with thermal photons!, with a beam lifetime of about 80 hours (the beam tube is at "room" temperature.) and to collisions with residual gas molecules in the beam tube, with a beam lifetime of about 200 hours.
To compensate for energy loss due to synchrotron radiation as the electrons and positrons are deflected in magnetic fields around the LEP ring, their energy is boosted by a high frequency (about 350 MHz) electrical power system located at several points around the ring. HR>

Joe Rothberg, , 12 March 1996