The Large Hadron Collider (LHC) is like an immensely powerful kitchen, designed to cook up some of the rarest and hottest recipes in the Universe, like the quark–gluon plasma (QGP), a state of matter known to have existed shortly after the Big Bang. While the LHC mostly collides protons, once a year it collides heavy ions – such as lead nuclei – a key ingredient for preparing this primordial soup. In the 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. Scientists believe the quark–gluon plasma filled the Universe briefly after the Big Bang, and its study offers a glimpse into the conditions of that early epoch of our Universe’s history.
However, the extremely short lifetime of the quark–gluon plasma, that is produced in heavy-ion collisions – around 10−23 seconds – means it cannot be observed directly. Instead, physicists study particles produced in heavy-ion collisions that pass through the QGP, using them as probes of the QGP’s properties. The top quark plays here a very special role: "The top quark decays faster into a W boson and a bottom quark than the time needed to form the quark-gluon plasma. However, the decay products of the W boson start to interact with the plasma only at a later stage", explains Matthias Schott, "This allows us to use the top quark as a sort of time marker, which gives us a unique opportunity to study the time evolution of the quark-gluon plasma for the first time".
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 from 2015 to 2018. 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 observation was made possible by ATLAS' 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.
"We are especially proud of the fact, that we do not rely on the identification of bottom-quark in those events. This allows us to use our analysis in the future for the notoriously difficult bottom-tagging calibration in heavy-ion collisions, which is necessary for the future measurements of the time evolution of the quark-gluon plasma", as Matthias Schott explains.
ATLAS physicists measured the top-quark-pair production rate, or “cross section”, with a relative uncertainty of 31%. The total uncertainty is primarily driven by the data-set size, meaning that new heavy-ion data from ongoing Run 3 will enhance the precision. The ATLAS researchers also compared their new measurement with theoretical predictions and previous measurement of the CMS experiment, where a good agreement was found.
Schott again: "A significant part of this important work was performed by Patrycja Potepa within her PhD thesis, and we are already preparing future measurements focussing on the decay of a top quark into two other quarks with new PhD and master students of the University of Bonn. This allows us to get a first glimpse at the time evolution of the QGP and understand fundamental properties of the early Universe."