School of Physics - Theses
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ItemDesigning and assessing model independent tests of the DAMA modulation( 2022-12)Particulate dark matter is a long hypothesised solution to various astrophysical observations seemingly at odds with a completely luminous universe. Despite the success of dark matter in explaining these observations, to date physicists have been unable to conclusively observe its interactions with Standard Model matter directly. This thesis will focus on trying to understand the results from the DAMA collaboration, which for the past two decades has reported a modulation signal consistent with dark matter, but in tension with other null experimental results under the usual dark matter assumptions. This study demonstrates the need for a model independent test of this signal to understand its origin, the requirements of such a test, and how different dark matter experiments can be compared or assessed to understand how sensitive they are to this elusive signal. This thesis examines such a study through the lens of a dark matter detector currently under construction in Australia: SABRE South. In particular, it will focus on purification techniques that can be used to produce benchmark low background equipment, detailed simulation studies that can guide the design of SABRE South, and the detailed analysis that must take place to understand how sensitive and or competitive SABRE South will ultimately be. It will also touch on interesting phenomenology studies that can be conducted with such a detector; examining non-standard or unusual dark matter models and signatures that are produced by relaxing the assumptions typically made about its fundamental nature, and distribution with the galaxy.
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ItemEstimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment( 2022)Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.
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ItemPhenomenology of particle dark matter( 2017)The fundamental nature of dark matter (DM) remains unknown. In this thesis, we explore new ways to probe properties of particle DM across different phenomenological 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 Chapter 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 demonstrate 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.
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ItemPhysics beyond the standard model( 2016)In this Thesis we present a collection of original bodies of work pertaining to a number of theoretical/phenomenological questions of the Standard Model, as studied from a "bottom-up" perspective. In Chapter 2: Higgs Sector, we consider the implications of extending the Standard Model Higgs sector by a very light (100 MeV < $m_s$ < $m_h/2$) real singlet scalar field. We identify the regions of parameter space which experiments at the Large Hadron Collider are uniquely sensitive to. There is a strong focus on low background displaced decay signatures. In Chapter 3: Naturalness, we show how a Higgs mass sensitivity measure can be rigorously derived from Bayesian probability theory. We use this measure to constrain the masses of various fermionic and scalar gauge multiplets, obtaining naturalness bounds of O(1-100) TeV. In Chapter 4: Neutrino Mass, we write down the minimal UV completions for all the Standard Model dimension 7 operators which might be responsible for the radiative generation of Majorana neutrino masses. A detailed collider study of a one-loop realisation is performed. In Chapter 5: Baryon Asymmetry of the Universe, we present a proof that the three-flavour Type I seesaw model cannot provide an explanation for neutrino masses and the baryon asymmetry of the Universe via hierarchical leptogenesis without introducing a Higgs naturalness problem. We then describe a minimal extension (the "$\nu$2HDM") which can avoid this conclusion. In Chapter 6: Strong CP Problem, we describe a very minimal model (the "$\nu$DFSZ") which can explain neutrino masses, the baryon asymmetry of the Universe, the strong CP problem, and dark matter, without introducing a naturalness problem for the Higgs. This model serves as an existence proof that weakly coupled high scale physics can naturally explain phenomenological shortcomings of the Standard Model. Lastly, in Chapter 7: Dark Matter, we consider the implications of a class of self-interacting "plasma dark matter" models for direct detection experiments. A number of qualitatively unique signatures (when compared to single component collisionless dark matter) are identified. We emphasise the prediction for a signal which modulates with sidereal day.
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ItemInteracting dark matter: decay and bremsstrahlung processes( 2013)Though there is substantial indirect astrophysical evidence for the existence of dark matter (DM), it has yet to be directly detected. Consequently, little is known about its internal structure. It is possible that there is a small but finite non-gravitational interaction between dark matter and the Standard Model (SM) which may have observable consequences. The purpose of this thesis is the exploration of some of these interactions and consequences. In particular we consider the possibility that dark matter is unstable on long timescales, as motivated by discrepancies between simulation and observation of structure on sub-galactic scales. We also consider the consequences of electroweak radiative corrections to annihilation processes involving dark matter, as such corrections are necessarily present in many well motivated models. We consider this possibility in the contexts of dark matter annihilation in galactic halos, and production in colliders. Chapter 1 provides an introduction to dark matter, including some of its astrophysical and particle aspects. As a motivation for the following sections, we begin by briefly outlining some of the observational evidence for dark matter. We go on to discuss structure formation, and the cold dark matter distribution on galactic scales. Next we discuss the possibility of non-gravitational interactions involving dark matter, including decay, annihilation, scattering off nuclei, and production. Finally we discuss the determination of the relic abundance in the early Universe, including a discussion of models involving coannihilation. Late decaying dark matter has been proposed as a solution to the small scale structure problems inherent to cold dark matter cosmology. In these models the parent dark matter particle is unstable, and decays into a daughter with near degenerate mass, plus a relativistic final state. In Chapter 2 we review the observational constraints on decaying dark matter, and construct explicit particle physics models to realize this scenario. To achieve this, we introduce a pair of fermionic dark matter candidates and a new scalar field, which obey either a Z4, or a U(1) symmetry. Through the spontaneous breaking of these symmetries, and coupling of the new fields to standard model particles, we demonstrate that the desired decay process may be obtained. We also discuss the dark matter production processes in these models. In Chapter 3 we investigate electroweak radiative corrections to dark matter annihilation into leptons, in which a W or Z boson is also radiated. In many dark matter models the annihilation rate into fermions is helicity suppressed. We demonstrate that bremsstrahlung processes can remove this helicity suppression, causing the branching ratios Br($\ell \nu W $), Br($\ell^+\ell^-Z$), and Br($\bar\nu \nu Z$) to dominate over Br($\ell^+\ell^-$) and Br($\bar\nu \nu$). We find this effect to be most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Finally, in Chapter 4, we investigate a mono-Z process as a potential dark matter search strategy at the Large Hadron Collider (LHC). In this channel a single Z boson recoils against missing transverse momentum attributed to dark matter particles, $\chi$, which escape the detector. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. We look for muonic decays of the Z, showing the Standard Model backgrounds to this process to be easily removable with modest selection cuts. We compare signal with Standard Model backgrounds and demonstrate that there exist regions of parameter space where the signal may be clearly visible above background in future LHC data.