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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 80
The synchrotron light source can also be used to measure bunch length and the
longitudinal beam profile. Part of the synchrotron radiation generated by the undu-
lators is collected by a separate mirror located behind the transverse optics. About
2 × 10 6 photons are expected to be collected for each bunch of 1.15 × 10 11 protons.
The photon flux reproduces the time and intensity profiles of the beam, so that lon-
gitudinal measurements can be made.
3.2.4 Beam position monitors
Beam position monitors (BPMs) measure the transverse position of the beam,
and are used extensively in any accelerator complex. Inside linacs and transfer lines,
they are used to measure and correct beam trajectories. They also measure beam
trajectories inside synchrotrons. In particular, the first turn trajectory measurement
is vital for closing the orbit on itself. Once a closed orbit has been established, BPMs
continuously monitor beam position on a turn-by-turn or even bunch-by-bunch basis.
Data from the BPMs is used for beam-related measurements such as Van der Meer
scans.
The basic concept behind most BPMs is electrostatic or electromagnetic pickup
from the beam by devices placed inside the beam-pipe. We will briefly describe the
most commonly used beam pick-up technology at the LHC, namely button pickups.
Button pickups: These are electrostatic pickups with button-shaped electrodes,
thus the name (Figure 3.8). As a beam of charged particles travels along the metal
beampipe, it induces an image charge on the pipe wall. The image charge mimics the
longitudinal profile of the beam and travels at the same speed as the beam. A button