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

Chapter 4: Data Collection and Event Reconstruction 105
Reconstruction of photon conversions
About 60% of all photons produced in pp collisions in ATLAS converts to an
electron-positron pair before reaching the calorimeter [17]. The conversion very often
happens outside the volume of the Pixel detector, giving rise to a vertex far removed
from the primary event vertex. The identification of conversion vertices is therefore
vital for physics studies involving final-state photons.
The conversion reconstruction mechanism proceeds in three steps. The first step
involves a preselection of track pairs as conversion candidates. Electron tracks are
selected by requiring that a large fraction of the TRT hits be high threshold hits 6 .
Tracks passing this criterion are grouped into oppositely charged pairs. The difference
in polar angle θ between the two tracks must be small, since the photon is massless 7 .
In addition, the distance of closest approach of the two tracks to each other must be
small.
In the second step, an initial estimate of the conversion vertex position is derived
using the perigee parameters of the track pair. The perigee parameters were defined
with respect to the primary vertex, and are not accurate for photon conversion tracks.
Once the vertex position has been roughly estimated, the perigee is redefined using
this estimate and the track perigee parameters are recomputed. These tracks are now
6 An electron emits transition radiation (TR) in passing through the TRT, while heavier particles
do not emit TR below a Lorentz factor of ≈100. In a TRT straw tube, a TR photon loses most of
its energy in a single Compton scattering interaction, thereby giving a large ionization pulse. By
contrast, the passage of the primary electron or other charged particle through the tube produces
a smaller ionization pulse. The two types of pulses can be distinguished by having two separate
detection thresholds. An electron traversing the TRT is therefore expected to give to a large number
of high threshold hits, while heavier charged particles should generate few such hits.
7 In the axial magnetic field of the inner detector, oppositely charged tracks will diverge in the
azimuthal angle φ, so that the tracks are not expected to have a small difference in this angle.