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

Chapter 4: Data Collection and Event Reconstruction 103
other vertex. The Adaptive Multi-vertex Fitter avoids this problem, in which all pre-
selected tracks are used to form one vertex candidate. The candidate is fitted; tracks
that are considered to be outliers in this first fit are not rejected but rather used to
form a second vertex candidate. The two candidates are then simultaneously fitted,
each track being weighted down with respect to the two vertices. The procedure
is repeated so that the number of vertices grows at each iteration, but the vertices
compete with one another to gain tracks. At the end of the process, a track that
does not enter any vertex with a χ 2 probability greater than 1% does not influence
the fit result [47]. This approach is the default one used for primary vertex finding
in ATLAS.
x Vertex Resolution [mm]
1
-1
10
-2
10
ATLAS Preliminary
s = 7 TeV
0 10 20 30 40 50 60 70
Number of fitted tracks
Figure 4.4: Position resolution in the x direction of reconstructed primary vertices as
a function of the number of tracks associated with the vertex.
Figure 4.4 shows the position resolution of reconstructed primary vertices in the
x direction as a function of the number of tracks associated with the vertex [96]. We
see that the resolution is ≈ 150 µm for vertices built from as few as 10 tracks.