This week, in a groundbreaking presentation at CERN, the ATLAS collaboration at the Large Hadron Collider (LHC) announced the first-ever observation of top quarks in lead-ion collisions. This achievement marks a major milestone in the study of heavy-ion physics and holds significant implications for our understanding of the quark–gluon plasma (QGP) — a unique state of matter believed to have filled the universe moments after the Big Bang.
Heavy-ion collisions, such as those between lead ions, are conducted at extremely high energies to recreate conditions similar to those of the early universe. In these collisions, the protons and neutrons within the colliding nuclei can momentarily break down into their constituent quarks and gluons. Under such extreme conditions, these fundamental particles exist in a “deconfined” state, forming what is known as QGP, an almost perfect, dense fluid. Studying QGP allows scientists to probe the behavior of the strong force, one of the four fundamental forces of nature responsible for holding atomic nuclei together.
The QGP’s extremely short-lived nature — lasting only about (10^{-23}) seconds — makes it impossible to observe directly. Therefore, physicists rely on indirect probes to study its properties. The particles generated during these collisions interact with the QGP, allowing researchers to glean insights into its behavior. Among these particles, the top quark has emerged as a particularly valuable probe due to its unique characteristics.
The top quark is the heaviest known elementary particle, with a mass significantly higher than that of any other quark. This immense mass leads to a remarkably short lifetime, decaying into other particles almost instantaneously, far quicker than the time it takes for the QGP to form. As a result, the decay products of top quarks have the potential to interact with the QGP after its formation, effectively serving as “time markers” that can offer a glimpse into the early stages of QGP evolution. This property positions the top quark as an exceptional tool for exploring the temporal dynamics of QGP in unprecedented ways.
The ATLAS experiment’s recent observations were conducted during Run 2 of the LHC, where lead-ion collisions were studied at an energy level of 5.02 teraelectronvolts (TeV) per nucleon pair. The researchers focused on detecting top quark pairs produced in these collisions. To identify these events, the team analyzed the “dilepton channel,” where each top quark decays into a bottom quark and a W boson, with the W boson further decaying into either an electron or a muon, accompanied by a neutrino. This specific decay channel was chosen because it is relatively clean and less prone to background noise, making it ideal for precise measurements.
The ATLAS collaboration reported a statistical significance of 5.0 standard deviations, which is the threshold for claiming a discovery in particle physics. This means that the likelihood of the observed signal being a mere statistical fluctuation is extremely low, confirming the presence of top-quark production in these heavy-ion collisions. Previously, the CMS collaboration had found evidence of this process in similar lead–lead collisions, but the ATLAS experiment’s findings are the first to reach the stringent standard required for a formal observation.
A key factor behind this successful detection was the ATLAS detector’s sophisticated lepton reconstruction capabilities. The team also leveraged the high statistics from the complete Run-2 lead–lead data set, along with new simulations and refined techniques for calibrating jets — streams of particles resulting from quark and gluon interactions. Notably, the analysis was conducted without relying on “bottom-tagging” methods typically used to identify jets originating from bottom quarks. This innovative approach not only simplifies the analysis but also paves the way for improved bottom-quark tagging techniques in future heavy-ion studies.
One of the most intriguing outcomes of the study was the measurement of the top-quark pair production rate, known as the “cross section.” The ATLAS team determined this rate with a relative uncertainty of 35%. The primary limitation on precision was the size of the available data set, suggesting that the ongoing data collection in Run 3 of the LHC could significantly reduce these uncertainties. With more data, researchers anticipate achieving higher precision, which would allow for even deeper exploration of QGP properties.
This discovery is more than just a new entry in the annals of particle physics; it opens up a whole new dimension for studying the quark–gluon plasma. The ATLAS collaboration’s future research plans include exploring the “semi-leptonic” decay channel of top-quark pairs, where only one W boson decays into a lepton while the other decays into quarks. This channel could provide an even richer dataset, allowing scientists to track how the QGP evolves over time and to extract more detailed information on its properties.
Additionally, the results have the potential to enhance our understanding of nuclear parton distribution functions (nPDFs), which describe how the momentum of protons and neutrons is distributed among their constituent quarks and gluons within nuclei. By studying how top quarks are produced in nuclear collisions, physicists can refine these distribution models, leading to more accurate descriptions of the strong force.
Overall, the observation of top-quark production in lead-ion collisions represents a pivotal achievement in particle physics. It not only deepens our understanding of the early universe but also expands the toolkit available to researchers studying the fundamental forces that govern the cosmos. With ongoing experiments and the prospect of more precise data from the LHC, the exploration of QGP and its interactions with top quarks promises to yield even more exciting discoveries in the years to come.
Source: CERN