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Uncovering the Origins of Mass: Discovery of the di-Higgs Process and Constraints on the Higgs Self-Coupling

Periodic Reporting for period 3 - HiggsSelfCoupling (Uncovering the Origins of Mass: Discovery of the di-Higgs Process and Constraints on the Higgs Self-Coupling)

Período documentado: 2022-06-01 hasta 2023-11-30

The Standard Model (SM) of particle physics describes the elementary constituents of matter and their interactions. In 2012, its last ingredient, the Higgs boson, was discovered at the Large Hadron Collider (LHC). The exploration of the Higgs boson is now one of the most exciting avenues to explore for New Physics beyond the Standard Model and allows some of the most pressing problems in theoretical physics to be addressed, such as the origins of the electroweak symmetry breaking mechanism. This mechanism was introduced into the Standard Model in 1964 by hand in order to find a prescription to give elementary particles mass without breaking the gauge symmetry equations of the SM. The postulated shape of potential energy of the Higgs boson in the SM is completely ad hoc, with no derivation from first principles. It is something simple that works, but theoretically, it is highly unsatisfactory. Fundamentally, we still do not understand why and how particles acquire mass because we do not know the origin of the mechanism for electroweak symmetry breaking. The shape of the Higgs potential is key to understanding the electroweak symmetry breaking mechanism and measuring it will lead not only to a better understanding of the origin of this mechanism, but will provide an answer to one of the most intriguing mysteries in science: Why and how do elementary particles acquire mass?

A particularly crucial measurement is the production cross-section of Higgs boson pairs, which provides unique information on the Higgs self-coupling and on the underlying nature of the electroweak symmetry breaking mechanism. The overall shape of the Higgs potential depends on the value of Higgs self-coupling. The HH--> bb decay channel has the highest branching ratio (~0.33) among all possible 2H decay channels. It is therefore a promising channel to use, given the fact that the production rate for Higgs pairs is already very low. This project aims to develop and complete the measurement of the di-Higgs cross section and most stringent bounds on the Higgs self-coupling. To achieve this goal new new experimental techniques to improve the background reduction rates and enhance the signal will be developed. The objectives are the development of novel bottom quark energy reconstruction algorithms, new bottom quark and Higgs identification techniques, and neural network analysis tools. Analysis of ATLAS data will then enable searches for New Physics and ultimately the di-Higgs cross section measurement to constrain the Higgs self-coupling. This landmark measurement will lead to the confirmation of how particles acquire mass and open new avenues to understand what lies beyond the Standard Model.
The project was significantly delayed due to the corona pandemic. The hiring of the research team took up to 9 to 12 months longer. Since December 2018 the Large Hadron Collider is in a technical shutdown (LS2) in order to upgrade the machine and the experiments. It was planned that LHC Run3 data taking starts begin of 2021. But due to the delays, because of the Covid pandemic, the start of the Run3 data taking had to be postponed by one year. All of this meant that the project was also delayed by one year.

The project team focused on getting ready for the start of the Run3 data taking period in 2022.
Because of the Covid pandemic the project has a delay of one year and is still in the beginning phase. I expect that the project will perform diHiggs searches with 300 fb-1 and constrain the Higgs selfcoupling.
This is an ATLAS data event recorded in 2018. It shows a diHiggs candidate decaying to 4 b-jets.