The ALICE Collaboration has made a groundbreaking discovery in the field of particle physics, shedding light on the formation of quark-gluon plasma (QGP) in proton-proton and proton-lead collisions. This finding challenges the long-held belief that small collision systems, such as protons, could not generate the extreme temperatures and pressures necessary for QGP creation.
In the early universe, shortly after the Big Bang, matter existed in a state known as QGP, characterized by extremely high temperatures and densities. Recreating this state requires high-energy collisions between heavy ions, such as lead nuclei. However, the ALICE Collaboration's research reveals a surprising pattern in proton-proton, proton-lead, and lead-lead collisions at the Large Hadron Collider (LHC).
One of the key signatures of QGP formation is anisotropic flow, where particles emitted in collisions are not evenly distributed but exhibit preferred directions. Interestingly, this flow pattern is more pronounced for particles composed of three quarks (baryons) compared to those made up of two quarks (mesons). The explanation lies in the process of quark coalescence, where quarks in the QGP combine to form larger particles, resulting in stronger flow for baryons.
The ALICE Collaboration's study measured the anisotropic flow of various meson and baryon species in proton-proton and proton-lead collisions. The results demonstrated that, similar to heavy-ion collisions, the flow pattern was more pronounced for baryons than mesons at intermediate momenta. This observation supports the hypothesis that an expanding system of quarks is present even in small collision systems.
Furthermore, the researchers compared their findings with simulations that assumed QGP formation and evolution. The models that incorporated the anisotropic flow of quarks and their subsequent coalescence into mesons and baryons successfully explained the observed flow pattern. However, the successful models still showed discrepancies with the data, primarily due to uncertainties in the modeling of proton substructure and initial collision geometry.
Looking ahead, the ALICE Collaboration anticipates gaining new insights into the nature and evolution of QGP across different collision systems with the upcoming oxygen collisions in 2025. This development bridges the gap between proton collisions and lead collisions, offering a more comprehensive understanding of QGP formation and its implications in various collision scenarios.
