Standard Model Extension (SME) is an extension of the SM that incorporates in a model independent way the effects of LIV and effects of the violation of the CPT symmetry, and allows to study the experimental signatures of these effects at the energies accessible by the collider experiments nowadays. The first part of the project was focused on the phenomenological study necessary to simulate the LIV effects in the process of top quark-antiquark pair production. Given that the currently available common Monte Carlo based software tools can not simulate the necessary processes according to a model that explicitly violates Lorentz invariance, I developed, a software tool that allows to modify the weight of the events that are simulated using the SM model for the top quark-antiquark production so that the distributions of an arbitrary observable based on these modified events match the expected distributions of the corresponding observables based on the events that would have been generated according to the SME model. This procedure has allowed to study the expected LIV effects for any value of the parameters of the SME model, without a need for explicit generation of the corresponding events. Furthermore, it has allowed to systematically study and identify the experimental observables that are most sensitive to the LIV effects in the process of interest, and that can provide the best discrimination between the SME and SM hypotheses.
The second part of the project included the studies that were necessary to perform the measurement using the proton-proton collision data collected with the ATLAS experiment during the LHC Run 2 in the period 2015 - 2018. The measurement is performed as a function of ATLAS detector orientation in the Sun-centred reference frame (with respect the so called side-real time) optimised to probe the modulations of the cross-section of the top quark-antiquark pair production as a function of the side-real time. While the measurement is significantly relying on ATLAS experiment centrally produced datasets, significant part of the work is invested to understand and validate the detector performance as a function of the side-real time, and determine necessary corrections to account for the side-real time dependence of the detector performance. In particular, the analysis has considered integrated luminosity, distribution of the pile-up events and trigger efficiencies to depend on the side-real time. Furthermore, the state of the art machine learning techniques are used to identify particles according to their signature in the detector, and to match them to their constituents at the interaction point. The SME signal events are obtained by reweighting the SM top quark-antiquark pair production simulated sample based on the derived reweighting function. The event selections are applied to enhance in the sample the events with the topology of the interest, and to suppress the events with other topologies. To extract the values of the top quark-antiquark cross section in each bin of the side-real time a maximal likelihood fit formalism is applied. All systematic uncertainties associated to the measurement are implemented as a nuisance parameters in the fit. Once the analysis and the result of the measurement are approved by the ATLAS collaboration, these will be submitted for publication in one of the major international scientific journal and linked from the project webpage:
https://www.ipb.ac.rs/delta/(odnośnik otworzy się w nowym oknie) The measurement is presented to the ATLAS collaboration attracting the interest of colleagues and establishing new avenue/searches (not involving the top quarks) for probing the physics beyond the Standard Model with the ATLAS experiment. As a part of the project, the international workshop dedicated to the possibilities to test the space-time properties with the collider experiment has been organised at the home institution, gathering across the world the key theorists and experimentalist from the field. During the period of the project, I have presented multiple times the research goals of this projects introducing them to the research field at the student seminars yearly events, subsequently supervised bachelor and master students, and took part in organisation, lecturing and tutoring of the high school students from Serbia at the CERNs High School Students Internship Programme. In addition, I have also disseminated the research related to this project and HEP in general at the public media.
