Search for Higgs bosons decaying to dark matter using advanced artificial intelligence techniques and upgrade of the silicon tracker at ATLAS

The Large Hadron Collider (LHC) is a fantastic success, having been in operation for more than a decade, collecting huge datasets and releasing hundreds of results; the highlight being the discovery of the Higgs boson. Searches for new physics and precision measurements of Standard Model (SM) parameters, including the properties of the Higgs Boson, continue and will get a huge boost once the upgrade of the LHC to the High-Luminosity LHC (HL-LHC) will be completed.

At HL-LHC, the machine and deliver 10 times larger datasets to ATLAS (and CMS) for a total integrated luminosity of 4000 fb-1 and will increase its centre-of-mass energy up to 14 TeV. This PhD offers a balance between commissioning the new silicon tracking detector in readiness for the HL-LHC and exploitation of the data currently being taken by searching for Higgs bosons decaying to dark matter using the most up to date artificial intelligence techniques.

Dark matter and dark energy are the most mysterious entities in the Universe. If dark matter obtains its mass from the Higgs mechanism (like the fundamental particles of the Standard Model) and is sufficiently light, we should expect the Higgs boson to decay to a pair of dark matter particles, which will travel through the detector undetected. This is the best way to discover light dark matter at the LHC.

Liverpool has had a long involvement in this channel. Having a student assigned to this analysis will allow us to continue this important work. The student will analyse the run 2 and run 3 data, which will be more than double the previously published dataset, using the process where the Higgs boson is produced in association with a Z boson, which decays either to electrons or muons.

The student will use the knowledge gained from their LIV-INNO training to use the latest artificial intelligence tools such as a Graph Neural Network (GNN) to improve the efficiency and separation from the main background of ZZ production. The student will modify the GNN, which has been shown by Liverpool researchers to greatly improve tau reconstruction.

In addition, the student will extend the analysis to include models where the Higgs boson decays to scalar particles that are long-lived but may partially decay in the detector, thus giving rise to some missing momentum. This unique signature has never been analysed at the LHC.

Due to the intense environment of the HL-LHC, where more than 200 collisions take place simultaneously, the central part of the ATLAS detector will be replaced. The new Inner TracKer (ITK), which measures the trajectories of charged particles, is constructed with silicon pixel and strip modules. The University of Liverpool are major contributors to many aspects of the design, build and commissioning of this new detector including the assembling of one of the outer pixel endcap (EC) detectors in the clean room at Liverpool.

A successful commissioning of the ITK will be crucial to continue to pursue most analyses at ATLAS including the Higgs to dark matter in association with a Z. An excellent reconstruction of the electrons or muons from the Z decay together with an optimal reconstruction of the missing momentum will be vital.

The student will work as an integral part of the Liverpool ATLAS group, helping in the development of the operational procedures for, and making the first system-level performance evaluation of, the UK pixel endcap detector. They will contribute to an intense programme of testing that will be needed to ensure that the delivered endcap performs to the stringent specifications.

This testing and evaluation will include the first measurements of the performance of pixel quad modules in their final configurations. Key to this work will be the development of an analysis framework which is tightly coupled with the production