Most comprehensive search for new exotic particles in dijet final states

The Oxford ATLAS group played a decisive role in a newly released analysis searching for exotic particles decaying to two jets in ATLAS. The ATLAS dijet search analysis team was led by Oxford student, Katherine Pachal member of the Oxford Exotics Group. This analysis is sensitive to the highest energies available at the LHC and to theories predicting the production of a pair of particles charged under the strong force – an enormous range of models. The spectrum of invariant masses of the dijet pairs follows a smooth distribution such that any new particles would appear as bump-like "resonances" on top. Experimentally this search pushed also the analysis techniques to extremes in order to use jets with energies well above 2 TeV. Figure 1: The observed dijet mass spectrum (black bullets) compared to the predicted smooth background expectation (red line). This is compared to a sample of excited quark signals (q*) with different masses. A discovery of excited quarks would indicate for example that quarks have a substructure.

Here, the results were used to set limits on excited quarks, heavy charged gauge bosons, colour scalar octets, excited bosons, and quantum black holes, but many more models could have been included. To allow theorists to easily reinterpret the analysis for examining other such models, limits were also set on generic resonances represented by Gaussian and, for the first time, Breit-Wigner functional forms.

Figure 2: Limits on excited quarks. Masses below the crossing point between the observed upper cross section limit (solid black line) and the theory prediction of q* cross sections (dashed blue line) are excluded.

Not only did the dijet search results provide highly competitive exclusion limits on the models studied, it also explored lower masses in search of lighter particles with a small cross-section. Masses below 1 TeV have been inaccessible to this search since the LHC collision energy increased in 2011. This new iteration has now reached the lowest dijet masses examined in ATLAS and provided the strongest limits over the entire mass range.