Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector

Abstract: A search for the associated production of the Higgs boson with a top quark pair ($t\bar t H$) is reported. The search is performed in multilepton final states using a dataset corresponding to an integrated luminosity of 36.1 fb$^{-1}$ of proton--proton collision data recorded by the ATLAS experiment at a center-of-mass energy $\sqrt{s} = 13$ TeV at the Large Hadron Collider. Higgs boson decays to $WW^*$, $\tau\tau$, and $ZZ^*$ are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the $t\bar t H$ production cross section is $\sigma(t\bar t H) = 790^{+230}_{-210}$ fb, in agreement with the Standard Model prediction of $507^{+35}_{-50}$ fb. The combination of this result with other $t\bar t H$ searches from the ATLAS experiment using the Higgs boson decay modes to $b\bar b$, $\gamma\gamma$ and $ZZ^* \to 4\ell$, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the $t\bar t H$ production mode.

Overview of the ATLAS ttH Production Study

The research paper titled "Evidence for the associated production of the Higgs boson and a top quark pair with the ATLAS detector" details an empirical investigation into the associated production of the Higgs boson with a top quark pair, denoted as ttH. This study employs multilepton final states using data from proton-proton collision experiments conducted by the ATLAS collaboration at the Large Hadron Collider (LHC). Utilizing a dataset of 36.1 fb^-1, collected at a center-of-mass energy of 13 TeV, the study targets the Higgs boson decays to WW*, ττ, and ZZ*.

Methodology

The search encompasses seven distinct final states, methodically categorized according to the quantity and type of charged-lepton candidates. The goal is to detect anomalies indicative of the Standard Model Higgs boson with a mass approximating 125 GeV produced alongside top quarks. The researchers have applied a robust selection process to analyze these states, supplemented by multivariate analysis techniques in most channels to enhance signal discrimination.

Results

The findings reveal an excess of observed events when juxtaposed with the expected background from Standard Model processes. The signal reached an observability of 4.1 standard deviations, marking a notable deviation from the anticipated 2.8 standard deviations under the SM hypothesis. The best-fit value for the ttH production cross section was found to be 790^{+230}_{-210} fb, which aligns with the Standard Model's prediction of 507^{+35}_{-50} fb.

Combination Analysis

In addition to the standalone multilepton analysis, the paper reports on a combined assessment incorporating parallel ttH searches focusing on the Higgs boson decay modes to bb̄, γγ, and ZZ* → 4ℓ. This consolidated approach yields an elevated observed significance of 4.2 standard deviations, compared to an expected 3.8 standard deviations. The combined best-fit cross section measured was 590^{+160}_{-150} fb, once again consistent with Standard Model predictions.

Implications and Future Perspectives

The results obtained provide substantial evidence for the ttH production mode, aligning closely with theoretical predictions, thus fortifying confidence in the Standard Model. This research elucidates the ttH interaction, a crucial aspect of understanding the Higgs boson's role in electroweak symmetry breaking. These findings also pave the way for nuanced explorations into new physics beyond the Standard Model and enhance precision in measuring the Higgs boson's coupling to top quarks. Future experimental endeavors could further inspect these interactions under varying conditions, potentially uncovering subtle discrepancies indicative of novel phenomena.