When two lead ions collide at the Large Hadron Collider (LHC), a remarkable phenomenon occurs. A hot and dense state of matter known as quark-gluon plasma is formed, where quarks and gluons are no longer confined within composite particles called hadrons. This unique fireball of particles, believed to have existed moments after the Big Bang, quickly expands and cools down. As a result, the quarks and gluons recombine to form hadrons, which are then detected by particle detectors. The shape of the overlap region between the ions, which occurs when they do not collide head-on, leaves a distinctive imprint on the flow of these hadrons. The study of this elliptic flow provides valuable insights into the properties of quark-gluon plasma.
The ALICE collaboration recently published a groundbreaking paper on the elliptic flow of heavy hadrons containing charm and beauty quarks. Unlike the majority of quarks and gluons in the quark-gluon plasma created during heavy-ion collisions, charm and beauty quarks are produced at the initial stages of these collisions. This means that they interact with the plasma throughout its evolution, offering a unique opportunity for study. The thermalization time, the time it takes for the quarks to equilibrate with the plasma, is inversely proportional to the quark’s mass. Consequently, it is expected that charm quarks will reach thermal equilibrium more rapidly than beauty quarks, due to their lower mass.
Once charm and beauty quarks thermalize with the plasma, they combine with the light quarks present in the medium to form D and B mesons, respectively. Prompt D mesons, which are produced immediately after the collisions, have been extensively studied and have shown an elliptic flow almost as strong as that of lighter hadrons such as pions. However, because beauty quarks take longer to thermalize than charm quarks, it is predicted that the elliptic flow of B mesons will be weaker than that of prompt D mesons.
In their recent analysis of non-head-on lead-lead collisions during Run 2 of the LHC, the ALICE collaboration was able to measure the elliptic flow of B mesons indirectly. They achieved this by studying the flow of non-prompt D mesons, which are produced in the decays of B mesons. To disentangle the signal from the prompt D mesons and suppress background particle processes, a machine-learning technique was employed. The results of this analysis confirmed the theoretical predictions, showing that the elliptic flow of non-prompt D mesons is indeed weaker than that of their prompt counterparts.
This new measurement of the elliptic flow of heavy hadrons provides valuable insights into the thermalization process of beauty quarks within the quark-gluon plasma. Furthermore, it paves the way for future investigations based on data from Run 3 of the LHC. With a significantly larger sample of lead-lead collisions, this new dataset will allow for even more detailed studies of the flow of charm and beauty particles, shedding further light on their dynamics within the quark-gluon plasma.
The study of heavy hadrons and their elliptic flow offers a window into the inner workings of quark-gluon plasma. By deciphering the behavior of charm and beauty quarks within this unique state of matter, scientists can deepen their understanding of the fundamental forces that govern our universe. The recent measurement of the elliptic flow of heavy hadrons by the ALICE collaboration marks an important milestone in the exploration of quark-gluon plasma and sets the stage for further discoveries in the years to come.
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