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How heavy quarks behave in the quark-gluon plasma

When two lead ions collide within the Large Hadron Collider (LHC), they generate an intensely hot and dense state of matter known as quark–gluon plasma. This exotic phase, believed to have filled the universe shortly after the , emerges as quarks and gluons exist without confinement within composite particles like hadrons. As this quark–gluon plasma expands and cools rapidly, the liberated quarks and gluons transform back into hadrons, which then emanate from the collision site toward detectors.

In instances where the lead ions don't collide head-on, the resulting overlap region assumes an elliptic shape, imprinting distinct characteristics on the flow of hadrons. The measurement of this elliptic flow serves as a potent method for studying quark–gluon plasma. Recently, the ALICE collaboration shared a new assessment of the elliptic flow in hadrons containing heavy quarks, offering profound insights into the plasma.

In contrast to gluons and light quarks that constitute the majority of the quark–gluon plasma, heavy charm and beauty quarks emerge in the initial collision stages, preceding plasma formation. These heavy quarks engage with the plasma throughout its —expansion, cooling, and transformation into hadrons.

Given multiple interactions with the plasma's constituents, heavy quarks attain thermal equilibrium with the medium inversely proportional to the quark's mass. Charm quarks, being lighter than beauty quarks, are expected to experience a shorter thermalization time and a higher degree of thermalization.

Upon thermalization, charm quarks combine with the medium's light quarks to form D mesons, while beauty quarks form B mesons. Previous findings indicate that the elliptic flow of “prompt” D mesons—generated immediately after collisions—is nearly as robust as that of the lightest hadrons, pions. Predictions suggest a weaker elliptic flow for B mesons due to the longer thermalization time for beauty quarks.

In their recent analysis of non-head-on lead–lead collisions during Run 2 of the LHC, the ALICE collaboration measured the elliptic flow of B mesons by assessing the flow of “non-prompt” D mesons resulting from B meson decays. Utilizing a machine-learning technique, the team distinguished non-prompt D mesons from prompt ones, while suppressing background processes mimicking D meson production and decay.

The outcome reveals that the elliptic flow of non-prompt D mesons is indeed weaker than that of prompt counterparts, aligning with expectations. This result provides fresh insights into the thermalization of beauty quarks within the quark–gluon plasma, paving the way for future ALICE measurements based on data from Run 3 of the LHC.

With a data sample from 2023 featuring 40 times more collisions than ALICE's prior heavy-ion data-taking periods, researchers anticipate delving deeper into the dynamics of charm and beauty particles in the quark–gluon plasma, unraveling more about their behaviors and interactions.

Source: CERN

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