University of Melbourne Doctoral Thesis Phenomenology of Particle Dark Matter by Rebecca Kate Leane ORCID: 0000-0002-1287-8780 A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy in the School of Physics Faculty of Science University of Melbourne May 5, 2017 iii Declaration of Authorship I, Rebecca Kate Leane, declare that this thesis: • comprises only my own original work, except where indicated in the preface, • was done wholly while in candidature; • gives due acknowledgement in the text to all other materials consulted; • and is fewer than 100,000 words in length, exclusive of Tables, Figures, Bib- liographies, or Appendices. Signed: Date: v “I was just so interested in what I was doing I could hardly wait to get up in the morning and get at it. One of my friends said I was a child, because only children can’t wait to get up in the morning to get at what they want to do.” – Barbara McClintock (Nobel Prize in Medicine) vii Abstract The fundamental nature of dark matter (DM) remains unknown. In this thesis, we explore new ways to probe properties of particle DM across different phe- nomenological settings. In the first part of this thesis, we overview evidence, candidates and searches for DM. In the second part of this thesis, we focus on model building and signals for DM searches at the Large Hadron Collider (LHC). Specifically, in Chapter 2, the use of effective field theories (EFTs) for DM at the LHC is explored. We show that many widely used EFTs are not gauge invariant, and how, in the context of the mono-W signal, their use can lead to unphysical signals at the LHC. To avoid such issues, the next iteration of a minimal DM framework, called simplified models, are considered. We discuss use of such models at the LHC in Chapter 3, and show that in the context of a renormalizable gauge-invariant theory, any isospin violating effects in mono-W signals cannot be large. In Chapter 4, we discuss an alternative search strategy to mono-X searches at the LHC — in the case that DM does not couple directly to hadrons, the mono-X signature does not exist, and instead a leptophilic DM signature can be probed. We focus on the prospects for leptophilic DM with a spin-1 mediator at the LHC, and discuss constraints from other experiments. In the third part of this thesis, we turn to astrophysical signals of DM. In Chap- ter 5, we show that a consequence of enforcing gauge invariance in simplified DM models provides a new dominant s-wave DM annihilation process for indirect detection searches, and set limits on the annihilation cross section from Pass 8 observations of the Fermi Gamma-ray Space Telescope. In Chapter 6, we demon- strate the impact of mass generation for simplified models, finding that the relic density and indirect detection constraints, along with the DM interaction types, are strongly dictated by the mass generation mechanism chosen. In Chapter 7, we show that the multi-mediator approach advocated in the previous two chapters can also lead to a new dominant signal, in the form of dark initial state radiation. Finally in Chapter 8, we look to the Sun to find that if DM annihilates to long-lived mediators, the gamma rays and neutrinos produced can be strongly probed by gamma-ray telescopes and observatories Fermi-LAT, HAWC, and LHAASO, as well as neutrino telescopes IceCube and KM3Net. Interestingly, these telescopes can provide the strongest probe of the DM spin dependent scattering cross section, outperforming standard high-energy solar neutrino searches and direct detection experiments by several orders of magnitude. ix Publications The following is a chronological list of journal publications completed during this PhD candidature: 1. N. F. Bell, Y. Cai, R. K. Leane∗ and A. D. Medina, “Leptophilic dark matter with Z0 interactions”, Phys. Rev. D 90, no. 3, 035027 (2014) [arXiv:1407.3001 [hep-ph]]. 2. N. F. Bell, Y. Cai, J. B. Dent, R. K. Leane∗ and T. J. Weiler, “Dark matter at the LHC: Effective field theories and gauge invariance”, Phys. Rev. D92, no. 5 053008 (2015) [arXiv:1503.07874 [hep-ph]]. 3. N. F. Bell, Y.Cai and R. K. Leane∗, “Mono-W Dark Matter Signals at the LHC: Simplified Model Analysis”, JCAP 01 (2016) 051 [arXiv:1512.00476 [hep-ph]]. 4. N. F. Bell, Y. Cai and R. K. Leane∗, “Dark Forces in the Sky: Signals from Z0 and the Dark Higgs”, JCAP 08 (2016) 001 [arXiv:1605.09382 [hep-ph]]. 5. N. F. Bell, Y. Cai and R. K. Leane∗, “Impact of Mass Generation for Simpli- fied Dark Matter Models”, JCAP 01 (2017) 039 [arXiv:1610.03063 [hep-ph]]. 6. R. K. Leane, K. C.Y. Ng, J. F. Beacom, “Powerful Solar Signatures of Long- Lived Dark Mediators” [arXiv:1703.04629 [astro-ph.HE]]. 