Physicists from the ATLAS Collaboration CERN’s Large Hadron Collider have uncovered strong evidence of the production of four top quarks. This rare process is expected to occur only once for every 70,000 pairs of top quarks created at the Large Hadron Collider.
Event display of a candidate four-top-quark event, where two of the top quarks decay leptonically — one with a resulting muon (red) and one with an electron (green) — and two top quarks decay hadronically (green and yellow rectangles). The jets — b-tagged jets — are shown as yellow (blue) cones. Image credit: ATLAS Collaboration / CERN.
“The top quark is the most massive elementary particle in the Standard Model, clocking in at 173 GeV, which is equivalent to the mass of a gold atom,” the ATLAS physicists said.
“But contrary to gold, whose mass is mostly due to the nuclear binding force, the top quark gets all its mass from the interaction with the Higgs field.”
“So when four top quarks are produced in a single event, they create the heaviest particle final state ever seen at the Large Hadron Collider, with almost 700 GeV in total.”
In their search for four-top-quark production, the physicists studied data recorded between 2015 and 2018.
“When produced through proton-proton collisions at the Large Hadron Collider, this process leaves signatures in the ATLAS detector,” they explained.
“The four top quarks produce four W bosons and four jets originating from bottom quarks.”
“The W bosons then, in turn, each decay into two jets or one charged lepton and an invisible neutrino.”
“As a final step, the tau leptons decay into a lighter lepton or a jet, with additional neutrinos.”
For this result, the researchers chose to focus on collision events producing two leptons with the same charge or three leptons.
“We trained a multivariate discriminant using the distinct features of the signal, including the high numbers of jets, their quark-flavor origin, and the energies and angular distributions of the measured particles,” they said.
“The main background processes that resemble the signal stem from the production of a pair of top quarks in association with other particles, such as a W or Z boson, a Higgs boson, or another top quark.”
Each background process was individually evaluated, primarily through dedicated simulations which included information from the best available theoretical predictions.
The most difficult background processes — the top-quark-pair production with a W boson and backgrounds with fake leptons — had to be determined using data from dedicated control regions.
“Fake leptons arise when the charge of a lepton is misidentified, or when leptons come from a different process, but are attributed to the signal,” the scientists said.
“Both had to be well-understood and precisely evaluated in order to reduce the systematic uncertainty on the final result.”
“We measured the cross section for the production of four top quarks to be 24+7–6 fb, which is consistent with the Standard Model prediction (12 fb) at 1.7 standard deviations.”
“The signal significance amounts to 4.3 standard deviations, for an expected significance of 2.4 standard deviations were the four-top-quark signal equal to the Standard Model prediction. The measurement provides strong evidence for this process.”
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ATLAS Collaboration. 2020. Evidence for tt¯tt¯ production in the multilepton final state in proton-proton collisions at s√=13 TeV with the ATLAS detector. ATLAS-CONF-2020-013
This article is based on a press-release provided by CERN.
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