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Search for Higgs boson pair production

Periodic Reporting for period 1 - hh (Search for Higgs boson pair production)

Reporting period: 2016-03-21 to 2018-03-20

The Standard Model (SM) of particle physics has been a tremendous success, particularly with the discovery of the Higgs boson in 2012 by the ATLAS and CMS collaborations. Since then, research has focused on measurement of the properties of the newly discovered particle. A deviation of a measurement from the SM prediction would be a sign of new physics “beyond” the SM (BSM). The BSM physics is required to answer many remaining open questions in the SM. Our project “hh” aims to observe new physics in the Higgs boson sector through the search for a Higgs boson pair, considering both resonant and non-resonant production. Many BSM theories predict the existence of heavy particles that can decay to a pair of Higgs bosons, such as minimal supersymmetric extension of the SM, twin Higgs and composite Higgs models. These heavy particles are identified as a resonance in the Higgs boson pair invariant mass spectrum. The enhancement of the non-resonant Higgs boson pair cross-section might occur through loop corrections involving new particles or through non-SM couplings. This project is based on the data collected in 2015 and 2016 by the ATLAS experiment in the Large Hadron Collider at CERN. The results of this project indicate non-significant deviations from the SM predictions. Tight upper limits are set in both the non-resonant and resonant modes in the mass range 260 GeV to 1 TeV, improving the sensitivity by a factor up to 10 compared to the previous ATLAS results. The Higgs boson self coupling, a fundamental parameter, cannot be directly probed with the currently available data but was significantly constrained in this project. The prospects of measuring the Higgs boson self-coupling in the high luminosity LHC (HL-LHC) were also studied within this project.
Our work consisted on the optimisation of the analysis, starting from the event categorisation to an improved statistical analysis. We also revisited in this project the signal and background modelling and assess the impact of the systematic uncertainties. We derived final results on the upper limits on the Higgs boson pair production cross-section for both the resonant and the non-resonant searches. In the case of the non resonant analysis, the 95% Confidence Level (CL) upper limit on the Higgs boson pair cross-section was found to be 22 times the SM hypothesis. The limits on the Higgs boson self coupling were set between -8.5 and 13. For the resonant production in the narrow width approximation, the observed limits range between 1.14 pb to 0.12 pb for masses between 260 and 1000 GeV.
These results show the importance of performing this search with the current data available as well as motivates the HL-LHC construction where one of the main motivations will be the measurement of the Higgs boson self coupling and consequently the measurement of the Higgs potential which is crucial for the understanding of the electroweak symmetry breaking. While no excess has been observed, the importance of this search relies on the restriction of the phase space of the possible new physics and also ensures an optimistic extrapolation to the HL-LHC environment with much more data to probe the Higgs boson self coupling.