Measuring the self-interaction cross-section of dark matter with astronomical particle colliders
Harvey, David Richard
The dark matter paradigm has been a great source of speculation in both the 20th and 21st Centuries. Since its proposed existence in 1933, the mounting evidence has led to this theoretical particle becoming one of the greatest mysteries of modern physics. However, despite its dominant presence in the Universe, little is known about its nature and how it behaves. In this thesis I critically analyse one particular property of dark matter: the self-coupling. The self-interacting dark matter paradigm hypothesises that dark matter is not collisionless as assumed in most cosmological simulations, and in-fact has some probability that it will scatter off itself. Such a self-coupling will resolve many discrepancies that exist between observations and theory, particularly on small, non-linear scales. Moreover, any detection of a self-interaction cross-section will place considerable limitations on the acceptable particle physics models of dark matter and hence has grown to become an important question. In this thesis I develop and implement a method to constrain the self-interaction cross-section of dark matter that exploits continually accreting and merging groups of galaxies as they fall into galaxy clusters. Utilising the ubiquitous nature of accreting substructure, I measure the offsets between dark matter and baryonic gas as they become separated due to their differing interaction properties. Studying this effect over a sample of events, I will be able to make the first ever statistical estimate of the cross-section of dark matter, while averaging over many different unknown merging scenarios. I begin my thesis by deriving an analytical description of sub-halo in-fall, allowing me to constrain dark matter self-interaction models directly from observations. In this study, I find that current archival data should be able to detect a difference in the dynamical behaviour of dark matter and standard model particles at 6σ, and measure the total interaction cross-section σDM/m with 68% confidence limits of ±1 cm2g-1. Having constructed a new method to derive constraints on the cross-section of dark matter I carry out a study into the potential systematics that may affect a measurement. I determine the accuracy of weak gravitational lensing, which is the distortion of light due to intervening mass, as a tool to estimate the positions of substructure in galaxy clusters. I find that the public Lenstool software can measure the position of individual 1:5 x 1013Mʘ peaks with ~ 0:3" systematic bias, as long as they are at least ~ 30" from the cluster centre. Finally, I develop a pipeline that can analyse a sample of inhomogeneous observations from The Hubble Space Telescope and the Chandra X-ray Observatory. By measuring the positions of dark matter, gas and galaxies for 68 individual merging events, from a total of 28 galaxy clusters, I detect a 7:4σ offset between gas and an unobserved dark mass. I make the first ever measurement of cross-section of dark matter from a sample of clusters finding σDM < 0:50cm2/g [95% CL], the best constraints to date. In addition to this I find that the brightest group galaxy in-fact tends to lead the dark matter halo during merging events. Although evidence for the existence of interacting dark matter, I conclude that the astrophysics of the BCG is complicated, and that this apparent directional bias should be considered in all galaxy cluster analyses. Moreover, I show that this technique is easily extendable for future surveys that have larger samples of galaxy clusters, with constraints of σDM < 0:001cm2/g potentially attainable.