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

Chapter 4: Data Collection and Event Reconstruction 109
ATLAS uses two methods for offline beamline reconstruction: one based on pri-
mary vertices, and the other on tracks. The vertex-based method estimates the
position and the Gaussian width of the beamline in all three coordinates and deter-
mines the slope along the x and y coordinates. Starting with primary vertices from
a suitably large number of bunch crossings, the algorithm first performs a simple
χ 2 minimization to roughly estimate the position and slope of the beamline. Then
it carries out a full parameter estimation using an unbinned log-likelihood method,
with specific selection criteria being applied to the positions and error matrices of the
primary vertices.
The track-based method determines the position and slope of the beamline, but
does not give width information. It starts with a suitably large number of tracks
and performs a χ 2 minimization [3]. Only tracks associated with primary vertices are
used in the fit.
Note that simplified versions of these two algorithms can also run online at Level-3
for fast beamline reconstruction on short time scales (≈ few seconds) and feedback
to the LHC.
Electron reconstruction in the inner detector
Depending on the pseudorapidity, electrons lose between 20% and 50% of their
energy in the form of bremsstrahlung before leaving the SCT [17]. Energy loss leads to
change in the track curvature of electrons, with the result that it becomes difficult to
reconstruct the tracks and accurately measure track parameters. The measurements
can be improved substantially, however, if the track fit takes possible bremsstrahlung
losses into account.