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

Chapter 4: Data Collection and Event Reconstruction 108
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Figure 4.6: Distribution of primary vertices in data in the xz (top left), yz (top right)
and xy (bottom) planes.
be parallel to the beampipe and centered on the nominal interaction point (IP), de-
fined as the origin in global detector coordinates. In reality, the beamline often has a
finite slope, and is off-center with respect to the nominal IP. For example, Figure 4.6
shows the distribution of primary vertices in 2010 data in the xy, xz and yz planes.
We see that the beamline is significantly displaced from the nominal position along
both x- and y-axes; additionally, a substantial slope is present in the xz plane [96].
Large jumps in the beam position of ≈mm can occur at the start of a run, while
during a run it can shift by up to 50 µm [4]. Accurate reconstruction of the beamline
is therefore necessary at least on a run-by-run basis, or even per luminosity block.
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