The positron excess and supersymmetric dark matter

Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and find that a boost factor >30 provides good fits to the HEAT data. Such an enhancement in the signal could arise if we live in a clumpy halo.Comment: 6 pages, LaTeX, proceedings of the 4th International Workshop on Identification of Dark Matter (idm2002), York, England, 2-6 September, 200

Dark matter and the first stars: a new phase of stellar evolution

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star'' may result: a giant ($\gtrsim 1$ AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion. Observational consequences are discussed.Comment: 5 pages, 2 figures; replaced with accepted versio

Dark matter powered stars: Constraints from the extragalactic background light

The existence of predominantly cold non-baryonic dark matter is unambiguously demonstrated by several observations (e.g., structure formation, big bang nucleosynthesis, gravitational lensing, and rotational curves of spiral galaxies). A candidate well motivated by particle physics is a weakly interacting massive particle (WIMP). Self-annihilating WIMPs would affect the stellar evolution especially in the early universe. Stars powered by self-annihilating WIMP dark matter should possess different properties compared with standard stars. While a direct detection of such dark matter powered stars seems very challenging, their cumulative emission might leave an imprint in the diffuse metagalactic radiation fields, in particular in the mid-infrared part of the electromagnetic spectrum. In this work the possible contributions of dark matter powered stars (dark stars; DSs) to the extragalactic background light (EBL) are calculated. It is shown that existing data and limits of the EBL intensity can already be used to rule out some DS parameter sets.Comment: Accepted for publication in ApJ; 7 pages, 5 figure

Mapping Nanoscale Metal-Insulator Phase Transition in NdNiO3 and Molecular Beam Epitaxy of SmTiO3 Thin Films

The subject of this thesis is the growth and characterization of quantum materials. Quantum materials are those in which correlated electron interactions result in functional emergent properties like high-temperature superconductivity, colossal magnetoresistance, ferromagnetism, and metal-insulator transitions. 3d transition metal oxides, particularly perovskites, have been found to be a fertile area of investigation in quantum materials. Rare earth nickelates and titanates fall into this category and are examined here. The bulk of this work concerns soft x-ray spectroscopy and imaging of a free-standing NdNiO3 thin film. NdNiO3 displays a metal-insulator transition as well as a magnetic transition, with the respective Curie and Néel temperatures coinciding. We employ x-ray absorption spectroscopy to characterize the electronic transition from metallic to insulating on cooling the sample, noting the magnitude of the hysteresis in the process; x-ray magnetic scattering to characterize the magnetic transition on heating; and scanning transmission x-ray spectromicroscopy to search for the formation of distinct domains of metallic and insulating phases during the progress of the electronic phase transition. Although these electronic domains were not observed, the sum of our experiments present confirmation that freestanding films of this novel configuration possess similar magnetic and electronic properties to those observed in their bulk counterparts. This finding is significant as it indicates that films of this type could be integrated into device applications in the same manner as bulk nickelates. A secondary thrust of this work is the development of the capability to synthesize thin films by molecular beam epitaxy (MBE) at the Canadian Light Source for future studies of quantum materials with an emphasis on interface effects and heterostructures. We briefly present results of an effort to grow SmTiO3 thin films by MBE with characterization by electron diffraction and spectroscopy

Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy

The previous proposal (by two of us) of chain inflation with the QCD axion is shown to fail. The proposal involved a series of fast tunneling events, yet here it is shown that tunneling is too slow. We calculate the bubble nucleation rates for phase transitions in the thick wall limit, approximating the barrier by a triangle. A similar problem arises in realization of chain inflation in the string landscape that uses series of minima along the monodromy staircase around the conifold point. The basic problem is that the minima of the potential are too far apart to allow rapid enough tunneling in these two models. We entertain the possibility of overcoming this problem by modifying the gravity sector to a Brans-Dicke theory. However, one would need extremely small values for the Brans-Dicke parameter. Many successful alternatives exist, including other "axions" (with mass scales not set by QCD) or potentials with comparable heights and widths that do not suffer from the problem of slow tunneling and provide successful candidates for chain inflation.Comment: 6 pages, 1 figur

Satisfiability in multi-valued circuits

Satisfiability of Boolean circuits is among the most known and important problems in theoretical computer science. This problem is NP-complete in general but becomes polynomial time when restricted either to monotone gates or linear gates. We go outside Boolean realm and consider circuits built of any fixed set of gates on an arbitrary large finite domain. From the complexity point of view this is strictly connected with the problems of solving equations (or systems of equations) over finite algebras. The research reported in this work was motivated by a desire to know for which finite algebras $\mathbf A$ there is a polynomial time algorithm that decides if an equation over $\mathbf A$ has a solution. We are also looking for polynomial time algorithms that decide if two circuits over a finite algebra compute the same function. Although we have not managed to solve these problems in the most general setting we have obtained such a characterization for a very broad class of algebras from congruence modular varieties. This class includes most known and well-studied algebras such as groups, rings, modules (and their generalizations like quasigroups, loops, near-rings, nonassociative rings, Lie algebras), lattices (and their extensions like Boolean algebras, Heyting algebras or other algebras connected with multi-valued logics including MV-algebras). This paper seems to be the first systematic study of the computational complexity of satisfiability of non-Boolean circuits and solving equations over finite algebras. The characterization results provided by the paper is given in terms of nice structural properties of algebras for which the problems are solvable in polynomial time.Comment: 50 page