ATLAS finds further confirmation of evidence for four top quark process
24 March 2021 | By
In a new result released this week, the ATLAS Collaboration studied the production of four top quarks at once in LHC collisions. This is the heaviest particle final state ever seen at the LHC, and provides physicists with a unique opportunity to study the top quark’s relationship to the Higgs boson. Its production rate could be affected by various new theories that go beyond the Standard Model, providing a unique window to search for new physics.
When four top quarks decay in the ATLAS detector, they can lead to some very complicated collision events! The four top quarks produce four W bosons and four jets – collimated sprays of particles – originating from bottom quarks. The W bosons then, in turn, each decay into two jets or one charged lepton (electron, muon or tau leptons) and an invisible neutrino. This leads physicists to search for 12 distinct particle signatures in the detector, with up to twelve jets produced in a single event.
For this result, ATLAS physicists studied the full LHC Run-2 dataset and focused on four-top-quark decays producing one charged lepton or two leptons with opposite electric charge. Despite accounting for the lion’s share of four-top-quark events (about 57%), these signatures are easily overshadowed by other, much more-common Standard Model processes with similar decay products.
Top quarks can be produced in association with additional jets, which is the main source of background for this analysis. This is made particularly challenging when the additional jets are initiated by b-quarks, a process which is still not well understood. To account for this difficult background, ATLAS physicists used simulations based on the best available theoretical predictions. These were then corrected using real data events with fewer jets from b-quarks, avoiding any contamination from four-top-quark events. They then extrapolated this correction to events with more jets from b-quarks. The result: a precise evaluation of the background and a reduced uncertainty on its contribution to the final result.
To best distinguish between four-top-quark and background events, ATLAS physicists used a multivariate discriminant called a Boosted Decision Tree (BDT). The BDT was trained on several distinct features of the signal, such as the large amount of transverse momentum of the many decay products, the kinematic properties of jets initiated by b-quarks, and the magnitude of the missing transverse momentum from the invisible neutrinos. The BDT ultimately gave each event a relative score between minus one and plus one, indicating the likelihood that an event is a four-top-quark decay. A four-top-quark signal would ideally peak at one, as shown in Figure 1.
ATLAS has found evidence of the four-top-quark process with an observed significance of 4.7 standard deviations – just shy of the conventional requirement of 5 standard deviations to claim an observation.
ATLAS’ new measurement of four-top-quark production (shown in Figure 2) was combined with a previous measurement that looked at events with multiple leptons with the same dataset. The combined four-top-quark production cross section is measured to be 25 +7-6 fb, which is consistent with the Standard Model prediction of 12.0 ± 2.4 fb within 2.0 standard deviations. The existence of the four-top-quark process is therefore favoured with an observed significance of 4.7 standard deviations. This provides stronger evidence for this process than the 2.6 standard deviations that had been expected and is just shy of the conventional requirement of 5 standard deviations to claim an observation.
The study of four-top-quark production will certainly benefit from further data from the LHC’s upcoming operation. However, the biggest improvements to the measurement’s precision will come from new developments in the theoretical understanding of the background processes and more sophisticated methods for signal separation. An approach that incorporates all of these improvements will allow scientists to explore this exciting process in even greater detail in the future and verify its compatibility with the Standard Model.
- Measurement of the tttt production cross section in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2021-013)
- Evidence for tttt production in the multilepton final state in proton–proton collisions 13 TeV with the ATLAS detector (Eur. Phys. J. C 80 (2020) 1085, arXiv: 2007.14858, see figures)
- Moriond EW presentation by Anil Sonay: Measurement of the 4-top quark production in 13 TeV pp collisions with the ATLAS experiment
- CMS Collaboration: Search for the Production of Four Top Quarks in the Single-Lepton and Opposite-Sign Dilepton Final States in Proton-Proton Collisions at 13 TeV, (arXiv: 1906.02805)
- ATLAS finds evidence of spectacular four-top quark production, Physics Briefing, 26th May 2020
- Summary of ATLAS results from Moriond 2021, ATLAS News, March 2021
- See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.