The search for pair production of Higgs Bosons at the ATLAS experiment

The aim of Iza's PhD project is to accomplish world-leading analyses looking for new phenomena and constraining the value of the Higgs self-coupling using di-Higgs production, specifically the four b-quark final state.
Higgs boson pair production is a critical channel, sensitive to many new physics phenomena and modifications to the Higgs sector, and also an important measurement of the Higgs self-coupling and the extended Higgs potential. Though a full measurement may need the full luminosity of the High Luminosity LHC, the large Run-2 datasets are sensitive probes, already able to constrain this vital coupling.
Iza will work within the Oxford ATLAS Exotics group in the department and also will collaborate and work in the respective analysis groups within the ATLAS collaboration. She will focus on analysing ATLAS data from LHC Run-2 (2015-2018) for evidence of di-Higgs production. Working on the di-Higgs decay to four b-quarks, she will search for evidence of new resonant and non-resonant phenomena. An important aspect of the latter will be to understand and improve how different values of the Higgs self-coupling can be constrained using the large datasets now available.
She will also work on the combination of the Run-2 search channels, the most sensitive of which are 4b, bbtautau, bbyy and bblnulnu. The combination of these channels, each using the full 140 fb-1 of Run-2 data (nearly four times that of the best previous result) will allow for much more powerful constraints on the SM coupling, or provide evidence of a non-SM coupling value. This is a major undertaking, requiring a great deal of effort from contributors working on each of the analysis channels to understand their relative sensitivities and contributions. She will need to understand the systematic and statistical correlations within the different channels and develop tools that properly account for them in this particular analysis. In particular, there has been a lot of theoretical progress, resulting in more precise predictions and ability to model BSM physics in an effective field theory approach, which will require considerably more work to understand fully, and optimise both the sensitivity and visualisation of the results.
To improve the sensitivity to final states containing Higgs decays to b-quark pairs, she will need to calibrate and develop new, advanced b-quark identification algorithms. She will work on means of calibrating b-tagging algorithms in-situ, using samples of high-pT gluon splitting to bb to provide correction factors in this topology directly. Dedicated advanced b-hadron taggers using machine learning techniques are being developed which gain significant performance. Often these cannot be calibrated by conventional means, but these techniques may provide an alternative route, improving Higgs jet tagging across the ATLAS physics programme.
This work will leave her ideally placed to analyse the first Run-3 data due in 2021 and 2022, at a new record collision energy of 14 TeV. She 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 this signature.
She will spend her second year at CERN to be as close as possible to the laboratory and data-taking, and to gain visibility within the ATLAS Collaboration.