CMS Detector Effects on Measured Transverse Momentum
Comparing Transverse Momentum Recorded in Cylinder and End-Cap Detectors
Kurt Hendrix and Jim Kantner


By using cuts of muon data along high-eta (n>1.2) and low-eta (n<1.2) regions, it is possible to distinguish muons detected by the cylindrical region of the CMS from those detected in the end-caps. By comparing the transverse momentums of these two groups of muons, the effects of detector geometry and composition on muon detection can be qualitatively described.



The Compact Muon Solenoid (CMS) is one of several detectors used to measure proton-proton collisions in the Large Hadron Collider. In order to accurately interpret the data gathered from collisions inside CMS, the selection effects of the detector itself must be understood. Two possible sources of these effects are the geometry of the detector and the composition of the detector. The shape of the CMS is that of a close-ended cylinder, with individual detectors arranged both along the cylinder's walls (hereafter the 'cylindrical region') and over the two ends (the 'end-caps'). While muons passing through the cylindrical region must travel through the detector's magnet, those passing through the end-caps are unimpeded.



Initially, a histogram of muon detections vs. eta was made in order to identify the two regions of the detector. A graph of the transverse momentum for all muons detected was made based on this data. Then, using cuts of the eta histogram, muons detected by the cylindrical region (eta<1.2) and the end-caps (eta>1.2) could be distinguished from one another. Histograms of transverse momentum could then be made based on these two seperate sets of muon detections.



The result of this analysis was to identify regions where detected muon transverse momentum was significantly different between cylindrical and end-cap detectors. In particular, the end-cap transverse momentum data displayed three peaks, at 1.1 GeV/c, 3.3 GeV/c, and 9 GeV/c. The cylindrical region displayed only the second two peaks, while the first was entirely cut out. 


Discussions & Conclusions

Based on the results of this analysis, we would conclude that there are significant detector effects present in the raw transverse momentum data. In particular, muons passing through the cylindrical region may be absorbed by the CMS' magnet, which is not present over the end-caps. This may account for the lack of a 1.1 GeV/c peak, and would also explain the shape of the graph of transverse momentum overall (which does not display a smooth exponential decay, as might otherwise be expected.) More complex quantitative analysis could determine models for comparison to the detected values, for future use in correcting for detector selection effects.