School of Physics - Theses

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    Designing and assessing model independent tests of the DAMA modulation
    Zurowski, Madeleine Jane ( 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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    Estimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment
    Mahmood, Ibtihal ( 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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    Testing Electroweak Baryogenesis at Colliders
    Friedrich, Leon ( 2021)
    Establishing a baryogenesis mechanism, a dynamical origin of the baryon asymmetry, remains an open problem in physics. Electroweak baryogenesis is one such mechanism that is often touted for its inherent testability at current and near future experiments. Taking this notion to heart, here we will examine the collider and dark matter phenomenology of three models motivated by electroweak baryogenesis and novel electroweak phase transitions. In chapter 2, we extend the standard model with two real scalar singlets and one vector-like lepton doublet and examine the collider phenomenology, phase transition history, and baryogenesis mechanism. We find that such a model is capable of generating sufficient baryon asymmetry while satisfying electron electric dipole moment and collider phenomenology constraints. In chapter 3, we study the phenomenology of a hypercharge-zero SU(2) triplet scalar whose existence is motivated by two-step electroweak symmetry-breaking models. If the neutral component of the triplet is stable, we find that this model is strongly constrained by disappearing charged track searches and dark matter direct detection experiments. Conversely, if it is unstable, we find that this model is constrained by multilepton collider searches, such that a triplet with a mass less than 230 GeV is almost excluded at 95% confidence. In chapter 4, we examine the collider and dark matter phenomenology of the standard model extended by a hypercharge-zero SU(2) triplet scalar and a gauge singlet scalar. In particular, we study the scenario where both of the new scalars are charged under a single Z2 symmetry. We find that such an extension is capable of generating the observed dark matter density, while also modifying the collider phenomenology such that the lower bound on the mass of the triplet is smaller than in minimal triplet scalar extensions to the standard model.
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    The origin of matter and dark matter
    Lonsdale, Stephen J. ( 2018)
    Why is the mass density of dark matter throughout the universe similar to that of ordinary matter? Asymmetric symmetry breaking models can explain this apparent coincidence by exploring how dark matter could naturally have a similar mass to the proton and how the number density of dark matter particles could be almost the same as ordinary nucleons. In this work models of high energy scale mirror symmetry connecting the standard model to an exact copy are spontaneously broken to produce models of asymmetric dark matter, with composite dark matter candidates, that naturally solve the dark matter mystery.
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    Phenomenology of particle dark matter
    Leane, Rebecca Kate ( 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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    Mono-X searches for simplified models of dark matter
    BRENNAN, AMELIA ( 2016)
    The identity of dark matter remains one of the big open questions in particle physics; while much is known about its distribution throughout the Universe, very little is understood about its particle nature. In particular, a small but non-zero coupling to the Standard Model (SM) sector has not yet been ruled out. WIMP-type dark matter (DM), with weak-scale mass and couplings, may therefore be produced in proton collisions with the Large Hadron Collider (LHC), and detected by the ATLAS experiment. Several collider searches are presented, which utilise the mono-X+ MET (missing transverse energy) topology, wherein DM (the presence of which is inferred through the observation of missing transverse energy) is produced in association with some object X. The mono-jet process has the largest cross section, however mono-boson analyses, the focus of this thesis, have other advantages. The mono-Z(l+l−) channel benefits from the straightforward identification of charged leptons within the detector and removal of the multi-jet background, while the mono-W/Z(jj) channel is able to utilise the growing collection of electroweak boson identification techniques which exploit the two-prong substructure of a large-radius jet. This thesis describes two ATLAS analyses that seek to constrain both Effective Field Theory (EFT) models and simplified models of DM. The ATLAS mono-Z(ll) analysis uses 20.3 fb−1 of data produced at 8 TeV and selects events with a leptonically-decaying Z boson produced back-to-back with a large amount of MET. A cut-and-count method finds that no excess above the SM prediction is observed, and so constraints are calculated for the suppression scale Λ of the EFTs, and for the quark-DM-mediator coupling of a simplified model with a scalar mediator exchanged in the t-channel. The ATLAS mono-W/Z(jj) analysis uses the first 3.2 fb−1 of data produced at 13 TeV, and selects events with a single large-radius jet produced in association with MET. A profile likelihood fit of the SM background estimation and data is used to extract a limit on the signal strength for a vector mediator s-channel simplified model, and converted to a limit on the suppression scale Λ for a ZZχχ contact operator. A reinterpretation of Run I results from ATLAS for three common simplified models is also presented, including a comparison of the results from the mono-jet, mono-Z(l+l−) and mono-W/Z(jj) channels. Limits on the model coupling strengths are discussed. The strongest constraints are obtained with the mono-jet channel, however the leptonic mono-Z channel is able to remove the large multi-jet background to attain limits that are weaker by only a factor of a few. It is essential that the reconstruction of objects within the ATLAS detector, along with their energy measurement and calibration, is well understood and that the performance is optimised. Along with a general discussion of the relevant objects in the detector (leptons, jets and MET), the in situ measurement of corrections to the energy scale of hadronically-decaying tau leptons is described.
