At a talk held at CERN this week, the ATLAS collaboration at the Large Hadron Collider (LHC) reported observing top quarks in collisions between lead ions, marking the first observation of this process in interactions between atomic nuclei. This observation represents a significant step forward in heavy-ion collision physics, paving the way for new measurements of the quark-gluon plasma (QGP) that is created in these collisions and delivering fresh insights into the nature of the strong force that binds protons, neutrons and other composite particles together.
In QGP, the fundamental components of protons and neutrons - quarks (matter particles) and gluons (strong force carriers) - are not bound within particles, but instead exist in a "deconfined" state of matter, forming an almost perfect dense fluid. Scientists believe that QGP filled the Universe briefly after the Big Bang and its study offers a glimpse into the conditions of that early epoch in the history of our Universe. However, QGP's extremely short lifetime when created in heavy-ion collisions - around 10−23seconds - means it cannot be observed directly. Instead, physicists study particles that are produced in these collisions and pass through the QGP, using them as probes of QGP's properties.
The top quark, in particular, is a very promising probe of QGP's evolution over time. As the heaviest known elementary particle, the top quark decays into other particles an order of magnitude faster than the time needed to form QGP. The delay between the collision and the top quark's decay products interacting with the QGP could serve as a "time marker", offering a unique opportunity to study the QGP's temporal dynamics. Additionally, physicists could extract new information on nuclear parton distribution functions, which describe how the momentum of a nucleon (proton or neutron) is distributed among its constituent quarks and gluons.
In their new result, ATLAS physicists studied collisions of lead ions that took place at a collision energy of 5.02 teraelectronvolts (TeV) per nucleon pair during Run 2 of the LHC. They observed top-quark production in the "dilepton channel", where the top quarks decay into a bottom quark and a W boson, which subsequently decays into either an electron or a muon and an associated neutrino. The result has a statistical significance of 5.0 standard deviations, making it the first observation of top-quark-pair production in nucleus-nucleus collisions. The CMS collaboration had previously reported evidence of this process in lead-lead collisions.
The observation was made possible by the ATLAS experiment's precise lepton reconstruction capabilities, coupled with a few other elements. These include the high statistics of the full Run-2 lead-lead data set, data-driven estimations of background processes that could mimic the signal, new simulations of top-quark events and dedicated jet calibration methods. Notably, the analysis does not rely on techniques that "tag" the jet originating from the bottom quark. This opens the possibility for the analysis to be used for the notoriously difficult bottom-tagging calibration in heavy-ion collisions, which would improve future measurements of the top quarks produced during these collisions.
ATLAS physicists measured the top-quark-pair production rate, or "cross section", with a relative uncertainty of 35%. The total uncertainty is primarily driven by the data set size, meaning that new heavy-ion data from the ongoing Run 3 will enhance the precision of the measurement.
The new ATLAS result opens a window into the study of QGP. In future studies, ATLAS scientists will also consider the "semi-leptonic" decay channel of top-quark pairs in heavy-ion collisions, which may allow them to get a first glimpse of the evolution of QGP over time.
