Top-Higgs coupling · Measurement of the top Yukawa coupling with the ATLAS detector

Measurement of the top Yukawa coupling with the ATLAS detector

The Large Hadron Collider (LHC) at CERN provides an unprecedented amount of high-energy proton-proton collisions which are used to probe the Standard Model of elementary particle physics. The discovery of the Higgs boson at the LHC has opened up new opportunities in revealing the mysteries of the Universe, such as the unknown building blocks of dark matter, the missing quantum description of gravity, or the abundance of the observed matter with respect to antimatter in the Universe. However, there is still no indication of new particles or interactions beyond the Standard Model (BSM) which would inevitably appear in any theory able to solve the puzzles mentioned above. To answer these questions, more measurements need to be performed. One of them is the measurement of the coupling of the top quark to the Higgs boson (called top Yukawa coupling) which is the topic of this research project. Yukawa couplings are linearly proportional to the fermion masses in the Standard Model, and thus the top Yukawa coupling is the most​ interesting​ one,​ since​ it​ is​ the​ largest​ of​ all the Yukawa​ couplings. A deviation of the measured top Yukawa coupling from the Standard Model prediction would be a proof of new phenomena beyond the Standard Model that could provide an answer to several open questions of contemporary physics. In this project, the top Yukawa coupling is estimated based on the measurement of the production cross section of the Higgs boson in association with​ a​ pair of​ top​ quarks (ttH). The main research objective of the project is to optimize the identification of ttH events to achieve a cross section measurement as precise as possible with the available amount of data. The focus is on the semileptonic decay of the pair of top quarks and on the Higgs boson decay to a pair of bottom quarks in events with collimated decays of the Higgs boson (so called boosted regime). The main project result is the improved precision of the cross-section measurement of the Higgs boson production in association with a top-antitop pair in the boosted regime. The findings in this project allowed to measure the cross section in the fiducial phase space defined at particle level for a Higgs boson transverse momentum above 300 GeV. The result is consistent with the Standard Model prediction within approximately 1 standard deviation. Another project result is a new method to mitigate the effects from simultaneous proton-proton collisions (pileup), called Iterative Constituent Subtraction. The new method significantly improves the reconstruction of jet observables in a high pileup environment which is essential for future LHC running. Additionally to the research objective, a training objective was also pursued, namely to provide the Researcher with new scientific and soft skills like project management, leadership, and presentation skills.

Data: CORDIS, © European Union