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

Chapter 4: Data Collection and Event Reconstruction 104
Reconstruction of secondary vertices and decay chains
The high granularity of the silicon detectors allows the identification of b-jets by
finding tracks from a displaced b-hadron decay vertex in the neighborhood of the
primary vertex. A b quark can decay into a c quark, giving rise to a secondary
vertex from the b hadron decay and one or more tertiary vertices from the c hadron
decay. Sophisticated algorithms are necessary to reconstruct such a decay chain. Two
vertex finding mechanisms exist for b-tagging in ATLAS, namely the inclusive and
the topological vertex finders.
Each vertex finder starts with a selection of displaced tracks that originate away
from the beamline. The inclusive finder attempts to fit all selected tracks into one
geometrical vertex [35]. Tracks with a large χ 2 contribution are iteratively removed
until the overall χ 2 falls below a threshold.
The topological vertex finder relies on identifying the primary vertex (PV)→ b → c
decay chain topology. By assuming that all selected tracks intersect a common PV →
b → c flight axis, the algorithm reduces the track clustering problem to one dimension.
The first fit 5 takes the b-hadron flight direction from the axis of a calorimeter jet that
is likely to have resulted from a heavy flavor decay. Intersection of a track with this
axis is searched for which, if found, gives the location of the second vertex. This
procedure is repeated, vertices being clustered in pairs, until a well-defined decay
chain has been reconstructed. In addition to b-tagging, this approach is likely to be
very useful for identifying decay chains of supersymmetric particles.
5 Note that the reconstruction of displaced vertices and decay chains in ATLAS employs constrained
vertex fitting. The constraint usually takes the form of the mass of the decaying particle
according to a particle hypothesis.