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Researchers involved in the ATLAS experiment at the Large Hadron Collider (LHC) have made a groundbreaking discovery by reporting the generation of top quark–antiquark pairs from collisions of heavy nuclei. These experiments, which involve colliding lead ions, recreate a transient state of matter known as quark–gluon plasma. This state is characterized by extreme temperatures and densities that result in a mixture of free quarks and gluons, reminiscent of the conditions present in the universe just moments after the Big Bang.
“Through heavy-ion collisions at the LHC, we are able to replicate the quark–gluon plasma in a controlled laboratory environment,” explains Anthony Badea, a postdoctoral researcher at the University of Chicago and a lead author of a recent study detailing these findings. This research not only enhances our grasp of the early universe but also poses significant implications for understanding quantum chromodynamics (QCD), the fundamental theory that describes the interactions between quarks and gluons.
While the quark–gluon plasma exists for an exceedingly brief period—approximately 10-23 seconds—scientists can examine its properties by studying how various particles produced through these collisions move within it. The top quark, the heaviest of all known elementary particles, serves as an invaluable tool for this exploration due to its short lifespan and distinctive decay patterns, which occur before the plasma dissipates.
“The top quark decays into lighter particles that undergo further decay processes,” notes Stefano Forte from the University of Milan, who was not part of the study. “The timing of these subsequent decays is altered when they happen within the quark–gluon plasma, making them a potential avenue to investigate its structure. The initial step to achieve this is determining how many top quarks are actually produced, which is the focus of this ATLAS research.”
Initial Discoveries
The ATLAS collaboration analyzed data stemming from lead–lead collisions to identify instances where pairs of top quark and antiquark were generated. Their investigation targeted a less common but clearer decay pattern referred to as the di-lepton channel. In this decay, each top quark transitions into a bottom quark and a W boson, the latter of which further transforms into a detectable lepton and a neutrino that remains invisible.
The findings not only confirmed the existence of top quarks produced in this complex environment but also demonstrated that the frequency of their production aligns with theoretical models based on current understandings of the strong nuclear force.
“This is a significant advancement,” states Juan Rojo, a theoretical physicist from the Free University of Amsterdam who was not involved in the study. “For years, we have investigated top quark production in simpler proton-proton collisions. This represents the first observation of such heavy particles generated within the intricate setting of colliding lead nuclei.”
Beyond validating QCD predictions regarding heavy-quark production in heavy-nuclei interactions, Rojo emphasizes that “this provides a novel method to probe the structure of the quark–gluon plasma.” Future research could shed light on peculiar effects, such as the differences between gluons within a heavy nucleus and those found in protons.
Important Milestone
“This is a critical initial step, but additional studies will necessitate a larger quantity of top quark events to investigate more nuanced effects,” Rojo adds.
Current observations of top quark production align closely with theoretical expectations. Future investigations could refine the understanding of quark and gluon behavior inside atomic nuclei, ultimately offering physicists the means to not only confirm existing theoretical frameworks but also to uncover new aspects of the quark–gluon plasma.
Rojo anticipates that upcoming experiments, particularly during the impending high-luminosity run of the LHC, could facilitate a deeper understanding of the quark–gluon plasma’s temporal structure, with improved measurements enhancing this exploration.
Badea concurs that the initial findings by ATLAS pave the way for extensive future research. “As we gather more data from nuclear collisions and enhance our comprehension of top quark dynamics in proton collisions, exciting prospects will emerge,” he asserts.
The detailed research findings were published in Physical Review Letters.
Source
physicsworld.com