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Searching for Dark Matter in the Higgs boson sector with the ATLAS detector

Periodic Reporting for period 1 - DarkMatterAndHiggs (Searching for Dark Matter in the Higgs boson sector with the ATLAS detector)

Reporting period: 2018-08-06 to 2020-08-05

The main goal of this project is to contribute to the efforts to uncover the particle nature of Dark Matter - a type of matter that is dominating the matter content of our universe but as of now poorly understood. Through its overwhelming abundance, Dark Matter played a crucial role in the evolution of our universe and thus of life itself, such that its thorough understanding is worthwhile.

In order to reach this goal, the data collected by the ATLAS experiment at CERN's Large Hadron Collider is analysed. This analysis focuses on events where Dark Matter particles are produced in association with a Higgs boson - the latest piece of the Standard Model of particle physics that was discovered. The decay of the Higgs boson to tau leptons is exploited as this allows for a different way to trigger the readout of the events compared to the more abundant decay to b-type quarks. This decay channel was hitherto unexplored by the ATLAS Collaboration such that this project allowed to widen the search portfolio of ATLAS.

By exploring this channel in addition to the on-going Dark Matter searches of ATLAS more insight can be gained as this channel focuses on a different part of the phase space of the data set. This point is strengthened by combining the results of the different ATLAS Dark Matter searches to set a common limit on Dark Matter models.

By the end of the project, no sign of Dark Matter was observed at the LHC, but more insights on how Dark Matter is not produced was gained.
The main fraction of the 24 month of this project was spent on developing the analysis of proton-proton collisions recorded with the ATLAS detector that contain the signature of two hadronically decaying tau leptons and so called missing transverse energy. The latter is a measure of the imbalance of transverse momentum in the detector and a typical signature of a Dark Matter (DM) particle escaping detection. The two tau leptons arise from the decay of the Higgs boson that is assumed to be produced in addition to the DM. In a first step, the latest models describing DM production in hadron collisions were reviewed in the beginning of the project. Two models were chosen as reference points for the analysis. The signature of these models was then simulated using Monte Carlo methods. This signal simulation was employed to develop the analysis strategy. The latter includes the best way to select the signal such that a region with high signal purity is reached. In addition, the best way to trigger the readout of the signal events has been studied. Different ways to estimate the background from Standard Model processes have been explored and the best ones chosen for the final analysis. Furthermore, it was studied which variables yield the best sensitivity for the signal and the statistical interpretation of the data was implemented.

This work was fully integrated in the ATLAS Collaboration with regular reports at internal meetings and a detailed supporting document written for internal use. In addition, contributions to the efforts of the collaboration to statistically combine several DM searches were made in order to exploit this analysis best.

Apart from the scientific work, I invested time in my personal development and the management of this fellowship. I took on coordinating tasks, attended conferences and supervised students. Furthermore, I initialised a network for female researchers at the subatomic group of my host institute. I organised 2 outreach events for high school students and wrote an article for the homepage of our research group. I was also the invited speaker of an event to encourage more researchers at my host institution to apply for external funding.
The analysis developed during this project extends the scope of DM searches performed by the ATLAS Collaboration as this final state was not studied before. The analysis relies on the full Run-2 data set collected with the ATLAS detector. This corresponds to the largest data set collected yet and also has the highest center-of-mass energy ever recorded - corresponding to 13 TeV. This data set allows for a far better reach than previously recorded data such that its analysis extends the state of the art. This analysis contributes to reveal in which part of the phase space Dark Matter is - or isn't - hiding such that one day we will have a more complete picture of how our universe, galaxy and planet came to be.