Non-Monotonicity of Transverse Momentum Correlations in Au + Au Collisions at RHIC

Abstract

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 ($p_T$) correlations for mid-rapidity charged particles in fixed-target Au+Au collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}} = 3.0--7.7$ 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 ($N_{part}$) is studied to test expectations from an independent-source scenario, where the correlations are expected to scale as $1/\sqrt{N_{part}}$. We observe a clear breakdown of the expected scaling behavior in central collisions and identify a statistically significant non-monotonic dependence of the $p_T$ correlations on collision energy, with a significance of approximately $5\sigma$. In contrast, transport-model calculations and data from mid-central collisions yield significances of only $2\sigma$ and $1.4\sigma$, respectively, insufficient to support a claim of non-monotonicity. 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.