Elizaveta Sitnikova

Doctoral project

Search for Dark Matter and other BSM phenomena with four tops

The PhD project consists of two parts – contributing to the ATLAS inner tracker upgrade and participating in searches for BSM phenomena in the four top final state.

Before the start of High Luminosity LHC the Inner Detector of the ATLAS detector will reach the end of its operating life. It needs to be replaced by a new Inner Tracker (ITk), able to cope with the higher luminosity of the machine. The ITk tracker is a worldwide collaboration involving more than 60 universities and research institutions, and DESY plays a prominent leading role. One of the two forward regions of the ITk strip sub-detector, the so-called "end-caps", will be assembled at DESY. The strip tracker consists of silicon microstrip sensor tiles, with their associated readout, power and control electronics, assembled together into a single unit, the so-called "modules". These modules require a very strict quality control (QC) procedure to demonstrate that they are suitable for installation in the final detector. Part of my PhD work consists of the development and testing of the silicon sensors and the modules at different stages of the module construction. One of the most demanding quality control steps of the strip modules involves the thermal cycling of every single module unit. Thermal cycling is defined as 10 thermal cycles between -35 and +20 degrees Celsius with electrical testing in between cycles to ensure the robustness and quality of modules. This is done in specialized custom-made setups, the so-called coldboxes, two of which are at DESY. As a part of this PhD software for coldboxes operation was developed with the goal of full automation of the process. Lund University is also participating in the module production chain of the strip tracker, and as such, it also utilizes the coldbox setup. Both DESY and Lund have been collaborating closely in the development and operation of these complex setups, and I have actively participated in this effort.

The other part of the PhD focuses on searches for beyond the Standard Model phenomena. The analysis provides a unique way to probe new physics, such as top-philic resonances (Z′) produced in association with top quarks (tt Z′) resulting in events containing four top-quarks. One study was performed using data of proton-proton collisions with center-of-mass energy of 13 TeV collected by the ATLAS experiment at the LHC in 2015-2018 with total luminosity of 139 fb−1. Selected events contained a single lepton in association with multiple jets and were categorized into signal regions according to the multiplicity of jets and how likely these contained b-hadrons. The next analysis will also include data, collected by the ATLAS experiment with center-of-mass energy of 13.6 TeV. Moreover, it will take into account different signal models and increase the studied signal mass range.