Undergraduate Honors Theses

Thesis Defended

Spring 2019

Document Type

Thesis

Type of Thesis

Departmental Honors

Department

Physics

First Advisor

Dennis Perepelitsa

Abstract

The sPHENIX experiment at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) will be a new detector with the purpose of studying the strongly interacting matter created in high energy Au+Au collisions. The high energy den- sity created in Au+Au collisions produces a medium of disassociated quarks and gluons called Quark Gluon Plasma (QGP), which is the same substance which ex- isted in the first microsecond of the universe. As a result of its incredibly strong interactions QGP is difficult to study; however, using the phenomena of jet quench- ing with photon jet pairs it is possible to gain insight into its nature. In order to use photon jet pairs to study QGP, the direct photon that is opposite the jet must first be correctly identified. Several algorithms are developed that use the sPHENIX calorimeters to reconstruct, identify, and isolate high energy direct photons that are created during the hard subprocess in high multiplicity Au+Au events. In this study events simulated by Pythia8 containing a photon jet pair are embedded into central Au+Au HIJING events and a full GEANT4 simulation of the sPHENIX detector is performed. Using reconstructed data from the simulated detector an algorithm is developed to determined how isolated an electromagnetic cluster is; direct photons tend to be more isolated that background sources. An algorithm is also developed to determine the probability an electromagnetic cluster resulted from a direct pho- ton rather than a hadron or photon producing decay. This algorithm takes tower information from clusters in the electromagnetic calorimeter, extracts information related to the shape and energy of the showers, and passes those variables to a trained machine learning algorithm. The machine learning algorithm then outputs the clusters likelihood of being a direct photon. Using the cluster isolation and identification it is possible to identify direct photons.

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