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Differential Higgs distributions as a unique window to New Physics at the LHC

Periodic Reporting for period 3 - HiggspT (Differential Higgs distributions as a unique window to New Physics at the LHC)

Reporting period: 2019-05-01 to 2020-10-31

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. Since then, 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 now opens a window to the discovery of New Physics in the Higgs sector through precision studies of the new particle.
A significantly larger dataset has now become available, collected between 2015 and 2018. 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 is 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->gammagamma) and to four leptons (H->ZZ->4l). The interpretation of the results includes an in-depth study of the low-pT region, where the differential distributions can be measured with the best statistical precision, and will also contain a search for contributions from new heavy particles in the high-pT region. These studies will be an important step in the study of how elementary particles acquire mass and whether this is consistent with the predictions of the SM or requires physics beyond the SM to be understood.
During the first half of the project, in a first step, the data taken by the ATLAS experiment in 2015 and 2016 was used to perform measurements of differential cross sections in the H->gammagamma decay channel, including measurements of more than 20 differential distributions. In a second step, the data taken from 2015 to 2017 was used to perform measurements of several differential cross sections in the H->gammagamma and H->ZZ->4l decay channels. Before, as preparation for these measurements on the larger dataset, the group had worked on the preparation of the reconstruction software used to process the data taken in 2017, as well as to reprocess the data taken in 2015 and 2016. In addition, to support both measurements in the H->gammagamma decay channel, the photon identification efficiency was measured using a clean sample of electrons selected from Z->ee decays. The measured differential cross sections were compared to a variety of theoretical predictions, showing in general good agreement between the predictions and the measurements. Currently, the measurements on the complete dataset taken between 2015 and 2018 are being carried out. The measurements in the H->ZZ->4l decay channel have been published, and preliminary measurements in the H->gammagamma decay channel are available, including constraints on the charm-quark Yukawa coupling derived from the low-pT region.
The dataset collected from 2015 to 2018 contains about ten times as many Higgs bosons as were available in the data recorded up to 2012. Since the precision of the measurements is still determined by the size of the available dataset, the measurements obtained are already much more precise than those previously available. In addition, the comparisons to theoretical distributions take advantage of theoretical predictions that have only become available recently and are improved and more precise as those available previously. The constraints derived on the charm-quark Yukawa coupling in the context of the preliminary measurements in the H->gammagamma decay channel was the first constraint on this quantity derived by the ATLAS experiment.