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A Statistical Framework for the Characterisation of WIMP Dark Matter with the LUX-ZEPLIN Experiment

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A statistical framework for the characterisation of WIMP dark matter with the LUX-ZEPLIN experiment Ibles Olcina Samblas Department of Physics A thesis submitted for the degree of Doctor of Philosophy November 2019 Abstract Several pieces of astrophysical evidence, from galactic to cosmological scales, indicate that most of the mass in the universe is composed of an invisible and essentially collisionless substance known as dark matter. A leading particle candidate that could provide the role of dark matter is the Weakly Interacting Massive Particle (WIMP), which can be searched for directly on Earth via its scattering off atomic nuclei. The LUX-ZEPLIN (LZ) experiment, currently under construction, employs a multi-tonne dual-phase xenon time projection chamber to search for WIMPs in the low background environment of the Davis Campus at the Sanford Underground Research Facility (South Dakota, USA). LZ will probe WIMP interactions with unprecedented sensitivity, starting to explore regions of the WIMP parameter space where new backgrounds are expected to arise from the elastic scattering of neutrinos off xenon nuclei. In this work the theoretical and computational framework underlying the calculation of the sensitivity of the LZ experiment to WIMP-nucleus scattering interactions is presented. After its planned 1000 live days of exposure, LZ will be able to achieve a 3σ discovery for spin independent cross sections above 3.0 10 48 cm2 at 40 GeV/c2 WIMP mass or exclude at × − 90% CL a cross section of 1.3 10 48 cm2 in the absence of signal. The sensitivity of LZ × − to spin-dependent WIMP-neutron and WIMP-proton interactions is also presented. All the sensitivity projections are calculated using the LZStats software package, which is discussed in detail in this thesis. In addition, this work classifies key systematic uncertainties by their impact on the WIMP sensitivity and motivates the inclusion of the highest-ranked into the analysis likelihood function. The effect of some of these systematics on the reconstruction of the WIMP cross section is also studied and it is found to be sub-dominant. Declaration I hereby declare that the contents in this thesis are my own work, except where explicit reference is made to the work of others. My specific contributions in each chapter are listed below. Chapter 2 reviews the main pieces of evidence favouring the dark matter hypothesis and the different types of WIMP searches are discussed. All the figures in this chapter are borrowed from the references indicated in the caption, except for Figure 2.5, which is my own. Chapter 3 describes direct detection WIMP searches with a liquid xenon time projection chamber and presents the main characteristics of the LZ experiment. In this chapter I compared the prediction of the liquid xenon yields between the two most used (until recently) versions of the NEST software package: libNEST 5.0.0 and NEST v2.0.0. I used NEST in combination with a parametrisation of the LZ detector system, which is based on the extensive body of simulations described in the LZ Technical Design Report [1] that were done by LZ collaborators, to generate probability density functions as a function of S1 and S2 observables. This allowed me to give an estimation of the electron recoil discrimination in LZ. Also, I presented the main background contributions to the WIMP search in Table 3.3, which are based on a similar table developed by the LZ collaboration in Ref. [2], but it contains the most up-to-date estimates. In Chapter 4 I presented the LZStats package, which I developed and has become a key analysis tool used by the LZ collaboration. Although I have been the main developer of the code for most of my PhD, I got valuable input from other members of the collaboration and the original design was partly based on previous work developed by Alastair Currie and Attila Dobi. I was responsible for implementing the spin-independent and spin-dependent calculations, described in Chapter 3, into LZStats. Then, and in collaboration with other LZ members, we calculated the sensitivity projections shown in Ref. [2] (submitted for publication). This chapter describes that calculation in detail and provides the most up-to-date sensitivity projections. 