Proton Collisions,Event Displays,Physics,ATLAS
Figure 1: A spectacular candidate ttZ production event display with four leptons. Three reconstructed muons are represented by the red lines. Two of those were identified as having originated from the decay of a Z boson. Two reconstructed jets were identified as originating from b-quarks. A reconstructed electron is represented by the green track together with calorimeter energy deposits. The missing transverse momentum, originating from neutrinos from a semileptonic top decay escaping the detector, is represented by the dotted white line. (Image: ATLAS Collaboration/CERN)

In the contest for the heaviest known elementary particle, the top quark and Z boson rank first and third, respectively. When a proton–proton collision produces a top-quark pair together with a Z boson – a process known as ttZ production – their total mass can reach an impressive 440 GeV! The discovery of this highly energetic process thus required the record collision energy and rate of the LHC; no previous collider could come close. 

The complexity of ttZ production has made it a wonderful benchmark test for the Standard Model. It allows physicists to directly probe the electroweak interaction between the Z boson and the top quark, sharing complementarity of information with the related top-pair production with the Higgs boson. The Standard Model predicts the strength of this interaction and thus how often ttZ production should occur in the LHC, and it predicts the energies and relative emission directions of the top quarks and the Z boson. As new physics phenomena may modify some (or all) of these properties, measurements of ttZ production could give profound new insight to the current understanding of particle physics. 

Physicists have come a long way since the process’ discovery in 2014. The outstanding performance of the LHC and the ATLAS detector during Run 2 (2015-2018) provided a large dataset of ttZ events, from which researchers could explore the process’ dynamics in great detail. The results of their efforts were presented today at the International Conference on High-Energy Physics (ICHEP 2020).

The high mass and electroweak interactions occurring in ttZ production make it very rare, even at the LHC. ATLAS physicists studied the full dataset collected during Run 2 of the LHC for a precise measurement of its cross section. 

The new ATLAS result focuses on the cleanest signatures of this process, studying events with two leptons (electrons or muons) from the Z-boson decay and either one or two additional electrons or muons from the top-quark pair decay. To complete their event selection, physicists also looked for the presence of additional jets stemming from b-quarks created in a top-quark decay.

The resulting sample of events contained little contamination from other processes mimicking ttZ production. ATLAS physicists could thus take a precise new measurement of the ttZ production cross section, found to be σtt̄Z =1.05 ± 0.05(stat.) ± 0.09(syst.) pb, which is in agreement with Standard Model prediction. 

Plots or Distributions,Physics,ATLAS
Figure 2: Kinematic properties of the Z boson in ttZ events: (left) momentum transverse to the LHC proton beam and (right) rapidity, which is a variable related with the direction of emission with respect to the beam. The measurement results will serve to improve theoretical predictions, some of which are also shown in the plots. (Image: ATLAS Collaboration/CERN)

Further, the abundance of ttZ events allowed physicists to study the kinematic properties of this process. The probability of producing ttZ events was examined as a function of ten different kinematic variables, such as the energy and direction of the Z boson. These differential cross-section measurements were compared to the currently most precise theoretical predictions, and were found to be well reproduced (see Figure 2).    

The Standard Model continues to accurately predict these results: another celebration of the most predictive theory ever constructed by humanity. Although this latest analysis does not hint towards new phenomena, the next 15 years will see the LHC provide 20 times the number of collisions seen thus far. Such a yield will significantly increase ATLAS’ statistical precision and sensitivity. Will the Standard Model survive these future tests as well?