Highest energy ATLAS search for new particles decaying into dijets

The Oxford ATLAS group played a key role in the first 13 TeV ATLAS search paper, which was recently released to the public. The analysis presented in the paper searches for new particles that decay into two back-to-back jets (dijets), using data collected by the ATLAS detector in 2015. The ATLAS dijet search team was led by Oxford Postdoctoral Research Assistant, James Frost, member of the Oxford Exotics Group, and also involved Oxford DPhil student, Lydia Beresford, and Oxford academics, Todd Huffman and Cigdem Issever.

In 2015, the LHC upgraded to a higher centre of mass energy than ever before, 13 TeV. This makes searches using this data extremely interesting, as there is the potential to create new particles or interactions that were not possible at a lower energy. The dijet analysis is sensitive to the very highest mass scales available, and uses jets with masses up to 6.9 TeV, so we really are at the energy frontier.

The figure shows the highest mass, central dijet event passing the dijet resonance selection collected in 2015 (Event 1273922482, Run 280673). The two central, high transverse momentum jets have an invariant mass of 6.9 TeV, and each have a transverse momentum of 3.2 TeV, making this a very well balanced event.

New particles or interactions can be searched for by investigating the invariant mass spectrum of the dijet pairs, and looking for bump-like features, known as resonances on the smoothly falling distribution. There are many theoretical models that predict such resonances, and the source of bumps could be for example, the creation of excited quarks or quantum black holes. The creation of quantum black holes could indicate that gravity can propagate in extra dimensions.

Dijet invariant mass distribution in 13 TeV with the full 2015 data set.: The observed dijet invariant mass spectrum (black bullets) compared to the smooth background prediction (red line). This is compared to quantum black hole (QBH) samples with different masses, which are produced using the Oxford BlackMax (BM) generator.

As no new resonances are observed, the experimental results are used to set very competitive limits on the following theoretical models: excited quarks, quantum black holes, and excited bosons, as well as model-independent Gaussian shapes which can be used by theorists to set constraints on their own new physics models.