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

Chapter 4: Data Collection and Event Reconstruction 101
are computed for these tracks, but no refitting is performed. The tracks are scored,
resolved for ambiguities, and stored in a third track collection.
Once all the track reconstruction sequences have run, the three separate track
collections are merged. Ambiguity-resolving is performed once more to select unique
tracks from the three collections, and the final tracks stored in one collection. At this
point, post-processing steps such as vertex finding, photon conversion reconstruction
and beam-spot finding can be executed.
Vertex reconstruction
At the LHC design luminosity of 10 34 cm −2 s −1 , each bunch crossing is expected
to have > 20 events on average, of which in general only one will be interesting,
the others being mostly low-pT events 4 . Hence, there will be a number of primary
vertices in the interaction region, as well as secondary vertices, vertices from photon
conversions, long-lived neutral particles such as KS, and decay chains. The various
vertex topologies are shown in Figure 4.3 [35].
The correct reconstruction of vertices is important for many physics studies. Pri-
mary vertices arise from promptly decaying particles, while displaced vertices indicate
the decay of long-lived particles. Distinguishing between the different vertex types is
crucial for discovery channels such as H → 4l, H → γγ, b-tagging, τ-jet identification,
measurement of the lifetime of long-lived particles, and so on. ATLAS inner detector
reconstruction includes dedicated vertex finders for the different vertex types, includ-
ing full decay chains. The various reconstruction techniques will be discussed in some
4 As of the writing of this thesis, the maximum instantaneous luminosity reached is 2.07 × 10 32
cm −2 s −1 , the peak number of interactions per bunch crossing being 3.78.