Pileup Uncertainties in the ATLAS Jet Calibration




Associate Professor, Department of Physics and Astronomy

Project Summary

This summer, I worked with Professor Evelyn Thomson’s group as part of the ATLAS collaboration. My research focused on jets, complex objects that result from proton-proton collisions at the Large Hadron Collider (LHC). Reconstructing these jets is essential for understanding the complex particle interactions that occur at the LHC. However, individual jet reconstruction faces interference from other jets, known as pileup. Proton-proton collisions occurring either at the same time or within a short period of the collision being investigated leave excess energy in the detector. In order to confront this issue, corrections are applied to try to eliminate the dependence of reconstructed jet transverse momentum on pileup. These corrections are based on truth jets in Monte Carlo simulation. My project for the summer was to validate their efficacy in real data, as well as calculate their uncertainty. To begin, I validated the pileup independence of two methods of calculating reconstructed jet momentum. The first method employed track jets, which can accurately measure the momentum of charged particles. The second method balanced the reconstructed jet momentum against the momentum of a recoiling Z boson. The dependencies of reconstructed jet momentum  on pileup were plotted against track jet and Z boson momentum in bins of η (pseudorapidity), a measure of the angle of the jet relative to the beam axis. These dependencies were found to be close to 0, validating the correction. The systematic uncertainty is calculated for each η bin as the difference of these dependencies between simulation and data. Upon calculating these uncertainties, I noticed that there were some fundamental issues with the original cuts creating the data I was working with. These cuts restricted the η range of my analysis and caused some strange behavior in the Z boson plots. At the conclusion of my summer, I was able to adjust the cuts to fix some of these issues.

Participating in this project gave me a great introduction to the world of physics research. Being a part of the ATLAS group allowed me to learn a great deal about particle physics. I also learned a lot about conducting computational research. Knowing how to code in Python and work with large datasets will help me continue to work in different areas of physics moving forward.