A collision between two atomic nuclei accelerated at a speed close to that of light creates a dense system of quarks and gluons. Interactions among them are so strong that they behave collectively like a droplet of fluid of ten-femtometer size, which expands into the vacuum and eventually fragments into thousands of particles. We report a new manifestation of thermalization in recent data from the Large Hadron Collider. Our analysis is based on results from the ATLAS Collaboration, which has measured the variance of the momentum per particle across $\text{Pb}+\text{Pb}$ collision events with the same particle multiplicity. This variance decreases steeply over a narrow multiplicity range corresponding to central collisions. We provide a simple explanation of this newly observed phenomenon: For a given multiplicity, the momentum per particle increases with increasing impact parameter. Since a larger impact parameter goes along with a smaller collision volume, this in turn implies that the momentum per particle increases as a function of density, which is a generic consequence of thermalization. Our analysis provides the first direct evidence of this phenomenon at the femtoscale.

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• Charged-particle multiplicity
• Collective flow
• Particle correlations & fluctuations
• Quark-gluon plasma