The φ/Z projection may be used to view 3D tracks and hits and to associate them to data in the TRT endcaps, the LAr endcaps, the HEC and to the TGC φ-strips. Indeed it is the only projection in which the TRT endcap straws may be viewed.
It is not intuitively understandable, but allows to get a good angular separation of data from the innermost detectors. Circles (projections of helices) which pass through the origin, transform into straight lines, with a gradient inversely proportional to Pz:
z-zv ∝ Pz × (φ-φ0)
where the constant of proportionality depends on the strength of the solenoidal magnetic field. For large path lengths, φ will increase (decrease) with increasing path length for negatively (positively) charged tracks.
Checking of pattern recognition is easier in the φ/Z projection than in the Y/X projection (see 5.1 Y/X Projection). In particular a Y-Skew rubber-band can be used to optimally select the region around a track for closer inspection. Ideal helices passing through the origin appear as a sequence of parallel lines, one for each turn. Even helices with slowly changing radius can be identified as such. For simulated and reconstructed tracks only the first half turn of the helix is drawn.
If the standard view is selected, data from the calorimeter barrel are not displayed (in order not to complicate the picture) as they fall on top of data either from the tracking chambers or from the calorimeter endcaps.
Alternatively, the view parameter may be used to select a single layer of the muon barrel detectors. In the case of RPC layers, the RPC Gas Gap parameter may be used to further select data from only a given sublayer of the detector.
The projection orthogonal to φ/Z is the ρ/Z projection (see 5.4 ρ/Z Projection).