No description available.

We consider the prospects to use polarized dark-matter detectors to discriminate between various dark-matter models. If WIMPs are fermions and participate in parity-violating interactions with ordinary matter, then the recoil-direction and recoil-energy distributions of nuclei in detectors will depend on the orientation of the initial nuclear spin with respect to the velocity of the detector through the Galactic halo. If, however, WIMPS are scalars, the only possible polarization-dependent interactions are extremely velocity-suppressed and, therefore, unobservable. Since the amplitude of this polarization modulation is fixed by the detector speed through the halo, in units of the speed of light, exposures several times larger than those of current experiments will be required to be probe this effect. / text

Despite the great successes of the Cold Dark Matter (CDM) model in explaining a wide range of observations of the global evolution and the formation of galaxies and large-scale structure in the universe, the origin and microscopic nature of this dark matter is still unknown. The most common form of CDM considered to-date is that of Weakly Interacting Massive Particles (WIMPs), but some of the cosmological predictions for this kind of CDM are in apparent conflict with observations (e.g. cuspy-cored halos and an overabundance of satellite dwarf galaxies). For these reasons, it is important to consider the consequences of different forms of CDM. We focus here on the hypothesis that the dark matter is comprised, instead, of ultralight bosons that form a Bose-Einstein Condensate (BEC), described by a complex scalar field. We start from the Klein-Gordon and Einstein field equations to describe the evolution of the Friedmann-Robertson-Walker (FRW) universe in the presence of this kind of dark matter. We find that, in addition to the phases of radiation-domination (RD), matter-domination (MD) and Lambda-domination (LD) familiar from the standard CDM model, there is an earlier phase of scalar-field-domination (SFD) which is special to this model. In addition, while WIMP CDM is non-relativistic at all times after it decouples, the equation of state of BEC-SFDM is found to be relativistic at early times, evolving from incompressible ($\bar{p} = \bar{\rho}$) to radiation-like ($\bar{p} = \bar{\rho}/3$), before it becomes non-relativistic and CDM-like at late times. The timing of the transitions between these phases and regimes is shown to yield fundamental constraints on the particle mass and self-interaction coupling strength. We also discuss progress on the description of structure formation in this model, which includes additional constraints on these parameters. / text

The PICASSO experiment located 2 km underground in SNOLAB is directly searching for dark matter signals by looking for interactions between dark matter particles and an active target made of superheated droplets of freon C4F10. During the interaction, energy is deposited to the freon triggering a phase transition, inducing pressure waves which are detected by piezo-electric sensors. A temperature dependent analysis of the amplitudes of the signals for detector 71 showed that, above 25 ◦C, between 20 and 80 % of the events were saturated implying that the preamplifiers had too high a gain. Decreasing this gain by a fixed factor was not found to be a suitable solution to the problem. Ideally, a temperature dependent gain should be established. In addition, some channels have intrinsic problems and should be repaired. A threshold analysis was used to establish the trigger efficiency which was found to be 90% above 25 ◦C but only 50% at lower temperatures with the current setting of the threshold. A temperature dependent threshold setting has been proposed. A new setup at Queen’s University has been built and a gamma calibration using three different radioactive sources (22Na,137Cs,57Co) was undertaken leading to a new detector response curve for gammas. For a proper analysis, new and more appropriate cuts were implemented. The analysis confirmed the expectation that the PICASSO detectors are mostly blind to gammas below 50 ◦C. However, the detectors appear to be more sensitive to 122 keV gammas than to 622 keV gammas by a factor of about 10. The sensitivity for 22Na also differs by a large factor from what was expected from old calibrations on detectors with much smaller bubbles. The rate plots exhibit a strong exponential increase in rate above 40 ◦C which is not due to any of the gamma sources used, but could be due to neutrons or low energy x-rays. This remains under investigation. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-24 18:28:21.308

Thesis (Ph.D.)--University of Melbourne, School of Physics, 2005. / Typescript (photocopy). Includes bibliographical references (leaves [171]-185).

abstract: The identity and origin of dark matter is one of the more elusive mysteries in the fields of particle physics and cosmology. In the near future, direct dark matter detectors will offer a chance at observing dark matter non-gravitationally for the first time. In this thesis, formalisms are developed to analyze direct detection experiments and to quantify the extent to which properties of the dark matter can be determined. A range of non-standard assumptions about the dark matter are considered, including inelastic scattering, isospin violation and momentum dependent scattering. Bayesian inference is applied to realistic detector configurations to evaluate parameter estimation and model selection ability. A complete set of simplified models for spin-0, spin-1/2 and spin-1 dark matter candidates are formulated. The corresponding non-relativistic operators are found, and are used to derive observational signals for the simplified models. The ability to discern these simplified models with direct detection experiments is demonstrated. In the near future direct dark matter detectors will be sensitive to coherent neutrino scattering, which will limit the discovery potential of these experiments. It was found that eleven of the fourteen non-relativistic operators considered produce signals distinct from coherent scattering, and thus the neutrino background does not greatly affect the discovery potential in these cases. / Dissertation/Thesis / Doctoral Dissertation Physics 2016

