CERN Physicists Observe Top-Quark Pairs in Lead-Lead Collisions

Top-quark pair production has been observed in lead-lead collisions at CERN’s Large Hadron Collider (LHC) with the ATLAS detector, marking the first observation of this process in interactions between atomic nuclei.

In quark-gluon plasma, 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.

Physicists believe that quark-gluon plasma 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, the quark-gluon plasma’s extremely short lifetime when created in heavy-ion collisions — around 10^-23 seconds — means it cannot be observed directly.

Instead, physicists study particles that are produced in these collisions and pass through quark-gluon plasma, using them as probes of the plasma’s properties.

The top quark, in particular, is a very promising probe of the quark-gluon plasma’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 quark-gluon plasma.

The delay between the collision and the top quark’s decay products interacting with quark-gluon plasma could serve as a ‘time marker,’ offering a unique opportunity to study the plasma’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 the new study, physicists with the ATLAS Collaboration 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.

“We measured the top-quark-pair production rate, or cross section, with a relative uncertainty of 35%,” the physicists said.

“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 result opens a window into the study of quark-gluon plasma,” they added.

“In future studies, we 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 quark-gluon plasma over time.”