18. November 2024

First Observation of Top Quarks in Heavy-Ion collisions First Observation of Top Quarks in Heavy-Ion collisions

CERN result opens a new window into the study of the early Universe

The ATLAS collaboration at the Large Hadron Collider (LHC) reported the first observation of top quarks in collisions between lead ions in a talk held at CERN last week. Members of the research group of Prof. Dr. Matthias Schott from the Physikalisches Institut at the University of Bonn have been contributing to this new study. The observation of top-quark pairs represents a significant step forward in heavy-ion collision physics, paving the way for new measurements of the quark–gluon plasma 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.

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: ATLAS/CERN

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 high -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.