Top Quarks in Search of Connection
New results from the ATLAS and CMS experiments at the Large Hadron Collider (LHC) show a signal that points to a short-lived, quasi-bound state consisting of a top quark and a top antiquark. This is a breakthrough that was long considered experimentally unattainable. It was made possible by precise analyses of the extensive data sets from both experiments. Research groups from DESY and the University of Hamburg were among those involved in the CMS measurement, which already provided initial indications in October. German research groups from DESY and the universities of Dortmund, Siegen, and Wuppertal played a central role in the current ATLAS measurement from July. The discovery marks the beginning of a new field of research into quasi-bound states of the heaviest elementary particle.
The top quark is known as an extremely short-lived particle, which decays almost instantaneously due to its high mass. Unlike its lighter sibling particles, such as the charm or bottom quarks, it cannot form long-term bonds.
It is produced in proton-proton collisions at the Large Hadron Collider at CERN and can be studied in more detail via its decay products. There, it very often occurs in pairs together with the anti-top quark. If the energy in the collision is low, the particles are almost at rest after production. This energy range is also called the “production threshold.” In this situation, the two quarks can briefly exchange gluons and thus exist in a quasi-bound state. However, this state is also short-lived and ends when the top quarks decaying one after the other. The mass of the state is slightly less than twice the mass of the top quark.
Until recently, it was assumed that experimental verification of this process was not possible because only a small fraction of top quark pairs are produced near the production threshold and the signal is difficult to identify in the data. The large data set recorded by the ATLAS and CMS experiments between 2015 and 2018, combined with sophisticated experimental methods, has now made the discovery possible. The ATLAS measurement was carried out with strong participation from German research groups at DESY and the universities of Dortmund, Siegen, and Wuppertal. The result was first presented in July at the EPS conference in Marseille and has a significance of 7.7 standard deviations.
However, this discovery marks only the beginning of a new field of research that will deal with this completely new state of the heaviest known elementary particle: In order to understand exactly how top quarks communicate with each other, further measurements will be carried out and theoretical predictions will be further developed. This discovery is a prime example of how close collaboration between theory and experiment can advance research beyond the expected possibilities.
Further links:
ATLAS-measurement: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2025-008/
CMS-measurement: https://arxiv.org/abs/2503.22382
ATLAS-briefing: https://atlas.cern/Updates/Briefing/Quasi-Bound-Tops
Press release CERN: https://home.cern/news/press-release/physics/elusive-romance-top-quark-pairs-observed-lhc