particular selection criteria, with each level refining the decision made by its
predecessor.
The recorded data is then analysed using complex algorithms which aim to
reconstruct the original event. Monte Carlo simulations of t heoretical models
predict the complex physics processes that could occur during collisions. These
simulations are compared to experimental data. When excesses in the data are
observed as compared to predictions, this indicates a potential discovery.
7
Figure 2-2: Schematic diagram of the particle interactions through a wedge of the
ATLAS detector.
2.1.
The ATLAS co-ordinate system
The ATLAS co-ordinate system [5] is defined around the interaction point being
the origin. The longitudinal z-axis coincides with the length of the beam line.
The “A-side” of the detector corresponds to the region with positive z -values,
and the “C-side” corresponds to the other half. The transverse x-y plane
corresponds to the region perpendicular to the beam pipe. The positive x -axis
points from the interaction point to the centre of the LHC ring. The positive y -
axis points upward to the surface of the earth.
The transverse plane is generally described in terms of r-φ coordinates. The radial
dimension “r”, measures the distance from the beam line. The azimuthal angle
“φ”, measures the angle around the beam pipe from the x-axis. The polar angle
“θ”, is the angle around the y-axis from the positive z-axis. The spatial co-
ordinate preferred within most hadronic colliders however, is pseudorapidity “η”
[6]. It is defined as
𝜂 = −𝑙𝑛 tan(𝜃 2
⁄ )
. The distance
∆𝑅
is defined in η − φ space
as
∆𝑅 = √∆𝜂
2
+ ∆𝜑
2
.
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