Open Access

Hans Beck

• Heavy-ion collisions at ultra-relativistic energies allow access to the Quark-Gluon Plasma, the deconfined phase of the strong interaction, a state which is believed to have existed fractions of seconds after the big bang. Two-particle correlations at small relative momenta, and particularly their dependence on pair transverse mass, are distinctly sensitive to the reaction dynamics of the fireball created in heavy-ion collisions. Being the heaviest system to extract a size of the particle emitting source, proton-lambda correlations extend the studied range in pair transverse mass and are therefore well suited to explore the dynamical behavior of the matter created in Pb-Pb collisions at the Large Hadron Collider. The centrality dependence of the extracted source radii affirms the expectations of a larger source for more central collisions. Source radii were attained over a span of more than 0.9 GeV/c2 in mean pair transverse mass with a source radius extracted for a mean transverse mass as high as 2.18 GeV/c2. The source radii decrease with increasing pair transverse mass, as expected in a hydrodynamical picture. The comparison with radii obtained from other particle species exhibits the clear breaking of an elsewhere proposed scaling behavior of source radii with mean pair transverse mass for all particle species. Gamma-gamma correlations possibly allow to look past the barrier of kinetic freeze-out. Additionally, they bear the potential to solve the puzzling observation in heavy-ion collisions of an excess of photons with a large temperature-like inverse slope parameter on the one hand and an elliptic flow coefficient of photons comparable to the one of hadrons on the other hand. A striking signal in the two-photon correlation function is observed; however it seems likely to not be of quantum statistical origin. A path for further studies is laid out.