Calorimeter data from a single sampling are best shown in a φ/η projection, where the energy deposits may be shown through Lego towers or other methods. There are also methods to show several samplings superimposed.
Data from 3D tracking chambers may be shown in this projection with optimal separation of tracks. However one faces the problem, that charge, track momentum and the distance of hits from the beam axis cannot be recognised.
One way out is the V-plot, which preserves all the features of the φ/η projection. It may be applied to both tracks and hits. The latter without using any hit to track association.
For a point in space (with coordinates φ, η, ρ) a pair of points is displayed on the V-plot picture. In the case of particles moving in a solenoidal field the two displayed points get the same φ as vertical position and get two different horizontal positions namely
η ± k × (ρmax-ρ)
The value of the parameter k (gradient) is set by default but may be changed interactively. The parameter ρmax is set automatically depending on the selected view. As k and ρmax are known, φ, η and ρ may be recalculated from the coordinates of a pair of displayed points, which means that the V-plot is a true 3-dimensional image.
The position ZVTx of the primary vertex along the beam axis must be known to calculate η.
The following rules apply to interpret the V-plot:
Helices transform into a V like pattern.
For helices pointing to the origin with not too low Pt the arms of the V's are straight.
For helices not pointing to the origin the arms of the V's are curved:
with the same sign of curvature for tracks separated from the origin in z,
with opposite sign of curvature for tracks separated in ρ.
Positive tracks give V's pointing down.
Negative tracks give V's pointing up.
The gradient of the V arms is proportional to 1/Pt:
high Pt tracks give V's with large opening angle,
low Pt tracks give V's with small opening angle.
View's
In the standard view the energy deposits in the calorimeters are represented by boxes with the area of the box proportional to the energy deposited. All samplings and all calorimeters are drawn superimposed. Furthermore, all energies are given a color according to which of six predefined energy ranges they belong. The cells belonging to each energy range are drawn sequentally with the most energetic range being drawn first. This provides a view of the jet structure of the event where energetic isolated particles may be distinguished from jets. Tracks and S3D hits may be drawn superimposed on the calorimeters with the apex of the V corresponding to the exit of the track from the S3D detectors.
In the S3D and TRT views the apex of the V corresponds to the exit of the track from the S3D and TRT detectors, respectively.
In the other views the calorimeters data corresponding to a single sampling are drawn according to their actual cell structure and the tracks which are superimposed are drawn with the apex of the V corresponding to the intersection of the track with the selected sampling of the calorimeters.
Selecting data from a single jet
Data corresponding to a single jet or isolated particle may be selected using the special rubberband options (see 3.1.3 Interaction Control) available in the φ/η projection.
Data selection
For convenience the data to be viewed in the V-plot (S3D, STr, RTr and SNP) may be selected directly in the φ/η projection.
Options
By default all η values used in this projection are calculated taking into account the z offset of the primary vertex (ZVtx). However, if η vertex is deselected η will be calculated from z=0.
The apex of a V may be drawn to a cylinder smaller than the default associated with a view by setting the ρMax and zMax parameters.
When tracks are being shown without the corresponding S3D hits, it may be useful to draw only the outermost portion of the V as determined by the Short V parameter.
The VPlot Island parameter is used to set the color of the island drawn to allow easier comparison of the relative locations of the hit cells from different samplings which are being displayed in different windows.
The Draw Apex parameter is used to draw an additional symbol at the apex of the V. This is useful for high momentum tracks where it may otherwise be difficult to distinguish the exact location of the apex.
In the V-plot where the S3D hits are normally nearby it is useful to have them represented by a symbol of a smaller size than in the other projections. This difference in symbol size is determined by the S3D Δ Size parameter.