To illustrate where the background particles can originate, the beam line from the
exit of the main linear accelerator (linac) to the interaction point (IP) is
schematically shown in Figure 14.1. A +7 mrad bending magnet section (200 m
long) downstream of the collimation section is needed in order to create a sufficient amount
of separation for two experimental halls and to prevent the background from the upstream linac
from directly hitting the detector. In the final focus system, beams are gradually deflected
so as to have a horizontal beam crossing angle of 
mrad at the interaction point (IP).
There are two major sections for beam collimation (1200 m long) and a final focus system
(1800m long) in the beam line to handle beam energies up to 0.75 TeV. While their main
purposes are to clip the beam tails, secondary particles are inevitably produced, namely:
(1) muons and (2) synchrotron radiation photons, respectively. In addition at the IP, (3)
pairs and (4) mini-jet are created through beam-beam interactions. They all cause
background hits in the detector facility.
In subsequent sections the first three kinds of background are discussed together with a possible design of the interaction region.
Figure 14.1:
Top view of the beam line from the exit of the main linac to the
interaction point (IP) at 
TeV.