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    Physics beyond the standard model
    Clarke, Jackson David ( 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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    Lyα emitters as a probe of galaxy formation and ionisation history
    BRUNS JR, LOREN ( 2016)
    Current observations suggest that the reionisation of hydrogen in the intergalactic medium had begun by z ∼ 10 and was completed around z ∼ 6. Directly observing this epoch is not possible with existing instrumentation, making it difficult to infer how the increased ionising background during this period affected galaxy formation. This thesis aims to put constraints on the galaxy formation history of the Universe with existing instruments, by modelling and observing the number densities of observed Lyα emitters in the ionised environments around z ∼ 2 − 3 quasars to mimic conditions found during the epoch of reionisation. The main work presented is a model for the ionisation state of the intergalactic medium around star forming galaxies in the vicinity of a luminous quasar, tuned by empirical relationships from conditions at z ∼ 2 − 3. This model suggests that the intense ionising radiation from a quasar offsets the increased density of the intergalactic medium found around it, implying that the direct detection of star forming galaxies by their Lyα emission in the vicinity of z ∼ 2 − 3 quasars is less obstructed by the intergalactic medium than galaxies in the field. The accuracy of this model is compared to existing Lyα galaxy surveys and found to be in good agreement. Discrepancies exist between the expected number of Lyα emitting galaxies this model predicts and the surveyed region around the super-luminous quasar PKS 0424-131, in which no Lyα emission was detected. The modelling done suggests that in order to be consistent with this null detection at the 68% (90%) level, galaxies below 2.5×10^12 M⊙ (4.2×10^12 M⊙) must be omitted. These results suggest that considerable radiative suppression of galaxy formation by PKS 0424-131 is taking place. This hypothesis is tested using observations made on the Baade telescope at the Las Campanas Observatory with the Maryland Magellan Tunable Filter. The unique suitability of tunable filters for the detection of high-redshift galactic Lyα emission is described in detail, along with their idiosyncratic calibration and data reduction processes. The adverse seeing conditions make it impossible to put limits on the impact of ionising radiation of galaxy formation using these observations, and an analysis of the factors that prevented detection is provided. Finally, suggestions are made for ways to improve the chance of success for future observations of this effect using tunable filters, as well as ways to remove spurious ghost reflections in the data analysis that are unique to tunable filter observations.
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    Direct shear mapping: the first technique to measure weak gravitational shear directly
    de Burgh-Day, Catherine Odelia ( 2015)
    This thesis develops and tests a new technique, called Direct Shear Mapping (DSM), to measure weak gravitational lensing shear, $\gamma$, directly from observations of a single background source. The technique assumes the velocity map of an un-lensed, stably-rotating galaxy will be rotationally symmetric. Lensing distorts the velocity map, making it asymmetric. The degree of lensing can be inferred by determining the transformation required to restore axisymmetry. This technique is in contrast to traditional weak lensing methods, which require averaging an ensemble of background galaxy ellipticity measurements, to obtain a single shear measurement. The accuracy of the fitting algorithm is tested in simulated data with a suite of systematic tests. It is demonstrated that in principle shears as small as $0.01$ can be measured. The shear is then fitted in very low redshift (and hence un-lensed) velocity maps, and a null result is obtained with an error of $\pm 0.01$. The high sensitivity achievable with DSM results from analysing spatially resolved spectroscopic images, including not just shape information (as in traditional weak lensing measurements) but velocity information as well. To investigate the prospects for making nonzero shear measurements with DSM in current and future surveys, a theoretical estimate is made of the frequency of galaxy-galaxy weak lensing at low redshift. The probability of weak lensing at low redshifts is found to be good (1 in 1,000 galaxies at $z\sim 0.2$). An algorithm is then presented to make an empirically driven estimate of the frequency of occurrence of weak lensing in existing low redshift galaxy survey data. This algorithm is applied to the Galaxy and Mass Assembly Phase 1 Survey Data Release 2 catalogue. It is estimated that to a redshift of $z\sim 0.6$, the probability of a galaxy being weakly lensed by at least $\gamma = 0.02$ is $\sim$0.01. A technique is then demonstrated to measure the scatter in the stellar mass-halo mass relation using a simulated sample of low redshift DSM shear measurements. It is estimated that for a shear measurement error of $\Delta\gamma = 0.02$, this measurement could be made with a sample of $\sim$50,000 spatially and spectrally resolved galaxies. Finally, the first step towards extending DSM to incorporate weak lensing flexion is made. Including flexion in a lensing analysis increases the sensitivity of weak lensing measurements, and facilitates measurement of the gradient of the gravitational potential. Weak lensing convergence, shear, and flexion field variables are derived for a generalised lensing mass, without assuming circular symmetry. It is shown that the equations reduce back to the correct expressions for simple lens mass distributions, and when solved numerically for circularly symmetric lenses reproduce the results obtained for analytical solutions. Finally, the equations are solved numerically for a simple non-circularly symmetric lens.
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    Structure of dark matter in galaxies
    Trott, Cathryn Margaret ( 2004-12)
    The origin, nature and distribution of dark matter in the universe form some of the biggest questions in modern astrophysics. Dark matter is distributed on a wide range of scales in the universe. This thesis concentrates on galactic scales, attempting to lower the veil and probe the structure of dark matter in galaxies. (For complete abstract open document)