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Final Report Summary - EWSB (Study of the ElectroWeak Symmetry Breaking and the Higgs Sector with the ATLAS detector at the LHC)

The Standard Model (SM) of particle physics is the most successful physical theory to date. Its main phenomenology has been confirmed, and it has withstood quantitative test of unprecedented precision in various facilities for 30 years now.

The only piece of the SM lacking direct experimental observation is the mechanism of electroweak symmetry breaking, providing with mass the weak gauge bosons and it is related to the appearance of the SM Higgs boson. Following the observation of a new particle by ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC) in summer 2012, the SM is possibly completed. The aim of this project is the elucidation of the Higgs sector through the experimental study of the properties of the recently observed Higgs boson and the search for new physics either by discovering additional Higgs bosons or deviations from the SM expectations.

The Higgs boson decays to two Z bosons, that subsequently decay to a pair of leptons (electrons or muons) each, played a crucial role in its observation. It is often referred to as the "golden channel" because, despite the low rate, it offers the highest signal-to-noise ratio and a complete reconstruction of the Higgs boson candidate with high precision. This powerful channel was the most sensitive for a Standard Model Higgs boson in the mass range around 125 GeV at the time of the observation. These features also make it an ideal channel for the study of the properties of the observed boson: we are using the H->ZZ*->4l decays both to measure the Higgs boson mass and probe its couplings to the Z boson, which is an important test of the electroweak symmetric breaking mechanism, and also to eventually use it as a reference with respect to the other Higgs decay channels.

The complete LHC Run I dataset was employed to measure the mass of the Higgs boson. The precision of this measurement with the H->ZZ*->4l channel is already at the 0.4% level. The combination of the mass measurements from ATLAS and CMS provides a measured mass of 125.09 +/-0.24 GeV, with an achieved precision of 0.2%. It is noted that the ATLAS H->ZZ*->4l channel provides, currently, the mass measurement with the lowest systematic uncertainty. Furthermore, we categorised the observed H->ZZ*->4l events to measure the Higgs boson production rates at exclusive final states, enhancing specific production mechanisms (vector-boson fusion, associated production with a vector boson), and probed its coupling properties. The measurement is dominated by the statistical uncertainty, however through the introduction of an optimised event categorisation and of a multi-observable likelihood fit, a substantial improvement of about 40% to the sensitivity to the vector-boson mediated couplings with respect to our earlier result was achieved. Currently, the obtained results are compatible with the SM expectation. At the same time, the first fiducial and differential cross-section measurements in the H->ZZ*->4l final state were obtained. These are important measurements for direct detailed comparisons of the kinematic characteristics of the observed Higgs boson with the theory predictions. Finally, it was also established that the observed Higgs boson is predominantly a scalar, as predicted by the Standard Model.

The natural next step was to combine all the ATLAS Higgs boson coupling studies in a common framework and thus probe the Higgs boson coupling structure in the most comprehensive way. Using the complete LHC Run 1 datase an overall combination of the available decay channels has been performed. The results of the studies have been further used to place constraints on new phenomena via Higgs boson couplings and invisible decays.

Moreover, new searches for pair Higgs boson production have been performed, which would probe the Higgs boson self-coupling, a crucial test for the mechanism of Electro-Weak Symmetry Breaking.

Having performed the above mentioned analyses using the available LHC Run I dataset, it was clear that the statistical uncertainty dominated most of these studies. Moreover, the searches for additional Higgs bosons, particularly in decays involving the observed Higgs boson, are severally constrained by the available phase-space. For these reasons the LHC Run 2 at the increased centre-of-mass energy of 13 TeV was a crucial development to enhance, substantially, the sensitivity of our experiment to new physics.

Subsequently, the effort has been focused in commissioning the detector for physics at the beginning of the LHC Run 2. The measurement of the W and Z boson production cross sections at the increased centre-of-mass energy of 13 TeV was not only an important test of the SM at the highest energies available, but allowed the prompt commissioning of the detector. Furthermore, aiming to discover new physics at the Higgs sector, while tackling the questions on the nature of dark matter, searches for dark matter production in association with a Higgs boson have been performed.

Further on the topic of characterising the newly observed particle, we obtained the fist direct experimental constraints on the the Yukawa coupling to the charm quark, through the exclusive decay of the Higgs boson to a J/psi and a photon. This is a completely novel approach in the Higgs sector studies. Also, the decay of the Higgs boson to an Upsilon and a photon was searched for, as well as, the analogous - previously unobserved - decays of the Z boson. These results, in conjuction with the first searches for the associated production of a Higgs boson with a top-quark pair, yielded preliminary evidence of non-universality in the Higgs couplings to quarks. This development opened up a completely new line of research in the Higgs sector. This programme was taken forward by performing the search for Higgs boson decays to a phi meson and a photon. This search can provide information on the Yukawa coupling of the strange quark, The analogous - previously unobserved - decays of the Z boson has also been sought for.

In parallel, we have collaborated with phenomenologists on new ideas to push forward our understanding of the Higgs sector, while we have contributed in the evaluation of the physics potential of new facilities for the future.

During the whole duration of the project, substantial effort has been devoted on the preparations of the ATLAS Inner Track upgrade for the High Luminosity LHC, including both the irradiation programme at the Medical Cyclotron of the University of Birmingham and performance studies of an upgraded ATLAS at the High Luminosity LHC.

More details on the aims and progress of this project can be found at the dedicated website:

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