Observation of four-top-quarks at the ATLAS experiment

The simultaneous production of four top-quarks in proton-proton collisions is a very interesting but rare Standard Model (SM) process (about 1 in 70 thousand top-quark pair events). It can occur either via pure strong interactions or through electroweak interaction of the SM Higgs boson with top quarks in the pair-produced top-quarks process. It probes the strong interactions at a large momentum scale (above 700 GeV) as well as provides direct constraints on the Higgs-top Yukawa coupling strength and structure.

ErUM-FSP ATLAS

Many particles are produced in the final state as a result of the prompt decay of each top-quark into a b-quark and a W boson.  Researchers at the ATLAS experiment analyzed the events from the full Run-2 dataset consisting of either two same-charged leptons or at least three leptons, in addition to six or more jets, of which at least two must be tagged as b-jets.  Modern machine learning techniques are applied using important kinematic variables, such as the sum of the scores representing the tagging efficiencies of each b-jet in the event, minimum angular distance among all possible pairs of leptons, and the sum of transverse momenta of leptons and jets, to improve a discriminating power for an otherwise very challenging signal separation.  Main backgrounds, such as the associated production of W boson with the top-quark pair, are carefully estimated using data.  The process has now been observed with a significance of 6.1 standard deviations compared to the background only prediction, with the consistency to the SM prediction at the level of 1.8 standard deviations (~30% uncertainty on the measured signal cross-section).  

Figure: (left) Distribution of the multivariate discriminant score obtained using Graph Neural Network (GNN) in the signal region. (right) Distribution of the sum of discriminant variable scores of four energetic b-jets in the signal region at high GNN score. The data are compared with total signal-plus-background predictions after the maximum-likelihood fit. In the left plot, dashed lines represent the total predictions before the maximum-likelihood fit. The red histogram shows the contribution from the measured four-top-quark signal after the fit.
Figure: (left) Distribution of the multivariate discriminant score obtained using Graph Neural Network (GNN) in the signal region. (right) Distribution of the sum of discriminant variable scores of four energetic b-jets in the signal region at high GNN score. The data are compared with total signal-plus-background predictions after the maximum-likelihood fit. In the left plot, dashed lines represent the total predictions before the maximum-likelihood fit. The red histogram shows the contribution from the measured four-top-quark signal after the fit. Foto © ATLAS Collaboration

 

Being the heaviest observed fundamental particle, the top quark can play an important role in electroweak symmetry breaking leading to also the possibility of large couplings to new heavy particles that may exist in nature. The existence of new heavy particles can alter the four-top-quark production rate substantially, therefore, a precise measurement of this process is desirable to get insight into potential underlying new physics.  The observation of the simultaneous production of four top quarks opens the door to the next stages of explorations for its deeper understanding and its connections to new physics.   

German institutes (Universtät Bonn, George-August-Universität Göttingen) have contributed to this observation in many ways, e.g improved calibration of physics objects, development of analysis strategies, and estimation of important backgrounds.       

Display of a candidate four-top-quark event containing seven jets (four of them are tagged as b-jets) visualised as cones; three of the top quarks produce leptons in their decay (two muons shown in red, and one electron, shown in blue with yellow energy deposits in the electromagnetic calorimeter), and fourth top quark decay to jets. The reconstructed tracks of the charged particles in the Inner Detector are shown as orange lines. The energy deposits in the calorimeters are shown as yellow boxes. The missing transverse momentum is shown by a dashed line.
Display of a candidate four-top-quark event containing seven jets (four of them are tagged as b-jets) visualised as cones; three of the top quarks produce leptons in their decay (two muons shown in red, and one electron, shown in blue with yellow energy deposits in the electromagnetic calorimeter), and fourth top quark decay to jets. The reconstructed tracks of the charged particles in the Inner Detector are shown as orange lines. The energy deposits in the calorimeters are shown as yellow boxes. The missing transverse momentum is shown by a dashed line. Foto © ATLAS Collaboration

 

More details:

Observation of four top-quark production in the multilepton final state with the ATLAS detector,  arXiv:2303.15061