Transverse-momentumptcorrelations on (η, phi) from mean-ptfluctuations in Au–Au collisions at GeV

We present first measurements of the pseudorapidity and azimuth (η, ϕ) bin-size dependence of event-wise mean transverse-momentum ⟨ p t ⟩ fluctuations for Au–Au collisions at GeV. We invert that dependence to obtain p t autocorrelations on differences (η Δ , ϕ Δ ) interpreted to represent velocity/temperature distributions on (η, ϕ). The general form of the autocorrelations suggests that the basic correlation mechanism is parton fragmentation. The autocorrelations vary rapidly with collision centrality, which suggests that fragmentation is strongly modified by a dissipative medium in the more central Au–Au collisions relative to peripheral or p–p collisions.