CERN’s ALICE Collaboration Finds Evidence of Quark-Gluon Plasma in Proton Collisions

Key Takeaways

• Quark‑gluon plasma observed in high‑multiplicity proton collisions
• Baryons show stronger anisotropic flow than mesons
• Findings support quark coalescence models for QGP formation
• Future oxygen‑ion runs will bridge proton and lead systems
• Simulations without QGP fail to reproduce observed flow patterns

Summary

The ALICE Collaboration at CERN’s Large Hadron Collider has reported the first clear evidence of quark‑gluon plasma (QGP) forming in high‑multiplicity proton‑proton and proton‑lead collisions. The analysis revealed anisotropic flow patterns similar to those seen in heavy‑ion collisions, with baryons exhibiting a stronger flow than mesons. These results validate quark coalescence models and challenge the long‑standing view that only large nuclei collisions can generate QGP. Researchers plan to use upcoming oxygen‑ion data to map the transition between small‑ and large‑system QGP behavior.

Pulse Analysis

The discovery that quark‑gluon plasma can emerge from proton‑proton and proton‑lead collisions marks a paradigm shift in high‑energy physics. For decades, scientists believed that only the extreme temperatures and pressures generated in heavy‑ion collisions, such as lead‑lead interactions, were sufficient to melt protons and neutrons into their constituent quarks and gluons. ALICE’s new measurements of anisotropic flow—a collective motion of particles—demonstrate that even comparatively modest collision systems can achieve the necessary energy density, opening a broader experimental landscape for probing the early universe’s primordial soup.

Central to the breakthrough is the observed difference in flow strength between baryons and mesons. Baryons, composed of three quarks, exhibit a markedly stronger directional preference than mesons, which contain two quarks. This pattern mirrors the behavior seen in traditional heavy‑ion experiments and aligns with quark coalescence theory, where quarks recombine within the plasma to form larger particles. By comparing the data to sophisticated simulations, ALICE confirmed that models incorporating QGP formation and coalescence accurately reproduce the flow signatures, whereas models that omit these processes fall short. The result solidifies quark coalescence as a key mechanism in the evolution of the plasma.

Looking ahead, the collaboration’s upcoming oxygen‑ion collisions—recorded in 2025—promise to bridge the gap between proton and lead systems, offering a gradient of system sizes to map QGP properties across scales. This will refine theoretical frameworks, improve predictions for future collider experiments, and deepen our grasp of quantum chromodynamics under extreme conditions. Beyond fundamental science, the insights gained may inform advanced materials research and high‑temperature plasma applications, underscoring the broader relevance of these findings.