Over the past decades, particle physics has explored the fundamental structure of matter and built up and tested a model of elementary particles and their interactions, called the Standard Model of Particle Physics (SM). Over the years, the model was tested with increasing precision and was found to marvelously account for the experimental observations. Yet, one piece was missing: In the SM, the Higgs mechanism is evoked to explain the masses of the elementary particles and was found to be in extremely good agreement with precision measurements performed at the LEP and SLC colliders and other experiments. The mechanism also predicts the existence of a neutral boson, the Higgs boson. Until July 2012, the Higgs boson was the only elementary particle predicted by the SM that had not yet been observed. In July 2012, the ATLAS and CMS experiments, operating their detectors at the Large Hadron Collider (LHC) in Geneva, first announced the discovery of a new neutral particle with a mass around 126 GeV, an intriguing candidate for the long-sought Higgs boson. Measurements performed with the data taken until the end of 2012 had confirmed the new particle to be a Higgs boson. Much larger datasets and detailed studies are necessary to determine whether it is the Higgs boson as predicted by the SM, or one of the Higgs bosons predicted by a different model beyond the SM. The discovery of the Higgs boson has opened a window to the discovery of New Physics in the Higgs sector through precision studies of the new particle.
A significantly larger dataset has become available in the meantime, collected between 2015 and 2018 by the ATLAS and CMS experiments. It allows for much more thorough property studies of the new particle, and therefore a much deeper look into what might be the mechanism of mass generation for elementary particles. The aim of the project was a detailed study of the Higgs boson transverse momentum spectrum and other differential distributions as a precision test of New Physics in the Higgs sector using Higgs boson decays to two photons (H→γγ) and to four leptons (H→ZZ*→4l). The measurements performed on the 2015-2018 dataset allowed to improve the statistical precision by about a factor of three compared to the results available before the start of the project and are in good agreement with predictions from the SM (see Figure 1, which shows the Higgs transverse momentum distribution measured in H→γγ and H→ZZ*→4l decays, and their statistical combination. The bottom panel shows comparisons to theoretical predictions, where the different predictions use different predictions for the dominant gluon fusion process and are normalized by the K-factors given in the legend. The figure is taken from ATLAS-CONF-2022-022.).
The interpretation of the results has included an in-depth study of the low-pT region, where the differential distributions can be measured with the best statistical precision, and also contains a search for contributions from potential new heavy particles and new interactions in the high-pT region. The study of the low-pT region led to constraints on the strength of the couplings between the Higgs boson and bottom- and charm-quarks (see Figure 2, which shows the best fit values and constraints on the couplings boson derived from the Higgs transverse momentum distribution measured in Higgs boson decays to two photons and Higgs boson decays to four leptons, and their statistical combination. The figure is taken from ATLAS-CONF-2022-022.). The latter have comparable precision as constraints from searches for Higgs boson decays to charm quarks.
