Search for new interactions of the top quark at the ATLAS experiment at the LHC
IDPASC Portugal - PHD Programme 2017
2017 / 2018
Experimental Particle Physics
Patricia Conde Muino
Universidade do Minho
LIP, UMinho and ULisboa
The top quark is the heaviest elementary particle discovered so far and the study of its properties and couplings provides not only an important test of the Standard Model (SM) of particle physics, but also a window to physics beyond it. At the run-1 and run-2 phases of CERN's Large Hadron Collider (LHC), both the ATLAS and CMS collaborations developed an extensive program devoted to precision measurements and searches in the top quark sector. Nonetheless several of the analysis done so far can benefit from more data, either because they are statistically limited, or because such increased dataset will allow the use of more elaborate techniques, not accessible so far. Given the increase in the collected luminosity expected for the next years, new exciting opportunities will be opened, allowing to expand our knowledge on the top quark sector and, consequently, on Particle Physics. The increase of luminosity will allow to probe with unprecedented precision rare events, such as the production of top quarks via flavour changing neutral (FCN) couplings to the SM gauge bosons. In the SM, the production or decay of top quarks via Flavour Changing Neutral Currents (FCNC) is extremely suppressed but some of its extensions predict a significant enhancement of the probability for such processes. A highly sensitive way of probing the FCN coupling tqZ (with q being a up or charm quark) is the search of tZ production via FCNC. A similar search would also allow us to probe the tqg FCN coupling (where g denotes a gluon). Furthermore, the study of the tZ production in the context of the SM is an important measurement, providing information relevant to many other important results (as background), such as the measurement of the ttZ and ttH cross-sections. In the scope of the current proposal, the candidate will study FCNCs through the single top production with a Z boson using data collected by the ATLAS detector. Furthermore, a phenomenological study of the interference between tZq (FCNC in top decays) and tZ (production via FCNC) can be performed and the obtained results will be incorporated in the global strategy of the ATLAS searches for these processes. A close collaboration with phenomenology experts is foreseen, allowing to fully explore the consequences of the obtained experimental results. The present proposal foresees a search analysis focused on the top FCNC process where a top quark and a neutral Z boson is produced. A topology with three leptons, two charged leptons coming from the decay of the Z boson and one charged lepton along with one neutrino and one b-tagged jet from the decay of the top quark will be considered. This topology was chosen since the final state consists in a clear signature of three leptons and just one jet. The consequent loss in acceptance is compensated with the gain in efficiency. The candidate will carry out this work integrated in the Portuguese ATLAS group, in close collaboration with other international institutes. The validation of the Monte Carlo simulation, the evaluation of systematic uncertainties, the testing of new strategies to control them and a close collaboration with the phenomenological community in the interpretation of the experimental results will be crucial to fully exploit the potential of the new LHC data. In a second stage the implementation of multivariate techniques, such as the matrix element method, will be considered with the goal of maximizing the sensitivity of this search. Being a member of the ATLAS collaboration, the candidate will be expected to contribute to the operation of the detector and to perform technical tasks, namely on the evaluation of the efficiency of dedicated triggers using data collected by the ATLAS Forward Proton (AFP) detector. The experience to be gained in AFP will also be used to develop a search for the photo-production of top quarks at the LHC. For such FCNC process, the ability to tag final state protons, produced at very low polar angles, is crucial to separate the signal from background events. Using such information, in conjunction with the data collected at larger polar angles by ATLAS, will allow to obtain extremely high background rejection rates and thus improve on the LHC sensitivity to FCNC processes. The current proposal aims at a comprehensive training program, allowing the PhD candidate to acquire a significant expertise in High Energy Physics. Most of the work is expected to be developed in Braga, but an one year stay in Lisbon and short stays at CERN are also expected, allowing to benefit from the close collaboration with experts in the different fields, as required by the current proposal. Either a discovery or the world's most stringent limits on the top quark FCNC processes are expected to be obtained within the scope of this working program.