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Measurement of the top Yukawa coupling with the ATLAS detector

Periodic Reporting for period 1 - Top-Higgs coupling (Measurement of the top Yukawa coupling with the ATLAS detector)

Reporting period: 2018-10-01 to 2020-09-30

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.
The measurement of the ttH production was performed within the ATLAS experiment at the Large Hadron Collider. The Researcher set up the full physics analysis chain in C++ programming language and he coordinated the work within the analysis team at the Institute of Physics at JGU Mainz. The Researcher investigated various Higgs boson identification techniques and chose the one which improves the most the measurement sensitivity. The Researcher implemented a reconstruction technique for the top-antitop quark pair reconstruction in ttH events which further improved the separation between signal and background. Based on an optimized event selection to ensure the best trade off between ttH events and background in the boosted regime, the Researcher implemented a machine learning technique (Boosted Decision Trees) to increase the measurement sensitivity. The usage of deep neural networks was also tested for future improvement of the measurement.
The Researcher developed a new pileup mitigation method for jets (Iterative Constituent Subtraction) which can be used to improve the jet reconstruction at the LHC experiments and consequently improve the ttH measurement in the long term.

The Researcher gained expertise in multivariate analysis techniques (Boosted decision trees and Deep neural networks), classification optimization and the profile likelihood fit method. The researcher attended workshops on presentation skills, career planning, and on leadership skills. The Researcher co-supervised bachelor, master and PhD students, which improved his project management and leadership skills. The Researcher improved his management skills by thorough planning of the measurement in this project, which also includes management of computing resources and personpower.

List of conferences attended:
- 40th International Conference on High Energy Physics, virtual conference (Prague, Czech republic), Oral contribution: Pileup and Underlying Event Mitigation with Iterative Constituent Subtraction, 30 July 2020
- DPG conference, Aachen, Germany, Oral contribution: Pileup mitigation with Constituent Subtraction, 26 March, 2019

Scientific publications:
- ATLAS Collaboration: Measurement of the Higgs boson decaying to b-quarks produced in association with a top-quark pair in pp collisions at sqrt(s)=13 TeV with the ATLAS detector, ATLAS-CONF-2020-058
Open access at: https://cds.cern.ch/record/2743685.
- P. Berta, L. Masetti, D. W. Miller, M. Spousta: Pileup and Underlying Event Mitigation with Iterative Constituent Subtraction, JHEP08 (2019) 175.
Open access at: https://link.springer.com/article/10.1007%2FJHEP08%282019%29175
- ATLAS Collaboration: Optimisation of large-radius jet reconstruction for the ATLAS detector in 13 TeV proton-proton collisions, arXiv:2009.04986. This preprint is already submitted to high-impact journal EPJC.
Open access at: https://arxiv.org/abs/2009.04986
- A. Basan, P. Berta, L. Masetti, E. Vryonidou, S. Westhoff: Measuring the top energy asymmetry at the LHC: QCD and SMEFT interpretations, JHEP03 (2020) 184.
Open access at: https://link.springer.com/article/10.1007%2FJHEP03%282020%29184

List of outreach activities:
- Poster presentation at a public event called “Institutstag”, Institute of Physics, JGU Mainz, Germany, 27.6.2019.
- Realization of Masterclasses for high school students, Frankfurt International School, Frankfurt am Main, Germany, 10.4.2019.
The project ensured a significant progress beyond the state of the art since the ttH cross section was not measured before in the fiducial phase space for Higgs boson transverse momentum above 300 GeV. The work carried out in this project ensured a possibility of measurement in this phase space. Another significant progress beyond the state of the art is a new competitive pileup mitigation method for jets called Iterative Constituent Subtraction.

The project belongs to fundamental research. Its main goal is the understanding of Nature and of its most fundamental constituents and their interactions. The proposed research can on the long-term give answers to important open questions in physics. These include establishing a connection between the Standard Model of elementary particles and general relativity, understanding the mass hierarchy of elementary particles and the particle-antiparticle asymmetry that leads to the existence of our Universe, origin of all fundamental physics​ constants,​ and​ discovering the nature of dark matter.
Feynman diagrams for the production of the Higgs boson in association with a pair of top quarks
Jet mass resolution for four pileup correction methods, arXiv:1905.03470
Signal-strength measurements as a function of Higgs boson transverse momentum, ATLAS-CONF-2020-058