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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 79
monitors 4 [1].
Synchrotron radiation monitors: Monitors based on synchrotron radiation emis-
sion provide a non-destructive and continuous way to map out the transverse density
distribution of the beam. In the LHC, synchrotron light is produced by 5T super-
conducting undulators 5 or ‘wiggler’ magnets that deflect the beam several times in
the transverse plane within a short interval [1]. Their positioning in IP4 is shown
in Figure 3.7. The synchrotron light is extracted 10 m downstream of the D3 mag-
nets using a telescope assembly and recorded with a CCD camera [39], providing a
2-dimensional transverse image of the beam. The transverse beam size resolution
using synchrotron monitors is expected to be 15-18%.
Q7 Q6 Q5 D4 D3 IP4 D3 D4 Q5 Q6 Q7
beam 1
beam 2
RF system
RF system
TV station SC undulators
H&V wire scanner
Light extraction
H&V gas monitor
Figure 3.7: Configuration of transverse profile monitors at LHC Interaction Point 4:
wire scanners, undulator magnets and gas monitors.
4 Particles in the tail of the beam transverse distribution are counted in the beam current measurement,
but contribute marginally to the luminosity; the core of the distribution accounts for most
of the luminosity. Hence, transverse tails introduce an error in the luminosity measurement, and
monitoring them is crucial for reducing this error.
5 An undulator is a periodic magnet structure that uses interference to concentrate synchrotron
radiation in a cone in the forward direction along the beam path. For details, see [39].