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

Chapter 4: Data Collection and Event Reconstruction 102
proton bunch
/V 0
vertex in jet
primary vertex
pile up vertex
secondary vertex
decay chain proton bunch
Figure 4.3: Vertex topologies expected in a typical bunch crossing at the LHC: the
primary vertex of interest, pile-up vertices, vertices in jets, secondary and tertiary
vertices from decay chains, and vertices from photon conversions and decays of longlived
neutrals (V 0 s).
detail in this section.
Primary vertex reconstruction
There are two approaches to primary vertex finding in ATLAS, the first of which
involves using a ‘sliding window’ approach to locate vertices. A number of tracks
compatible with the expected luminous region are selected and ordered by z0, the z
projection of the track impact parameter. Then track clusters are looked for in the
region using a sliding window along the z axis. All clusters thus found form candidates
for primary vertices. Each candidate is iteratively fitted using one of several vertex
fitters [47], outlying tracks being rejected at each iteration based on their contribution
to the fit χ 2 . The vertex with the highest pT -sum of tracks is defined as the primary
vertex.
In this approach, a track rejected from one vertex candidate is not included in any