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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 89
threshold is 2.5 GeV for pions and 10 MeV for electrons. The tubes are read out by
PMTs. When the instantaneous luminosity increases above 10 33 cm −2 s −1 , the plan
is to increase the number of tubes on each side to 168, and to replace the PMTs by
scintillation fibers [23].
The fast response time of LUCID (≈ 3 ns) allows it to measure bunch-by-bunch
luminosity. Signals due to secondary particles are minimal owing to two factors:
firstly, the Cherenkov threshold suppresses low-energy particles; secondly, the projec-
tive geometry leads to a shorter path length in the gas for secondary particles than
for primary particles from the IP. Since the number of Cherenkov photons is directly
proportional to the path length in gas, secondaries produce a relatively small signal.
LUCID measurements must be calibrated with absolute luminosity measurements
obtained in parallel from some other source. Once LUCID has been calibrated, its
measurements can be used over a wide range of beam conditions that may not be
suitable for absolute measurement devices such as ALFA. As discussed before, in the
2010 run the calibration to absolute luminosity was done using beam parameters.
Upon calibration, the overall accuracy of the luminosity measurement was 11%, the
dominant contribution coming from beam current measurement [94]. From 2011
onwards, the calibration is expected to use ALFA measurements, with an achievable
accuracy of ≈3%. Over the long term, physics processes with well-measured cross-
sections can also be used for the calibration, but the accuracy of this method depends
on theoretical uncertainties in the cross-sections, as discussed in Section 1.5.