5.4 ρ/Z Projection

The ρ/Z projection may be used to view 3D tracks and hits and to associate them to data in the calorimeters. It may also be used to provide a rough association of tracks to the MDT, RPC z-strips and TGC ρ-strips data from all sectors superimposed. To make this possible the muon data are not drawn at ρ but rather at ρ' determined by projecting ρ onto the φ-axis of the corresponding sector.

Although not a linear projection, the ρ/Z projection is intuitively understandable. Circles (projections of helices) which pass through the origin are recognised as approximately straight lines:

ρ (Pt/Pz)×(-Zvertex)

The picture of the detector is the same as if a cut through the beam axis would have been applied. For a point with coordinates ρ and φ, this is achieved by defining:

ρ = +(X2+Y2)     if     φρZ-90° < φ < φρZ+90°

and

ρ = -(X2+Y2)     otherwise.

Due to the cylindrical structure of ATLAS, the ρ/Z projection is the only one, where all main detector units may be displayed without overlapping. The different radial sub-elements of the muon detectors do overlap, but may be shown separately in the X'/Z projection (see 5.7 X'/Z Projection).

In the Y/X (see 5.1 Y/X Projection), φ/ρ (see 5.5 φ/ρ Projection) and φ/Z (see 5.6 φ/Z Projection) projections tracks are seen from the side of maximum curvature. In the ρ/Z projection (see 5.4 ρ/Z Projection) they are seen from the "orthogonal" side, i.e. the side of minimum curvature. Therefore ρ/Z is said to be orthogonal to Y/X, φ/ρ and φ/Z.

In order to check in space if a track passes through a set of 3D hits, or if it points to a set of calorimeter hits, the hits and the track must be displayed in at least two 2-dimensional projections which are orthogonal to each other i.e. in ρ/Z and in one of the projections Y/X, φ/ρ or φ/Z. Instead of using a pair of projections the 3-dimensional V-plot (see 5.2 φ/η Projection) is sufficient to check pattern recognition and track extrapolation.

Charge and momentum cannot be estimated in ρ/Z.

TRT data cannot be shown, unless very few TRT straws can be selected, which appear as lines, as neither Z in the barrel nor ρ in the endcap are known.

The inner detectors may be enlarged without enlarging the outer ones by using the fisheye interaction (see 4.5 Fisheye).

The angle φ ρZ may be modified in any φ defining projection while simultaneously being viewed in the ρ/Z projection as described in the Synchro Cursors interaction (see 4.4 Synchro Cursors).

The ρ/Z Projection Menu (see 3.3.2 Projection Menus)