Tom Melia

At the smallest distance scales we can currently probe in experiment, it is a fact that the world is quantum mechanical. Physical phenomena are understood through the language of quantum field theory (QFT). Recently my collaborators and I posted a series of papers that showed that in electromagnetism and general relativity, the classical equations that emerge from the underlying quantum description are not automatically the full set of Maxwell and Einstein equations. This shows up as a freedom in the initial conditions of the universe, and would result in a background electric charge or a background gravitational fluid in the universe. It is perfectly possible that the dark matter we see is in fact this background gravitational fluid. I am interested in testing this hypothesis . 

Effective field theory is a quantitive and organised way of ignoring the microscopic details of a theory below a certain length scale. Much is then determined by symmetry, e.g., whether a symmetry is spontaneously broken, or if one emerges upon coarse graining the system. In recent years, the scope of what we call a symmetry has been significantly enlarged, to include symmetries that act upon extended objects in the theory. Such ‘generalised symmetries’ are in fact responsible for the massless-ness of the photon in QED. I am interested in mapping out the details of this story in specific microscopic theories of spin-ice which, through coarse-graining, look like electromagnetism. I also work on the mathematical structure of effective field theory, through connections to conformal representation theory, and commutative algebra. 

I am a member of the COSI satellite collaboration, and am particularly interested in its potential to detect dark matter that decays or annihilates to MeV photons. COSI will address the long-standing 511 keV excess - the observation that the galaxy glows from electron positron annihilation - and could be an excellent probe of ‘shadow charge’ in the cosmos. I work closely with Tad Takahashi and his astroparticle physics group at IPMU, and with Shigeki Matsumoto on this project. I have also worked with Tad Takahashi on an analysis of Fermi data looking for the albedo flux of photons from the Jupiter Trojan asteroids. 

I am a member of a Japan-US collaboration that aims to build a quantum sensor using a chemical crystal known as a single molecule magnet (SMM). SMMs are molecular crystals, where each molecule has a large individual spin. We proposed the idea that SMMs could be used as single quanta particle detectors (including for low-mass dark matter). The detection mechanism is very similar to that of a conventional bubble chamber, but utilises a spin-flip magnetic avalanche that occurs after a particle deposits energy in the crystal. In principle this mechanism could work below the current energy thresholds of other quantum sensors. We are designing and building a working prototype of such a detector.