Hidden sector or dark sector states appear in many extensions to the Standard Model (SM), to provide particle mediators for dark matter in the universe or to explain astrophysical observations such as the positron excess in the cosmic microwave background radiation flux. A hidden or dark sector can be introduced with an additional U(1)d dark gauge symmetry. The discovery of the Higgs boson in 2012 during Run 1 by the Large Hadron Collider (ATLAS and CMS) opens a new and rich experimental program for Beyond Standard Model physics (BSM) based on the Higgs Portal. This exotic discovery route uses couplings to the dark sector at the Higgs level, which were not experimentally accessible before. This thesis presents the searches of possible exotic decays: H → ZdZ(d) → 4` where Zd is a dark vector boson. It had been initiated in the Run 1 period of the LHC using the ATLAS detector at CERN. The results showed (tantalizingly) two signal events where none were expected, so that in the strict criteria of High Energy Physics, the result was not yet statistically significant. The Run 1 analysis for a 8 TeV collision energy is further developed in Run 2 with a 13 TeV collision energy, to expand the search area, take advantage of higher statistics, a higher Higgs production cross section, and substantially better performance of the ATLAS detector. In this work, the search is further broadened and includes allowing the mass of the originating boson (the dark Higgs S) to vary from the SM value. This allows the search for the dark vector boson to also explore higher or lighter masses than the SM Higgs boson. This extended search is efficient and could include a more general class of models, with the mass constraint of the SM Higgs portal lifted. This thesis reviews the analysis results from Run 1 and Run 2, and presents its iteration in the full Run 2 search by focusing on its new channel where the additional scalar S (with mS 6= mH) decays to 4` via two dark vector boson states Zd . The case where the Higgs decays to 4` via two Zd (H → ZdZd → 4`) and also called high mass channel, has been just unblinded. Nineteen data events are observed where 14 were predicted. In overall, the data are consistent with the Monte Carlo prediction. No evidence of deviation from the Standard Model expectations are observed. / College of Engineering, Science and Technology / Ph. D. (Physics)

Thesis (Ph.D.)--Boston University / The existence of Dark Matter was first proposed by Fritz Zwicky in 1933, based on the observed velocity distribution of galaxies in the Coma Cluster. Subsequent studies of visible mass and velocity distributions in other galaxies have confirmed Zwicky's original observation; there is now little doubt that Dark Matter exists. However, due to the fact that Dark Matter interacts very weakly through non-gravitational forces, nothing is known about the nature of Dark Matter. It is believed that Dark Matter particles are streaming toward the Earth, in the Earth's rest frame, from the direction of the constellation Cygnus. Observation of this so-called Dark Matter 'wind' with a direction-sensitive dark matter particle detector would be compelling evidence that Dark Matter does consist of a gas of discrete particles as a new form of matter. The DMTPC collaboration is developing such a detector, and this thesis describes R&D work in support of that project. The DMTPC technique for looking for Dark Matter relies on Dark Matter particles interacting with atomic nuclei, causing the nuclei to recoil and to leave optical signals that can be detected. Since neutrons are electrically neutral and collide with nuclei, they can mimic Dark Matter signals. Therefore, the reduction of neutron background is critical to the successful detection and identification of Dark Matter particles. One important aspect of this thesis is to fully understand and quantify neutron interactions with our detector. In addition to providing information for understanding Dark Matter experiments, this work also allows us to understand how our device can be used as a neutron detector. We have been able to measure a number of neutron events in a variety of experimental runs both with and without neutron sources such as a neutron generator and 252 Cf. From these runs, we have obtained data for both elastic and inelastic interactions of neutrons of various energy ranges with detector gas nuclei. In this thesis, I will also discuss our current background data taking for the Dark Matter research and our plan for scaling up the detector to 100 m 3 for a competitive Dark Matter search.

No description available.

<p>There is very strong evidence that ordinary matter in the Universe is outweighed by almost ten times as much so-called dark matter. Dark matter does neither emit nor absorb light and we do not know what it is. One of the theoretically favoured candidates is a so-called neutralino from the supersymmetric extension of the Standard Model of particle physics.</p><p>A theoretical calculation of the expected cosmic neutralino density must include the so-called coannihilations. Coannihilations are particle processes in the early Universe with any two supersymmetric particles in the initial state and any two Standard Model particles in the final state. In this thesis we discuss the importance of these processes for the calculation of the relic density. We will go through some details in the calculation of coannihilations with one or two so-called sfermions in the initial state. This includes a discussion of Feynman diagrams with clashing arrows, a calculation of colour factors and a discussion of ghosts in non-Abelian field theory. </p><p>Supersymmetric models contain a large number of free parameters on which the masses and couplings depend. The requirement, that the predicted density of cosmic neutralinos must agree with the density observed for the unknown dark matter, will constrain the parameters. Other constraints come from experiments which are not related to cosmology. For instance, the supersymmetric loop contribution to the rare <i>b</i> -><i> sγ</i> decay should agree with the measured branching fraction. The principles of the calculation of the rare decay are discussed in this thesis. Also on-going and planned searches for cosmic neutralinos can constrain the parameters. In one of the accompanying papers in the thesis we compare the detection prospects for several current and future searches for neutralino dark matter. </p>