ATLAS Observes Top Quarks in Lead-Lead Collisions - A Major Step Towards Measuring QGP Time Evolution
"The ATLAS experiment achieves a breakthrough by observing top quarks in lead-lead collisions, paving the way for studying the quark-gluon plasma's evolution over time."
Image credits: © ATLAS/CERN
On November 12, at the LHC TOP Working Group meeting at CERN, the ATLAS collaboration announced a groundbreaking achievement: the observation of top quarks in lead-lead collisions with a statistical significance of five sigma. This marks the first definitive observation of top-quark-pair production in heavy-ion collisions and opens new possibilities for studying the quark-gluon plasma (QGP), a state of matter believed to have existed moments after the Big Bang.
The QGP is a “deconfined” state where quarks and gluons, normally bound within protons and neutrons, flow freely as a nearly perfect fluid. This extreme state only exists for fleeting moments when created in high-energy nuclear collisions. Understanding the QGP provides insight into the early universe and the strong force, one of the four fundamental forces of nature.
Top quarks, the heaviest known elementary particles, play a pivotal role in these studies. Because they decay almost instantaneously, their interaction with the QGP occurs after a brief delay, offering a unique "time marker" to probe the plasma's evolution. This capability, proposed in 2018 by researchers from LIP’s Phenomenology Group, allows scientists to explore the QGP’s temporal dynamics for the first time.
The ATLAS team achieved this result using advanced techniques, including precise lepton reconstruction and innovative simulations, applied to data collected during Run 2 of the Large Hadron Collider. They observed top-quark pairs in the "dilepton channel," where decay products include electrons or muons. With a 35% uncertainty, the measured production rate lays a foundation for future studies, which will benefit from new data collected in the ongoing Run 3.
This breakthrough was complemented by a presentation from LIP researcher Guilherme Milhano, who reviewed the time-differential measurement proposals from LIP’s Phenomenology Group. This underscores LIP’s key role in bridging theory and experiment to tackle fundamental questions in particle physics.
The discovery not only advances our understanding of the QGP but also strengthens the connection between experimental and theoretical research, exemplifying the collaborative spirit that defines modern physics.
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