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.
Display of a lead-lead collision at 5.02 TeV per nucleon pair that resulted in a candidate pair of top quarks that decay into other particles. The event contains four particle jets (yellow cones), one electron (green line) and one muon (red line). The inlay shows an axial view of the event. Image credit: ATLAS / CERN.
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.”
Source : Breaking Science News