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, which is connected to the mechanism proposed to explain the mass generation for the force carriers, in 2012 by the ATLAS and CMS Collaborations at the CERN Large Hadron Collider.
Following the discovery, a substantial body of accumulated experimental evidence suggests that this mechanism, as postulated in the Standard Model, is realised 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, no explanation of the large variations of the observed masses of the elementary matter particles is provided. Case in point, the top-quark - the heaviest matter particle - is approximately 330,000 times heavier than the electron - the lightest matter particle. Several theories beyond the Standard Model predict different mechanisms for mass generation, aspiring to explain the observed mass hierarchy.
The ExclusiveHiggs project explored the interactions of elementary matter particles with the Higgs boson using the ATLAS detector. Searches for rare exclusive Higgs boson decays and the direct search for Higgs boson decays to second generation quark--anti-quark pairs were undertaken. These new channels can shed light to the least studied part of the Standard Model, and map the challenges and the opportunities in tackling these questions in future facilities. At the same time, an extensive set of searches for analogous processes in the decays of the massive force carriers was 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 and elucidating the origin of mass.
Although no deviation from the Standard Model was observed to-date, within the ExclusiveHiggs programme a legacy of new searches and measurements was delivered. These results prompted other experiments to perform similar searches and the theory community to investigate the properties of these decays. Thus, the ExclusiveHiggs project established a sub-field of research into the Higgs sector. The developed techniques are now used in a variety of physics investigations, and the way was paved for future research that may potentially lead to the observation of new physics phenomena.