6 Chapter 5 describes an analysis of the systematic uncertainties that are expected to be most relevant for the projected performance of the LZ experiment. I presented the relative impact of each systematic uncertainty ranked by the combined impact when the nominal value is shifted by 1σ in either direction. This is my own work, but it was inspired by similar studies conducted by the ATLAS and CMS collaborations, amongst others. The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Re- searchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. Ibles Olcina Samblas November 2019 Acknowledgements There are so many people that have accompanied me in this journey and whose support was vital to make the writing of this thesis possible. I would like to thank first my colleagues at Imperial: Brais, Nellie and Rob (and our beloved plant Lidia too!). You became my second family, and you made the PhD experience a great deal more exciting. Also, I would like to extend my gratitude to those who I shared the corridor with at some point, and who made me grow both professionally and personally: Alfredo, Francis and Alastair. I also would like to thank my supervisor Henrique, who has been my physics (plus grammar, math and politics) educator during the entire duration of the PhD. You have been a source of constant inspiration and I feel fortunate of having had the opportunity to work closely with you. Special thanks go to all the members of the LZ collaboration, whose dedication and hard work have been inspiring since the very beginning. With an almost certain 100% probability that I will forget to mention someone, I would like to acknowledge those people who I have interacted with more closely over the past years and who I have enjoyed dearly working with: Asher, Jim, Hugh, Maria Elena, Aaron, Carmen, Theresa, Cees, Umit, Sally, Nicolas, Amy, Maria Francesa, Elizabeth, Athoy, Curt, Ben K. , Björn, Greg, Jordan, Luke, Quentin, Andrew S. , Andew N. , Scott H. and Billy. Also, I would like to extend my thanks to Glen Cowan, who put everything in order in my very disorganised mental section on statistics. I would like to thank the President’s PhD Scholarship for kindly funding my studies at Imperial College London. I am grateful of the friends I have made because of this incredible opportunity and of the warm community of fellow scholars that I found right from the beginning of my PhD. I would also like to thank, hoping not to appear too pedantic, those great composers who have brought so much enjoyment to my life. Vivaldi, Beethoven and Sarasate always gave me the strength back when I most needed it, while Brahms, Schubert and Bach made me levitate from my seat in those long hours sitting in front of the computer. Also, thank you Liam for teaching me how to enjoy playing music. 8 Thanks Pedro for always bringing that aura of knowledge around you, from which I somehow still managed to learn one thing or two. For starters, I would not be writing this thesis in Vim (and secretly enjoying it), if it wasn’t for you. Thanks also to all my former flatmates for all the great moments that we shared: Andy, Dani and Chris (plus the regular visitors to the Fort Comfort: Alan, Ignacio, Mada, Dylan, Irene, Erica and Zhiyu). I learned something valuable from each of you. This would not have been possible without that amazing group of friends that I feel so fortunate to have around me. Thanks for all the support you have given me and for holding on there waiting during my (embarrassingly frequent) long silences during the writing period. Thanks Guillem, Arianna, Alex C. , Alex B. , Ruben, Elias, Irene, Bea and Sarah. I don’t have words to describe my gratitude towards Julie and Duncan. You have done so much for me and I will always carry your affection wherever I go. Also, my special thanks to Alan, Katie and Polly for turning the big and scary city of London into a cosy and friendly environment. Thanks also to my family for the immeasurable love you have given me. I always felt your proximity despite the distance, and your support and undertanding has been my refuge for years. Gràcies Freix, Violeta i Joansa de tot cor. And of course you, senyoreta meua. You deserve half the credit for this thesis (maybe more). Tú encengueres la flama. “Tot està per fer, tot és possible avui.” Miquel Martí i Pol Contents List of figures 11 List of tables 13 Symbols and Acronyms 15 1 Introduction 19 2 Dark matter 23 2.1 A jigsaw puzzle . 23 2.2 Particle landscape . 32 2.2.1 WIMPs . 33 2.2.2 Axions . 36 2.2.3 Sterile neutrinos . 38 2.3 WIMP detection . 38 2.3.1 Direct searches . 39 2.3.2 Indirect searches . 44 2.3.3 Collider searches . 47 3 Direct detection of WIMPs with dual-phase xenon 49 3.1 WIMP-induced nuclear recoil rate . 49 3.1.1 WIMP astrophysics .
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