Measurement of the Z boson cross-section in - Harvard University ...

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 77
two cones of forward OTR (Figure 3.5) [66]. Consequently, if the screen is placed at
45 o to the beam direction, the photons can be collected as shown.
(a) (b)
Fig. 19 (a) Backward and forward OTR patterns with their imaging schemes.
Figure 3.5: (a) Backward and forward optical transition radiation patterns, (b) An
example of a 2-dimensional OTR image taken every four turns at injection at the
CERN SPS. The transverse beam dimensions can be determined from the image.
The photon emission probability scales with γp, the Lorentz factor of the beam
particles. The angular distribution of the emitted photons is peaked at 1 . Hence,
γp
OTR screens are suitable for high-energy beams. They can also withstand higher
intensities compared to scintillator screens. The LHC employs scintillator and OTR
monitors in the injection lines, in the ring and in the beam dump lines [5].
Figure 3.6 shows the image of a beam spot in the transverse plane after a single-
bunch beam has passed through a scintillator screen. The image is produced on a
so-called BTV (Beam TeleVision). The bunch contained 2 × 10 9 protons, and was in
fact the first beam circulated in the LHC, on September 10, 2008.