Searches for Particle Dark Matter: An Introduction

The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.

💡 Research Summary

This thesis presents a comprehensive program for searching for particle dark matter, focusing on four complementary approaches: stellar effects, gamma‑ray observations of dwarf galaxies and primordial minihalos, and global supersymmetric model fits that incorporate all available data. The introductory chapters review the overwhelming astrophysical evidence for dark matter—from galaxy rotation curves and cluster dynamics to gravitational lensing and the cosmic microwave background—setting the stage for a discussion of the leading particle candidates. Axions, sterile neutrinos, and a variety of weakly‑interacting massive particles (WIMPs) are described, with particular emphasis on supersymmetric WIMPs because they naturally arise in the Minimal Supersymmetric Standard Model (MSSM) and its constrained variants (CMSSM, NUHM, etc.).

The thesis then surveys detection strategies. Direct detection experiments aim to measure nuclear recoils from ambient WIMPs, but their sensitivity is limited by background and detector mass. Indirect detection looks for annihilation or decay products—gamma rays, neutrinos, and antimatter—originating in regions of high dark‑matter density. Accelerator searches at the LHC probe supersymmetric particles directly, yet the reach is model‑dependent. The most novel avenue explored here is the impact of dark matter on stellar physics. When WIMPs are captured by a star, their subsequent annihilation deposits energy in the core, altering the star’s temperature, luminosity, and evolutionary timescale. This effect is strongest in environments with high dark‑matter density, such as the Galactic Centre (GC) and the first generation of “dark stars” that may have formed in the early Universe.

Chapter 4 discusses nuisance parameters that affect any dark‑matter analysis: the spatial distribution of dark matter (NFW versus cored profiles), cosmic‑ray propagation models, and uncertainties in standard cosmological parameters. By quantifying these systematics the author ensures that the subsequent constraints are robust.

The core scientific contributions are presented in six peer‑reviewed papers, each summarized below.

1. Zero‑Age Main Sequence of WIMP Burners (Paper I) develops a stellar‑evolution code that includes WIMP capture and annihilation. The study shows that even modest WIMP‑nucleon cross sections can significantly inflate the radius and cool the surface of a zero‑age main‑sequence star, providing a potential observable signature.

2. Dark Stars at the Galactic Centre – The Main Sequence (Paper II) applies the same framework to main‑sequence stars orbiting close to the GC. By comparing model predictions with observed stellar parameters, the author derives competitive limits on the spin‑independent WIMP‑nucleon cross section (down to ∼10⁻⁴⁴ cm²), comparable to the best direct‑detection bounds.

3. Gamma‑Rays from Ultracompact Primordial Dark‑Matter Minihalos (Paper III) investigates the gamma‑ray flux from tiny, high‑density minihalos that could have formed from early‑Universe density perturbations. Using analytical density profiles and Fermi‑LAT sensitivity curves, the work demonstrates that such objects could be detectable as point sources, offering a novel indirect‑detection channel that is largely independent of the usual dwarf‑galaxy targets.

4. Direct Constraints on Minimal Supersymmetry from Fermi‑LAT Observations of the Dwarf Galaxy Segue 1 (Paper IV) combines the non‑detection of gamma rays from Segue 1 with a full MSSM parameter scan. The analysis yields upper limits on the annihilation cross section for neutralino masses between 10 GeV and 1 TeV, and excludes large swaths of the CMSSM “focus‑point” and “co‑annihilation” regions.

5. A Profile Likelihood Analysis of the Constrained MSSM with Genetic Algorithms (Paper V) tackles the computational challenge of exploring a 19‑dimensional supersymmetric parameter space. By employing a genetic‑algorithm optimizer that simultaneously maximizes the likelihood across all experimental constraints (collider limits, relic density, direct and indirect detection, and precision observables), the study achieves faster convergence and a more complete mapping of high‑likelihood regions than traditional Markov‑Chain Monte‑Carlo methods. The results confirm that current data favor neutralino masses around a few hundred GeV but leave open the possibility of heavier WIMPs.

6. Finding High‑Redshift Dark Stars with the James Webb Space Telescope (Paper VI) projects the observational signatures of the first dark stars, which would be powered primarily by dark‑matter annihilation rather than nuclear fusion. Using stellar‑atmosphere models, the author predicts distinctive spectral energy distributions and broadband colours that JWST could detect at redshifts z ≈ 10–15, providing a direct probe of dark‑matter physics in the epoch of reionization.

The thesis concludes that the combination of stellar‑physics constraints, gamma‑ray observations of both dwarf galaxies and primordial minihalos, and sophisticated global fits of supersymmetric models yields a powerful, multi‑messenger framework for probing particle dark matter. While present data already exclude large portions of the simplest supersymmetric parameter spaces, significant viable regions remain, especially for heavier neutralinos. Future facilities—JWST, the Cherenkov Telescope Array, and next‑generation direct‑detection experiments—will dramatically sharpen these constraints. Moreover, the genetic‑algorithm approach demonstrated here offers a scalable solution for the increasingly complex statistical challenges posed by high‑dimensional theoretical models, positioning it as a valuable tool for the next era of dark‑matter research.