Search for physics beyond the Standard Model in Higgs pair final states which decay into two pairs of b-quarks with the ATLAS detector

Beojan will analyse ATLAS data from the data-taking period of 2015 to 2018 to search for evidence of new physics beyond the Standard Model (SM) of Particle Physics and set upper cross-section bounds on SM Higgs pair production in the 2H -> 4b final state, where both Higgs bosons decay into a pair of b-quarks. This final state has the largest rate among the possible di-Higgs final states and consequently is an important channel for the future Higgs self-coupling measurement. If the Large Hadron Collider (LHC) performs as expected Beojan will analyse datasets containing up to 150 fb-1, comprising an order of magnitude more than current results. He will work closely with the Oxford ATLAS Exotics and Higgs groups in the department and also will collaborate and work in the respective analysis groups within the ATLAS collaboration. He will also work on feasibility studies with phenomenologists in Oxford and at CERN to improve the sensitivity to this decay channel for a future Higgs self-coupling measurement at the LHC. He will work on the understanding of the kinematics of this process and its backgrounds using Monte Carlo simulations and control regions in data. He will develop selection criteria for this process to maximize the signal (S) over background (B) and the discovery significance S/sqrt(B). The aim is to develop an analysis strategy which will enable him to combine the searches in the different kinematic regimes of this process coherently (resolved, intermediated and boosted regimes). These regimes are defined by the momentum of the two Higgs bosons and the level of collimation of their final state products. Their combination will improve analysis sensitivity, especially for new resonances at higher masses. One of the most important backgrounds for this final state is the multi-jet background and he will develop data-driven background estimation methods in order to minimize the uncertainty on the backgrounds. The theoretical uncertainties on multi-jets are too large and would degrade the sensitivity of the analysis. He will need to develop new experimental techniques to identify highly energetic Higgs bosons in these different kinematic regimes for both Higgs bosons of the 4b final state. The challenges of this final state will be to mitigate the signal efficiency loss due to the fact that the b-quarks collimate more strongly as the energy of the decaying Higgs bosons increases. The collimation of the b-quarks of the H->bb decay will also be an important problem to overcome. He will need to calibrate and develop new and advanced b-quark identification algorithms using variable-R cone sizes and also re-optimise the multivariate b-tagging algorithms. In order to maximise the Higgs boson discrimination against background he will use algorithms and multivariate analysis techniques to select the Higgs jets. As Beojan's final state complements that of Santiago Paredes' thesis in that both share a Higgs boson decaying into a b-quark pair, both students will be able to work together on the H-> bb decay identification aspects. Beojan will focus on the development of advanced b-tagging algorithms, their calibration and systematic uncertainties, whereas Santiago will focus on the jet energy and mass calibration and uncertainties for b-quark jets. If he makes a discovery, he will focus on measuring the mass of the new state and the rate of its production. In the case, there is no discovery he will compute exclusion limits on new physics models and upper cross-section bounds on the SM di-Higgs cross section. The goal is the publication of his analysis and newly developed experimental techniques in the form of ATLAS papers during his PhD. He will spend his second year at CERN to be as close as possible to the laboratory and data-taking, and to gain visibility within the ATLAS collaboration.