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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 76
directions. Several sets of scans have been performed in ATLAS during the 2010
data-taking [94], the results from one of which is shown in Figure 3.4. In this case,
the rate of production of primary vertices was monitored in the ATLAS inner detector
as the beams were displaced against each other. Each distribution was fitted with
a double Gaussian with a common mean. The beam sizes thus determined were
47.3 ± 0.2 µm and 56.3 ± 0.3 µm respectively in the horizontal and vertical planes.
We see that the accuracy from the fits is at the per mille level.
3.2.3 Beam profile measurement
Several technologies are used at the LHC for measuring beam profile (bunch shape)
in the transverse and longitudinal directions. We will briefly discuss three of them:
• scintillator and optical transition radiation monitors
• wire scanners
• synchrotron light monitors
Scintillator and optical transition radiation (OTR) monitors: The basic
concept here is to place a thin metal screen in the beam path. A scintillator screen
is usually a doped alumina foil, usable only for low-intensity beams. Beam particles
produce scintillation photons, which can be observed on a TV screen. An OTR
monitor consists of a very thin titanium screen. Charged particles traversing the
interface between vacuum and the metal screen emit transition radiation photons.
Two cones for backward OTR are produced around the angle of reflection, as well as