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
.
