Study of Excess Electronic Recoil Events in XENON1T

Evan Shockley

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This thesis summarizes the original analysis work performed by the author on data from XENON1T, a search for dark matter with a to...

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This thesis summarizes the original analysis work performed by the author on data from XENON1T, a search for dark matter with a ton-size noble liquid detector operated at Gran Sasso Underground Laboratory in Italy. The nature of dark matter is one of the most open and pressing questions of modern physics, and the unique data acquired with this detector allows the exploration and investigation of several potential scenarios. The analysis of Dr. Shockley searches for a class of elusive elementary particles that interact with the electrons of ordinary atoms, instead of the nucleus. Results of the analysis present, with high confidence, an excess with respect to the expected background. Beyond more mundane explanations, this additional rate of electron-mediated interactions might be a first hint of physics beyond the standard model. This accessible thesis provides details on the detector, the data, and the theory, delivering to the reader an in-depth and coherent picture of the search for physics beyond the standard model.
Nominated as an outstanding PhD thesis by the University of Chicago Provides an accessible introduction to searches for new physics with the XENON1T detector Presents data analysis indicating the possible detection of a candidate particle for dark matter
Auteur
Evan Shockley is currently a postdoctoral scholar at UC San Diego. He obtained his PhD from the University of Chicago in 2020 after earning a MS from the University of Chicago in 2016 and a BA from Saint Mary's University of Minnesota in 2014. In his final year at University of Chicago he was granted a Grainger Fellowship by the physics department for outstanding experimental research. A member of the XENON collaboration, his research interests center around the search for physics beyond the standard model using some of the world's most sensitive particle detectors.
Contenu
TABLE OF CONTENTSLIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viLIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiiACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xivABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv1 SEARCHING FOR NEW PHYSICS WITH XENON1T . . . . . . . . . . . . . . . 11.1 The XENON1T Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.1 The S1 signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.2 The S2 signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.3 Position reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 81.1.4 Particle discrimination: electronic vs. nuclear recoils . . . . . . . . . 81.1.5 Background reduction . . . . . . . . . . . . . . . . . . . . . . . . . . 101.2 Physics reach of XENON1T . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2.1 Solar axions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2.2 Neutrino magnetic moment . . . . . . . . . . . . . . . . . . . . . . . 201.2.3 Bosonic dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 EVENT RECONSTRUCTION AND SELECTION . . . . . . . . . . . . . . . . . 262.1 Energy Reconstruction and Resolution . . . . . . . . . . . . . . . . . . . . . 262.1.1 Energy Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 262.1.2 Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.2 Detection Eciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.2.1 Single Photoelectron Acceptance . . . . . . . . . . . . . . . . . . . . 422.2.2 Reconstruction Eciency . . . . . . . . . . . . . . . . . . . . . . . . . 482.2.3 Eciency as function of charge . . . . . . . . . . . . . . . . . . . . . 562.2.4 Eciency as function of reconstructed energy . . . . . . . . . . . . . 572.3 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 BACKGROUND + SIGNAL MODELING AND STATISTICAL METHODS . . . 673.1 Background Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.2 Signal Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813.3 Statistical Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834 RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.1 Background-only t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.2 Investigating the excess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004.3 Potential Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.3.1 Xenon x-rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.3.2 37Ar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110iv4.3.3 Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.4 Searches for new physics in XENON1T ER data . . . . . . . . . . . . . . . . 1174.5 Search for solar axions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.5.1 Main Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.5.2 Interpretation under specic QCD models . . . . . . . . . . . . . . . 1244.5.3 Including tritium as a background . . . . . . . . . . . . . . . . . . . . 1304.6 Search for an enhanced neutrino magnetic moment . . . . . . . . . . . . . . 1334.6.1 Main Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1334.6.2 Including tritium as a background . . . . . . . . . . . . . . . . . . . . 1364.7 Bosonic Dark Matter Results . . . . . . . . . . . . . . . . . . . . . . . . . . 1364.8 Science Run 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1404.9 Time Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424.10 Updated Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1454.10.1 Summary of updates . . . . . . . . . . . . . . . . . . . . . . . . . . . 1454.10.2 Results of updated analysis . . . . . . . . . . . . . . . . . . . . . . . 1485 CONCLUSIONS AND OUTLOOK . . . . . . . . . . . . . . . . . . . . . . . . . . 1525.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1525.2 Outlook: XENONnT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1555.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157v