Search for new phenomena in dijet angular distributions at √s = 8 and 13 TeV

A new energy regime has recently become accessible in collisions at the Large Hadron Collider at CERN. Abundant in hadron collisions, the two-jet final state explores the structure of the constituents of matter and the possible emergence of new forces of nature, in the largest momentum transfer collisions produced. The results from searches for phenomena beyond the Standard Model in the dijet angular distributions are presented. The data were collected with the ATLAS detector in proton-proton collisions at centre-of-mass energies of 8 and 13~TeV, corresponding to integrated luminosities of 17.3 fb$^{-1}$ and 3.6 fb$^{-1}$, respectively. No evidence for new phenomena was seen, and the strongest 95\% confidence level lower limits to date were set on the scale of a range of suggested models. This work details the limits on the compositeness scale of quarks in a contact interaction scenario with two different modes of interference with Standard Model processes, as well as on the threshold mass of quantum black holes in a scenario with 6 extra spatial dimensions, and on the mass of excited quark states. It also includes new exclusion limits on the mass of a dark matter mediator and its coupling to fermions, as derived from the contact interaction limits using an effective field theory approach. The performance in ATLAS of the jet-area based method to correct jet measurements for the overlaid energy of additional proton-proton collisions is also presented. It removes the dependence of the jet transverse momentum on overlaid collision energy from both simultaneous interactions and those in the neighbouring bunch crossings, and was adopted as part of the jet calibration chain in ATLAS.