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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 81
pickup collects this charge at certain locations in the beampipe. The amplitude of
the signal is proportional to the beam (or bunch) intensity and inversely proportional
to the distance of the beam from the pickup device.
(a) (b)
Figure 3.8: (Left) Structure of a 24 mm LHC button electrode; (right) configuration
of a mounted BPM consisting of four button electrodes, two each for horizontal and
vertical beam position monitoring.
The button-type BPM used in the LHC consists of two pairs of button electrodes
placed symmetrically in the beam pipe, one pair for measuring the horizontal beam
position and the other for the vertical position. Since the signal induced in each
electrode in a pair depends on the distance of the beam from the electrode, the
beam position can be determined by comparing the signals from the two electrodes.
First, however, the signals must be normalized in order to make them independent
of beam intensity. At the LHC, this is done by a process known as Wide Band Time
Normalization (WBTN) [66]. The signal from each electrode is split and recombined
with a delayed signal from the other electrode. This results in two signals whose
relative zero-crossing time depends on the beam position. The electronics works at
40 MHz, enabling bunch-by-bunch position monitoring.