Searching for Dark Matter using data from LHC Run 2 and Run 3 at the ATLAS experiment

The aim of Tom's PhD project is to make world-leading analyses searching for evidence of Dark Matter (DM) and new particles mediating its interactions with the Standard Model (SM). Overwhelming astrophysical evidence now suggests the existence of DM, yet nothing is known of its particle nature: it cannot be accounted in the SM and remains one of the largest open questions in physics.

Several extensions of the SM postulate stable, electrically neutral, weakly interacting massive particles as DM candidates, which could be produced in the high energy collisions of the LHC. Once produced, this DM would escape detection, producing an imbalance in the measured transverse momentum (ETmiss) of the detector. A wide class of models probed at the LHC postulate processes wherein one or more SM particles are produced recoiling against DM, resulting in a "SM + ETmiss" signature.
A highly interesting and topical signature is that of a dark Higgs boson, responsible for giving masses to dark matter and a potentially extended dark sector and interacting with SM matter via a scalar portal. These two-mediator dark matter models are not well explored, and yield some interesting new and uncovered signatures. Specifically, for dark Higgs masses above 250 GeV, the decay to two Higgs bosons represents the second largest contribution, and one in which both high-momentum Higgs decays and the ETmiss of the dark matter can be easily and separately reconstructed.

This signature of a resonant di-Higgs production with significant ETmiss in the event through the mediator decay to dark matter has no dedicated experimental analyses. The main initial focus of the studentship is to analyse this final state, and developing a first ATLAS analysis for these resonant di-Higgs plus ETmiss signatures, building on the existing DM searches for mono-Higgs and supersymmetric Higgsinos in which two Higgs bosons are produced independently in an event. Due to the complicated final state and multiple kinematic handles on the signal, exploitation of machine learning techniques, such as classification and mass regression, will be investigated to provide additional sensitivity.

The interpretation of this result will be in common with other dark Higgs final states, such as bb at low mass, and WW at high mass, allowing for a combination or summary to give the full sensitivity of ATLAS to these models. This signature is important, a general feature of many models with extended Higgs sectors mediating interactions with dark matter. Alternatively, he will have the opportunity to follow this analysis work with contributing to flagship dark matter searches using Run 3 data in the latter half of his PhD, as the high luminosities and sensitivities from Run 3 in 2023 and 2024 become available for experimental analysis.

Maximising our sensitivity to final states containing Higgs decays to b-quark pairs is an interesting problem at the LHC. At high Higgs or boson momentum, the two b-quarks are close together, and can be captured in one large jet. The identification of high momentum b-quark jets is important, as is use of the full information present in these 'Higgs jets' - the presence of two sub-jets each identified as likely to contain a b-hadron, the overall structure of the jet and its kinematics. Substantial progress has been made in ATLAS with the development of an 'X->bb' tagger and ongoing work using advanced machine learning algorithms to provide a means of identifying and classifying these jets, finding jets from Higgs boson decays efficiently and with much improved rejection of top and light-quark QCD backgrounds. Tom will have the opportunity to be amongst the first analyses using these algorithms and to understand and contribute to the machine learning algorithms driving those.

He will spend his second year at CERN to be as close as possible to the laboratory, his analysis team and Run-3 data-taking, and to gain visibility within the ATLAS collaboration.