Primary Analysis by Dr. James Keating
Notes by Leland Dunwoodie

Exploration of DiElectron Data
One thing we notice in plotting various parameters from the 100K dielectron data on ManyEyes is that when phi1 (representing the angle in the x-y plane) is plotted along the vertical-axis, and eta 1 (representing the pseudorapidity along the z-axis) is plotted on the horizontal-axis, two vertical "stripes" that contain fewer events are present:
One stripe is found at eta = -1.5, and the other at eta = +1.5. A similar phenomenon is present in the graph of phi2 vs eta2, as seen below:

Once again in this plot, there are fewer events at eta = -1.5 and eta = +1.5.
The electrons are detected in the electromagnetic calorimeter, which has an upper limit of 2.6 in eta, as seen in the figure below and in the two plots above. One possibility is that electrons are actually present at +/- 1.5 but are not detected because these positions represent the "gap" at the intersection between the barrel portion of the calorimeter and the endcaps of the calorimeter. As there are no detectors In this small gap, electrons present in these locations cannot be detected.
To evaluate this possibility, we looked at a drawing of the side view of the CMS detector with accompanying eta values:


The electromagnetic calorimeter portion of CMS is shown in green above. "EF" represents the endcap of this calorimeter, while "EB" represents the barrel. Note the gap between these two portions of the calorimeter. The dashed line near this gap projects up and to the left, showing that the eta value for this location is approximately 1.5.
This diagram confirms our idea that the "stripes" on the ManyEyes scatter plots shown above represent "gaps" between the barrel and endcap portions of the electromagnetic calorimeter, as the eta values of the stripes correspond with the eta values of the gaps in the calorimeter (-1.5 and +1.5).

(For completeness sake, a diagram of phi, representing the angle in radians in the transverse (x-y) plane, is shown below:)


Note: There is a gap between the barrels and the endcaps because the detector's operators at CERN need space to run cables and wires to the detector.