Nonmonotonicity of Transverse Momentum Correlations in Au + Au Collisions at RHIC

Event-by-event transverse momentum correlations are sensitive to the equation of state of strongly interacting matter and are expected to exhibit anomalous fluctuations in the vicinity of the QCD critical point. We report the first measurements of two-particle transverse momentum (pT) correlations for mid-rapidity charged particles in fixed-target Au+Au collisions at nucleon-nucleon center-of-mass energies sqrt[sNN]=3.0-7.7 GeV, measured by the STAR experiment during the Beam Energy Scan (BES) Phase II program. The dependence of the scaled correlator on the number of participating nucleons (Nₚₐᵣₜ) is studied to test expectations from an independent-source scenario, where the correlations are expected to scale as 1/sqrt[Nₚₐᵣₜ]. We observe a clear breakdown of the expected scaling behavior in central collisions and identify a statistically significant nonmonotonic dependence of the pT correlations on collision energy, with a significance of approximately 5σ. In contrast, transport-model calculations and data from mid-central collisions yield significances of only 2σ and 1.4σ, respectively-insufficient to support a claim of nonmonotonicity. These observations provide new constraints on the equation of state at high baryon density and may be sensitive to the presence of a QCD critical point.