A unique experiment at the Large Hadron Collider (LHC) has brought science closer to unraveling the mysteries of the early Universe. Physicists at the ATLAS Collaboration have for the first time recorded the formation of top-quark pairs in collisions of heavy lead nuclei. This discovery not only confirmed theoretical predictions, but also opened a new avenue for the study of quark-gluon plasma, a state of matter that existed in the first microseconds after the Big Bang.
“Soup” since the beginning of time
When lead nuclei collide at incredible speeds, the LHC creates extreme conditions where a quark-gluon plasma – a glowing and extremely dense “broth” of free quarks and gluons – emerges. It was this substance, according to scientists, that filled the universe in the first moments of its existence.
“Our experiment is a time machine that replicates in the laboratory processes that took place 13.8 billion years ago,” explains Anthony Badea of the University of Chicago, one of the study’s authors.
The study of plasma allows us not only to better understand the evolution of the cosmos, but also to test the foundations of quantum chromodynamics, a theory describing the interaction of quarks.
Massive witnesses of the past
Top quarks, the most massive known elementary particles, played a special role in the study. Their uniqueness lies in their extremely short life (about 10-²⁵ seconds) and distinct decay pattern. Since top quarks decay even before the quark-gluon plasma disappears, they become ideal probes to study its structure.
“The particle decays into lighter components, which in turn continue to decay. By analyzing the time delays between these processes, we can see how the plasma affects the particles,” comments Stefano Forte of the University of Milan.
First results
The ATLAS team analyzed millions of collisions, looking for rare top-quark birth events. The researchers focused on the so-called di-lepton channel, where each quark decays into a W boson (the carrier of the weak interaction) and a bottom quark. The W boson, in turn, transforms into a lepton (such as an electron) and a neutrino, which are detected by detectors.
The results showed: the number of top quarks formed is fully consistent with the predictions of modern physics. “This is the first step. We now know that we can generate top quarks in the complex environment of nuclear collisions. The next step is to study exactly how plasma affects their behavior,” notes Juan Rojo of the University of Amsterdam.
Future of research
Currently, the amount of data is still insufficient to analyze the more subtle effects in detail. However, in just a few years, after the LHC is upgraded to work at higher energies, scientists will be able to collect significantly more statistics. This will allow:
- Determine whether the properties of gluons in nuclei differ from those in free protons;
- Investigate the temporal dynamics of the quark-gluon plasma;
- Detect new phenomena in strong interaction.
This research not only expands the boundaries of our understanding of particle physics, but also reminds us: the answers to the biggest mysteries of the Universe are hidden in its smallest constituents.
We previously reported on how neutron stars can have quark nuclei.
According to physicsworld.com
