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

Periodic Reporting for period 4 - HiggspT (Differential Higgs distributions as a unique window to New Physics at the LHC)

Reporting period: 2020-11-01 to 2021-10-31

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. 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.
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→γγ decay channel, including measurements of more than 20 differential distributions. In a second and third step, the data taken in 2015-2017 and then in 2015-2018 were used to perform more precise measurements of differential cross sections in the H→γγ and H→ZZ*→4l decay channels. The measurements on the complete dataset have been statistically combined to further improve the statistical precision.
To support these measurements, the team worked on improvements to the reconstruction of photons that convert to electron-positron pairs in the detector material and on software tools to improve the modeling of the shape of electromagnetic showers in the simulation. In addition, 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. For the first time within the ATLAS Collaboration, the low-pT region has been used to derive constraints on the strength of the couplings between the Higgs boson and bottom- and charm-quarks. The latter have comparable precision as constraints from searches for Higgs boson decays to charm quarks. A search for potential new heavy particles or new interactions is currently still in development.
The results were presented in several conference notes and publications, as well as a PhD thesis. Members of the team also presented the results at several conferences and workshops.
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 largely determined by the size of the available dataset, the measurements obtained are much more precise than those previously available, with a statistical uncertainty reduced by about a factor of 3 compared to measurements available before the start of the project.
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 were the first constraints on this quantity derived by the ATLAS experiment and of comparable precision as the constraint from searches for Higgs boson decays to charm quarks that have been derived in the meantime.
Measured Higgs transverse momentum spectrum, compared to theoretical predictions (Copyright CERN)
Best fit values and constraints on bottom and charm Yukawa couplings (Copyright CERN)