5.5 φ/ρ Projection

The φ/ρ projection may be regarded as a modified Y/X projection (see 5.1 Y/X Projection). The same data are displayed in both. This projection may be used to view 3D tracks and hits and to associate them to data in the TRT, LAr and TILE barrels, and to the RPC φ-strips.

The φ/ρ projection is not intuitively understandable, but allows to get a better angular separation of data from the innermost detectors. It has a singularity at ρ=0, so that the primary vertex cannot be displayed. Circles (projections of helices) which pass through the origin, transform into approximately straight lines, with a gradient inversely proportional to Pt:

ρ       Pt × sin(φ-φ0)       Pt × (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. Tracks not pointing exactly to the origin (d00) show a distinct non linearity when approaching ρ=0. Low momentum tracks curve far from the origin, due to the approximation in the above formula.

Checking of pattern recognition is easier in the φ/ρ projection, where helices are approximately straight lines, 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. This would correspond to a - non-existent - curved rubber-band pattern in the Y/X projection.

If the standard view is selected, data from the calorimeter endcaps and the muon endcaps are not displayed, in order not to complicate the picture, as they may fall on top of data from the tracking chambers or data from the calorimeter barrel.

Alternatively, the view parameter may be used to select a single layer of the muon endcap detectors (see 5.1.1 Muon Detector Endcaps in Y/X Projection). In this case, the TGC Gas Gap parameter may be used to further select data from only a given sublayer of the detector. The view parameter may also be used to select layers from the FCAL, LAr and HEC Endcaps and MBTS, it can also display the summed LAr and HEC Endcaps.

The φ/ρ projection is special in that it allows both the particle's charge and Pt to be estimated from the track slope.

The projection orthogonal to φ/ρ is the ρ/Z projection (see 5.4 ρ/Z Projection).