The ATLAS coordinate system (X,Y,Z) is defined as a right-handed system:
X = horizontal axis, pointing to the centre of the LHC ring,
Y = vertical axis, pointing upward,
Z = horizontal axis, following the beam direction.
Additionally, the coordinate system (φ,η,ρ) is defined as:
ρ = sqrt(X2+Y2),
φ = arctan(Y/X), the azimuthal angle, φ = 0 corresponds to the positive X-axis,
η = arctan(ρ/Z).