7. N. F. Bell, Y. Cai, J. B. Dent, R. K. Leane∗ and T. J. Weiler, “Enhancing Dark Matter Annihilation Rates with Dark Bremsstrahlung” [arXiv:1705.01105 [hep-ph]]. ∗Author list is alphabetical, as per convention in particle physics. xi Acknowledgements My time as a PhD student has been one I will always treasure. For that, I have many people to thank. Firstly, I thank my supervisor, Nicole Bell. I could not have been more lucky to have such a sharp, discerning and understanding person as my mentor during my PhD. I have learnt so much from you, not just about physics, but also all the little things that are important along the way. You have supported many things I wanted to do during my PhD candidature, allowing me flexibility, while holding a high standard for work. I am deeply grateful. To my advisory panel, Ray Volkas and Andrew Melatos (and informally, also Matt Dolan), thank you for providing me with invaluable gifts of perspective and support. To my collaborators, John Beacom, Yi Cai, James Dent, Anibal Medina, Kenny Ng, and Tom Weiler, it has been a pleasure working with and learning from you. A highlight during my PhD has been the numerous adventures abroad. To Tom Weiler, thank you for hosting my stay in Nashville, and for your extensive generosity and support. To John Beacom, thank you for inviting me to work with you at Ohio State after our brief initial meeting, and opening many doors. I am deeply grateful for your ongoing support and mentorship. To the graduate stu- dents and postdocs I have met abroad at conferences and summer schools, thank you for your gifts of friendship. I already look back at many of our adventures fondly. I was very fortunate to be a member of CoEPP during my PhD, which has provided significant resources, both financial (particularly for travel (and many servings of Thursday morning cake and tea)), as well as a supportive work en- vironment. I thank all the staff and students in CoEPP, as well as the School of Physics at the University of Melbourne, for making my time here so enjoyable. Finally, to my family, thank you for your ongoing unconditional love, support, and understanding. I would not be where I am today without you. xiii Contents Declaration of Authorship iii Abstract vii Publications ix Acknowledgements xi List of Figures xx List of Tables xxi List of Abbreviations xxiii Preface xxvii I Dark Matter and the Universe1 1 Introduction3 1.1 Astrophysical Evidence for Dark Matter................3 1.1.1 First Observation in Galaxies..................3 1.1.2 Galactic Rotation Curves.....................4 1.1.3 Gravitational Lensing and Merger Events...........4 1.2 Cosmological Evidence for Dark Matter................6 1.2.1 Cosmological Parameters....................6 1.2.2 CMB, BAO, BBN and Supernovae...............8 1.2.3 Large Scale Structure....................... 12 1.3 Theories and Candidates......................... 15 1.3.1 Adding to the Standard Model................. 15 1.3.2 Theories for Dark Matter..................... 17 1.3.3 Weakly Interacting Massive Particles and Thermal Freezeout 20 1.4 Searches for Particle Dark Matter.................... 22 1.4.1 Interpreting Searches Within a Minimal Theory Framework 22 1.4.2 Collider Searches......................... 24 1.4.3 Direct Detection Searches.................... 25 1.4.4 Indirect Detection Searches................... 28 Gamma-ray Signals........................ 28 xiv Positron and Neutrino Signals................. 31 II Dark Matter at the Large Hadron Collider 33 2 Effective Field Theories and Gauge Invariance 37 2.1 Introduction................................ 37 2.2 EFT operators and gauge invariance.................. 38 2.2.1 Scalar operator.......................... 38 2.2.2 Vector operator:.......................... 39 2.3 Mono-W and SU(2)L invariance.................... 39 2.4 Renormalizable models and EFTs.................... 42 2.5 Conclusion................................. 45 3 Mono-W Simplified Models and Gauge Invariance 47 3.1 Introduction................................ 47 3.2 Simplified Models for the Mono-W ................... 48 3.2.1 t-channel Colored Scalar Mediator............... 48 3.2.2 s-channel Z0 Mediator...................... 49 3.3 LHC Constraints and Reach....................... 50 3.3.1 Mono lepton channel....................... 50 3.3.2 Mono fat jet channel....................... 52 3.3.3 Results............................... 53 3.4 SU(2) Breaking Effects and Enhancements from WL Production.. 55 3.4.1 Isospin Violation in the t-channel Model........... 56 Cross Section Enhancement from WL Contribution..... 57 SU(2) Breaking and the MT Spectrum............. 58 3.4.2 Isospin Violating Effects in s-channel Models......... 58 3.5 Conclusion................................. 59 4 Leptophilic Dark Matter with Z0 Interactions 61 4.1 Introduction................................ 61 4.2 Leptophilic Model............................. 63 4.3 Dark Matter Relic Density........................ 64 4.4 Constraints from (g 2) , LEP and other searches.......... 65 − ` 4.5 LHC Phenomenology........................... 66 4.5.1 Z0 Decay to Leptons......................