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Search for New Physics in First and Second Generation Quark Yukawa Couplings through Rare Exclusive Decays of the Observed Higgs Boson

Periodic Reporting for period 2 - ExclusiveHiggs (Search for New Physics in First and Second Generation Quark Yukawa Couplings through Rare Exclusive Decays of the Observed Higgs Boson)

Reporting period: 2018-09-01 to 2020-02-29

Where does mass come from?

In the last century great advances have been achieved in our understanding of the microcosm. These culminated in the development of the Standard Model of particle physics, which describes the fundamental building blocks of nature, the elementary matter particles, and their interactions, mediated by force carrier particles. The experimental observation of the particles of the Standard Model was completed with the discovery of the Higgs boson in July 2012 by the ATLAS and CMS Collaborations at the CERN Large Hadron Collider. The Higgs boson is connected to the mechanism proposed to explain the mass generation for the force carriers.

In the years since the discovery, a substantial body of accumulated experimental evidence suggests that this mechanism, as postulated in the Standard Model, is realized in nature and results in the observed masses of the force carriers. Within the Standard Model, the elementary matter particles, the quarks and the leptons, acquire their mass through a separate mechanism, which also involves the Higgs boson. Alas, the Standard Model does not provide a satisfactory explanation of the large variations of the observed masses of the elementary matter particles. Point in case, the top-quark - which is the heaviest matter particle - is approximate 330,000 times heavier that the electron - which is the lightest matter particle. Several theories beyond the Standard Model predict different mechanisms for the generation of these masses, aspiring to explain the observed mass hierarchy.

The ExclusiveHiggs project aims to experimentally explore the interactions of elementary matter particles with the Higgs boson, at the ATLAS detector at the CERN Large Hadron Collider. Novel experimental techniques will be implemented, including the exclusive Higgs boson decays and the direct search for Higgs boson decays to quark--anti-quark pairs. These could shed light to the least studied part of the Standard Model, and map the challenges and the opportunities in tackling these questions in possible future particle physics facilities. At the same time, an extensive set of measurements of analogous processes for the massive force carriers will be performed, most of these for the first time, further enhancing the scientific value of the proposed research programme. Progress in this area is crucial to complete our understanding of the Standard Model, as well as elucidate the origin of mass, and could potentially lead to the observation of new physics phenomena.
The CERN Large Hadron Collider completed its second data taking campaign in the end of 2018, which resulted in a dataset of ten quadrillion proton collisions being delivered. The ExclusiveHiggs team has contributed in ensuring that interesting events in this dataset have been selected and recorded – with high quality - for further analysis. Moreover, the team has led the design, development, maintenance, and improvement of the highly selective algorithms dedicated to select and record interesting events for the rare exclusive decays of the Higgs boson and the carriers of the weak force. A major challenge in this undertaking is that the algorithms need to select as many of the interesting events as possible, while recording only approximately 1 out of 40,000,000 collision events. These algorithms enabled the collection of an unprecedented dataset, that would otherwise be lost. Based on this dataset, to-date, the team has led the search for several rare exclusive decays of the Higgs and Z bosons. The realization of these searches require novel techniques to overcome several experimental challenges, including the modeling of other processes that result in similar signatures in the detector but are not related to the sought physics signal.

Furthermore, the team made major contributions in several high profile publications, including the first search for Higgs boson decays to a pair of charm-quarks and the first ATLAS search for decays of the Higgs boson to electron-positron pairs and anomalous decays to electrons and muons. These searches were complemented with searches for new physics in the Higgs sector of the Standard Model, which remains the least studied part of the theory underpinning our understanding of the microcosm.

Looking towards the future, the team has made contributions to the ATLAS upgrade programme in view of the High Luminosity Large Hadron Collider, due to begin data-taking in mid-2027. More specifically, the ExclusiveHiggs team is involved in research and development efforts for the innermost parts of the ATLAS detector, where particle densities are the highest, including in the qualification of irradiation facilities and inter-facility comparisons, as well as in evaluating the potential of the upgraded ATLAS detector in reconstructing electrons and photons in the anticipated challenging environment. Finally, the team has contributed studies to the world-wide discussions for the future direction of particle physics, and the physics potential of proposed future particle collider facilities, as part of the on-going European Particle Physics Strategy Update 2018 – 2020.
The matter particles, the quarks and the leptons, are organized in three generations, each a heavier copy of the previous one. To-date, only the Higgs boson interactions with matter particles of the third and heaviest, generation have been established. The ExclusiveHiggs project aims to lead the way in the study of the Higgs boson interactions with matter particles of the first two generations.

The ExclusiveHiggs team has been instrumental in the publication of the first direct search for Higgs boson decays to a pair of charm-quarks, the dedicated searches for exclusive Higgs boson decays to a meson - a system composed by a quark and an anti-quark - and a photon, and searches for similar decays of the Z boson, as well as the first ATLAS search for Higgs boson decays to electron-positron pairs and anomalous decays to electrons and muons. These searches, to a large extent, set the state of the art in our current understanding of the Higgs boson interactions with matter particles of the first and second generations. In these efforts the team has contributed novel techniques in various aspects of the analyses, including novel background modelling and particle reconstruction techniques.

Until the end of the project, the ExclusiveHiggs team aims to use the unprecedented dataset of proton collisions, that we have contributed in collecting, to further our understanding of Higgs boson interactions with matter particles. We plan to consolidate and improve the already established searches, and to expand to new and promising ideas that could help unveiling new physics in the Higgs sector.