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

Chapter 3: Luminosity Measurement at the LHC and in ATLAS 92
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Figure 3.15: GEANT4 simulation showing an ATLAS BCM station with the four
sensors placed symmetrically about the beampipe on the support structure.
2010 running when the mean number of interactions per bunch-crossing was small.
A spacepoint-based minimum bias trigger was used to reject bunch-crossings with
no interactions. The trigger used spacepoint multiplicity in the silicon detectors to
determine whether a hard interaction occurred [70]. The total number of spacepoints
in triggered events in a given luminosity block 7 scales with the integrated luminosity,
after correction for trigger deadtime and prescales.
Primary vertex counting: The number of non-diffractive events per bunch crossing
has a Poisson distribution with a mean λ given by:
λ = σndLb
(3.19)
where σnd is the non-diffractive cross-section and Lb is the bunch luminosity. Once
σnd at the LHC is measured independently of ATLAS (e.g., using the TOTEM de-
7 A luminosity block is the smallest length of time during which the integrated luminosity can be
calculated, with corrections for trigger deadtime and prescales. The exact duration of a luminosity
block depends on operational conditions; in particular, the duration must be long enough that
the statistical uncertainty in the integrated luminosity measured for the block is smaller than the
systematic uncertainty. In the 2010 proton run, 2-minute long blocks were used.