We present the first observation of the simultaneous production of four top quarks (tttt) at the LHC by both the ATLAS and CMS experiments. The analyses are performed using the full LHC Run 2 data with a centre-of-mass energy of √s = 13 TeV, with an integrated luminosity of 140 fb-1 from the ATLAS experiment and 137 fb-1 from the CMS experiment. The observation was achieved by re-analysing events with two leptons with the same electric charge and jets. Both collaborations explored improved lepton and b-jet identification algorithms, as well as re-optimised event selections. The ATLAS experiment adopted a new data-driven estimate for one of the main backgrounds, ttW, a novel powerful multivariate discriminant based on a graph neural network, and a set of improved systematic uncertainties.
The measured tttt production cross sections are: 22.5 +4.7-4.3 (stat) +4.6-3.4 (syst) fb from the ATLAS experiment and 17.9 +3.7-3.5 (stat) +2.4-2.1 (syst) fb from the CMS experiment, both are consistent with the SM prediction within 2 standard deviations. The observed (expected) significance with respect to the SM background-only hypothesis from the two experiments are: 6.1 (4.3) standard deviations for the ATLAS and 5.5 (4.9) standard deviations for the CMS. The results from the ATLAS experiment are re-interpreted to obtain limits on the cross section of three-top-quark production, as well as various BSM scenarios within the effective field theory framework and a CP violating top-Higgs Yukawa coupling.
Didar Dobur is a CMS experimental physicist. After a PhD in ZEUS experiment at HERA, she fully enjoyed early years of LHC data searching for Supersymmetry. In 2013 she moved to a Belgian university with a Marie Curie fellowship after which she became a professor at Ghent university, where she now leads a research team in CMS. Her research interest remains to be driven by searching for new physics either throuh direct searches (heavy neutral leptons, dark matter, supersymmetry) or through precision measurement in top quark sector (tt+W,Z, tZq, 4-tops…).
Quake Qin is a post-doctoral researcher at the University of Manchester. He is currently leading the Heavy Quarks and Top (HQT) working group of the ATLAS Collaboration. He obtained his PhD degree in 2016 from the University of Manchester. He has been dedicated to measurements and new physics searches in the top quark and Higgs boson sectors as well as their intersection. As a post-doctoral researcher, he has led several new physics searches, including heavy charged Higgs boson, heavy scalar/pseudo-scalar/vector bosons in top-quark final states. His recent publication established the first observation of the simultaneous production of four top quarks at the LHC.