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An unforeseen feature in proton-proton collisions previously observed by the CMS experiment at CERN's Large Hadron Collider (LHC) has now been confirmed by its sister experiment ATLAS.

The result, yesterday at the European Â鶹ÒùÔºical Society's High-Energy Â鶹ÒùÔºics in Marseille, suggests that top quarks—the heaviest and shortest-lived of all the elementary particles—can momentarily pair up with their antimatter counterparts to produce a "quasi-bound-state" called toponium. Further input based on complex theoretical calculations of the strong nuclear force—called quantum chromodynamics (QCD)—will enable physicists to understand the true nature of this elusive dance.

High-energy collisions between protons at the LHC routinely produce top quark–antiquark pairs. Measuring the probability, or cross section, of this process is both an important test of the Standard Model of particle physics and a powerful way to search for the existence of new particles that are not described by the theory.

Last year, CMS researchers were analyzing a large sample of top quark–antiquark production data collected from 2016 to 2018 to search for new types of Higgs bosons when they observed something unusual. The team saw a surplus of top quark–antiquark pairs, which is often considered as a smoking gun for the presence of new particles.

Intriguingly, the excess appeared at the very minimum energy required to produce such a pair of top quarks. This led the team to consider an alternative hypothesis of something that had long been considered too difficult to detect at the LHC: a short-lived union of a top quark and a top antiquark.

The top quark is typically a loner. While other quarks can get together to form bound states called hadrons, the top quark's extremely short lifetime means that it typically decays almost instantly—disappearing before it can form a bound state.

But quantum mechanics makes it possible for the top quark-antiquark pair to occasionally survive long enough that, if produced almost at rest with respect to each other, they can exchange gluons (messengers of the strong force) that bind them into the toponium state.

Basing itself on a simplified toponium production hypothesis, CMS measured the cross section for the top quark–antiquark excess to be 8.8 picobarns (pb) with an uncertainty of about 1.3 pb. This passed the "five sigma" level of certainty required to claim a discovery in particle physics and made it extremely unlikely that the excess over the background-only prediction is just a statistical fluctuation.

"The observation of a non-relativistic QCD effect that was thought to be too difficult to detect is a great triumph for the LHC experiment program," said CMS spokesperson Gautier Hamel de Monchenault. "We keenly anticipate further rich interactions with our theory colleagues so that we may learn more about this fascinating corner of the Standard Model."

In examining the full LHC Run-2 dataset collected from 2015 to 2018, the ATLAS collaboration has now seen the same effect. The ATLAS data rejects models that ignore the formation of a quasi-bound-state with a significance of 7.7 sigma and determines the production cross section of the top quark-antiquark excess to be 9.0± 1.3 pb, in close agreement with CMS.

While there is no doubt that an unforeseen phenomenon is present in the LHC data, the challenge is to be certain of its underlying cause. An alternative or additional possibility to the formation of toponium could be, for example, the existence of a new particle with a mass close to twice that of the top quark which is produced in collisions between gluons and decays to a top quark-antiquark pair.

The conclusive interpretation of this new phenomenon will rely on accurate modeling of how quarks and gluons behave in the complex environment of high-energy proton-proton collisions, involving state-of-the art QCD calculations.

"For a long time, it was considered experimentally unfeasible to measure this subtle effect at the LHC, since events close to the production threshold make up only a small fraction of the top-pairs produced and are difficult to spot in the data," said ATLAS spokesperson Stéphane Willocq.

"However, thanks to the wealth of proton-proton data recorded during Run 2 of the LHC and thanks to advances in analysis techniques, this long-held assumption is now being overturned."

If the toponium hypothesis is confirmed, its discovery would add a new twist to the story of quarkonia– quarkonium is a term for unstable states formed from pairings of heavy quarks and antiquarks of the same flavor. Charmonium (charm–anticharm) was discovered in 1974, sparking the "November Revolution" in particle physics, and bottomonium (bottom–antibottom) was discovered three years later, both at laboratories in the United States.

"These impressive results from ATLAS and CMS prove that there is still much to learn about the Standard Model of Particle Â鶹ÒùÔºics at high energies," said CERN Director of Research and Computing, Joachim Mnich. "They show that high-precision measurements, many of which were never thought possible at a hadron collider, can reveal remarkably subtle phenomena that deepen our understanding of nature."

With the ongoing Run 3 of the LHC due to deliver significantly more data, the ATLAS and CMS collaborations are set to deepen the exploration of the strong force via top quark-antiquark interactions in the non-relativistic regime.

The was accepted for publication in Reports on Progress in Â鶹ÒùÔºics.