ATLAS released a new preprint yesterday on a search for new physics using monophoton events with missing transverse momentum. The paper mentions that this type of search is sensitive to various models of new physics, including some variations of large extra dimensions, dark matter, and supersymmetry.
In particular, these events could indicate the collision of two quarks leading to invisible particles, with a photon coming from initial state radiation. Final state radiation generally won’t be allowed in many of these models since an invisible particle won’t interact with photons at tree level. Collisions have basically no transverse momentum, so a high energy photon without anything else to balance out the transverse momentum is a strong indication that some invisible particle was also present in the final state. This channel is not free of backgrounds. As the paper notes, one of the most important backgrounds is Z production where the Z decays to neutrinos. If a photon is emitted in conjunction with this, then the even will look identical to the signal event from new physics. This represents an irreducible background since even a perfect reconstruction process can’t eliminate it. Instead, new physics must be found on top of this background (as well as others).
As with more or less every other paper from the LHC so far, no significant deviation from the Standard Model is found, so ATLAS is able to set exclusion limits for a number of different models.