Explore the "discovery channels" of the Higgs boson

This week, at the European Society of Physics High Energy Physics (EPS-HEP) conference in Ghent, Belgium, the ATLAS collaboration at CERN published new measurements of the properties of the Higgs boson at 39. using the complete LHC Run 2 dataset. Critically, the new results examine two of the Higgs boson decays that led to the discovery of the particle in 2012: H → ZZ * → 4ℓ, where the Higgs boson is disintegrates into two Z bosons, then decomposing into four leptons (electrons or muons); and H → γγ where the Higgs boson decays directly into two photons.

Although rare, these channels are easily identified and well measured in the ATLAS detector, allowing physicists to study the properties of the Higgs boson with great accuracy. In particular, they provide new measurements of the transverse momentum of the Higgs boson, which can be used to probe different mechanisms of Higgs production and possible deviations from standard model interactions.

On the road to gold: a new overview of the four lepton channel

The H → ZZ * → 4ℓ decay is what is called the "gold channel" of the Higgs boson, because it has the clearest and most distinct signature of all the possible decay modes of the boson of Higgs. Higgs. Thanks to the increased luminosity of the second stage, ATLAS has recorded about 300 "gold channel" candidate events between 2015 and 2018, a third of which should be due to ZZ background processes. The invariant mass spectrum for selected events at four leptons for the complete series 2 data can be viewed in Figure 1.

In addition to increasing the number of events collected, ATLAS physicists have improved their analyzes. While the ZZ background noise level was previously estimated using simulations, coupled with theoretical uncertainty, the new ATLAS result uses data to directly assess the background contribution. While keeping the total uncertainty about basically the same, this has greatly reduced the theoretical uncertainty and dependence of the model on the measure.

The ATLAS team has also introduced deep learning neural networks to distinguish events in the Higgs boson from which production mechanisms. This technique allows the ATLAS team to more closely identify whether a Higgs boson was produced by the joint fusion of a pair of gluons (ggF – accounting for 87% of the Higgs boson decay), or by the rarer fusion of two W or Z vector bosons (VBF – 7% of decays) or radiation of a W or Z boson (VH – 4%). Once successfully identified, ATLAS physicists could then measure the effective cross section of production for each.

The modes of production VBF and VH are distinguished quite well by the separation and the mass of "jets" of particles that they produce. For the VBF, the vector bosons are radiated by two quarks, which form energetic jets in the detector in the direction of the beam and in the opposite hemispheres. At the same time, the VH production mode also produces two jets, with the mass of the W (80 GeV) or Z (91 GeV) boson.

However, the gluons of the ggF production can also emit additional jets, thus mimicking the VBF and VH jet pairs. It's here that Deep Learning Neural Networks intervene. They have proved flexible enough to simultaneously separate ggF, VBF and VH with less overlap than previous machine learning techniques. The cross-sections of Higgs production measured in the four-lepton channel are shown in Figure 2, where there is a 20% improvement in VBF cross-section measurement using the neural network technique.

Two lights to see the Higgs: study the two-photon channel

ATLAS physicists have also incorporated new and improved analytical techniques into their study of the decay of the Higgs boson into a pair of photons (H → γγ). In particular, the improved identification of photons and the calibration of jet energy have reduced the associated systematic uncertainties. The electromagnetic shower shape criteria used to identify photons and remove unwanted photon candidates from hadronic decays have now been optimized in the sub-ranges of the photon's transverse moment, as the showers generated in the detector depend on the amount of light. photon energy. This resulted in a sensitivity improvement of a few percent.

Physicists have measured several differential cross sections for observables sensitive to the production and decay of the Higgs boson, including the kinematic distributions of the jets produced in association with the Higgs boson. Beyond the standard model, interactions between the Higgs boson and gauge bosons should modify these variables, providing an excellent test for new physics. Inclusive and differential cross-sectional measurements, determined from the signal peak event yields in the invariant diphoton mass distribution (see Figure 3), were found to be in good agreement with standard model predictions. ATLAS physicists used these measurements to limit the strength of the hypothetical interactions beyond the standard Higgs boson model with the gage bosons (see Figure 4).

In addition, ATLAS physicists were able to examine the interaction between the Higgs boson and the charm quark. The Higgs boson has not yet been seen charming the quarks, which, according to the standard model, should have a rate twenty times lower than that of the quarks, observed for the first time by ATLAS and CMS in 2018. However, if the strength (or "coupling") of the Higgs interaction with the charm quark were much greater than expected due to new physical processes, this would affect the measured momentum distribution of the boson of Higgs. Physicists have looked for the signature of this effect: an excess of data over theoretical predictions in the Higgs boson's dead moment region (see Figure 5). Such an excess was not observed in the data.

Combined Preview

The total production cross section of Higgs boson was measured at 56.7 ± 6.3 bp with the H → γγ decay channel and at 54.4 ± 5.6 bp with the H → ZZ * → 4ℓ channel. Combining the two channels, the total section is 55.4 ± 4.3 bp, which is consistent with the standard model prediction of 55.6 ± 2.5 bp. The differential cross section for the transverse moment of the Higgs boson in the two channels is also in agreement, as shown in Fig. 5, and their combination is in accordance with the predictions of the standard model.

Thanks to the excellent performance of the LHC and the ATLAS detector during the second stage, the ATLAS Higgs boson studies go beyond discovery, to enter a new era of precision measurements to better understand this particle. The trip is just beginning!

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