ATLAS experiment finds evidence of spectacular production of four top quark

The ATLAS Collaboration at CERN has announced strong evidence of the production of four major quarks. This rare Standard Model process is expected to occur only once for every 70,000 pairs of superior quarks created in the Large Hadron Collider (LHC) and has proven to be extremely difficult to measure.

The top quark is the most massive elementary particle in the Standard Model, registering 173 GeV, which is equivalent to the mass of a gold atom. But unlike gold, whose mass is mainly due to the nuclear bonding force, the top quark derives all of its mass from interaction with the Higgs field. So when four top quarks are produced in a single event, they create the heaviest particle end state ever seen at the LHC, with nearly 700 GeV in total. This is an ideal environment to search for new physics with still unknown particles that contribute to the process. If they exist, physicists will see an additional production of four superior quarks above what the Standard Model predicts, which will motivate a detailed study of the process.

In their new search for top four quark production, ATLAS physicists studied the full Run 2 dataset recorded between 2015 and 2018. When produced through proton-proton collisions at the LHC, this process leaves spectacular signatures in the ATLAS detector. The top four quarks produce four W bosons and four jets (collimated particle aerosols) that originate from the bottom quarks. The W bosons then, in turn, decompose into two jets or a charged lepton (electron leptons, muons, or tau) and an invisible neutrino. As a final step, tau leptons are broken down into a lighter lepton or jet, with additional neutrinos.

For this result, physicists chose to focus on collision events that produce two leptons with the same charge or three leptons. Despite accounting for only 12% of all top four quark decays, these signatures are easier to distinguish from background processes in the ATLAS detector. However, detecting a signal required a detailed understanding of the remaining background processes and the use of sophisticated separation techniques.

ATLAS physicists trained a multivariate discriminant (powered decision tree) using the distinctive characteristics of the signal, including high jet numbers, its quark flavor origin (bottom quark or not), and the angular energies and distributions of the measured particles. The main signal-like background processes come from the production of a pair of higher quarks in association with other particles, such as a W or Z boson, a Higgs boson, or another higher quark. Some of these processes have recently been observed by the ATLAS and CMS collaborations.

Each background process was evaluated individually, primarily through dedicated simulations that included information from the best available theoretical predictions. The most difficult background processes (production of the top quark pair with a W boson and bottoms with false leptons) had to be determined using data from dedicated control regions. False leptons arise when the charge of a lepton is misidentified, or when the leptons come from a different process, but are attributed to the signal. Both had to be well understood and accurately evaluated to reduce systematic uncertainty about the final result.

ATLAS measured the cross section for the production of four top quarks in 24 ⁺⁷_–6 fb, which is consistent with the prediction of the standard model (12 fb) at 1.7 standard deviations. The significance of the signal amounts to 4.3 standard deviations, for an expected significance of 2.4 the standard deviations were the signal of four quark-top equal to the prediction of the Standard Model. Measurement provides strong evidence of this process.

Additional data from the upcoming LHC run, along with further developments of the analysis techniques employed, will improve the accuracy of this challenging measurement.