Mono-dark-Higgs searches

The aim of Tim'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 (missing ET) 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 + missing ET" signature.
Recently there has been much interest in a set of 'Dark Higgs' models, in which a similar mechanism and boson - the Dark Higgs boson - is responsible for giving dark matter its large mass, and which are little constrained by the observed DM relic density.

Tim will work within the Oxford ATLAS Exotics group in the department and will collaborate and work with the respective ATLAS analysis groups. He will focus on the analysis of ATLAS data from LHC Run-2 (2015-2018) for evidence of dark Higgs production. He will work on the dark Higgs decay to a pair of Higgs bosons, in association with dark matter inferred through missing ET in the detector. This is a final state only now starting to be investigated by experimentalists and, in addition to dark Higgs models, is sensitive to many models with extended Higgs sectors or additional scalars. Initially the work will focus on designing and optimising a first ATLAS analysis for these resonant di-Higgs plus MET signatures, building on the existing searches for supersymmetric Higgsinos in which two Higgs bosons are produced independently in an event.

He will also be able to work on the combination of dark Higgs results - a combination of the main dark Higgs decay channels: a pair of b-quarks, vector bosons and Higgs bosons depending on the mass is foreseen for inclusion in the ATLAS dark matter summary paper on extended Higgs models. This will be a large effort, covering the many ATLAS Run-2 analysis channels that are sensitive to aspects of these models and will summarise to the general particle physics community what LHC Run-2 has brought to our understanding of dark matter.

To maximise our sensitivity to final states containing Higgs (or other heavy boson) decays to b-quark pairs, the identification of high momentum b-quark jets will be important. Tim will need to understand and develop the new, more advanced machine learning algorithms being used to identify b-quark pair decay. At high Higgs or boson momentum, the two b-quarks are close together, and can be captured in one large jet. The best approach to this is to 'double tag' the large jet - identifying it as having two b-quark jets within it consistent with a heavy boson decay, rather than a top quark decay or QCD process. There are many subtleties, from ensuring that the b-quark pair invariant mass (indicative of the decaying boson) is unbiased, to ensuring and fine-tuning a robust means of calibration of these taggers (such as from high momentum gluon splitting to a b-quark pair), where the conventional ones cannot be used.

This work will leave him well placed to analyse the first Run-3 data due in late 2021 or 2022, at a new record collision energy of 14 TeV. He will be able to ensure that b-tagging is as performant as possible, which will be vital for world-leading analyses using large luminosities of Run-3 data in these signatures.