This summer, I worked with Professor Evelyn Thomson’s group in the search for new elementary particles with ATLAS. Our study focused on the discovery of the Lightest Supersymmetric Particle, which we assume to be the Chargino. I used Truth Level Monte Carlo simulation to conduct my studies. My summer consisted of two main projects.
I began by looking into event acceptances for 4 lepton events. Charginos would be produced in pairs, and would decay independently of one another. Each Chargino has a certain probability to decay to a lepton and a Z boson, a neutrino and a W boson, or a lepton and a Higgs Boson. Our primary goal was to focus on decays that would end up producing 4 leptons, with three of these coming from one Chargino. However, due to limitations in the detector, not all of these leptons would be easily found. When looking through actual data, we will look for 3 lepton resonances, and attempt to reconstruct a Chargino from these particles. My first job was to determine both the proportion of events in which all 4 leptons pass certain Kinematic cuts, and the proportion of events in which the 3 leptons from one Chargino pass all of the cuts. This was a great introduction to both the framework and the project.
We worked closely with Theorists at Penn, who happened to deliver quite a bombshell midway through the summer. They showed that the branching ratio of the Chargino to 4 leptons was much lower than we anticipated, and that the branching ratio of the Chargino to a Higgs and a lepton was much higher than expected. This prompted me to look into these types of events and determine our ability to properly reconstruct the Chargino when it decays in this way. Of the events where the Chargino decayed to a Higgs boson, I focused on the events where the Higgs decayed to a b quark and an anti-b quark.
This project was a wonderful experience that gave me an excellent introduction to both particle physics and research. Learning about the Large Hadron Collider and elementary particles peaked my interest, and gifted me a unique perspective on what research really is. Furthermore, I learned what skills it takes to be successful in this field, and worked on developing them as much as I could. I look forward to continue on working with Professor Thomson through the next semester.