Correlated Pion-Proton Pair Emission off Hot and Dense QCD Matter

In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at root(S)(NN) = 2.42 GeV collected with HADES. In this energy regime the population of Delta(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d(3)N/dM(pi +/- p)dp(T)dy distributions of correlated pi(+/-)p pairs have been determined by subtracting the pi p combinatorial background using an iterative method. The invariant-mass distributions in the Delta(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1 < M-inv(GeV/c(2)) < 1.4, the yield is found to be similar for pi(+)p and pi(-)p pairs, and to follow a power law (A(part))(alpha), where (A(part)) is the mean number of participating nucleons. The exponent alpha depends strongly on the pair transverse momentum (p(T)) while its p(T)-integrated and charge-averaged value is alpha = 1.5 +/- 0.08(st) +/- 0.2(sy). (C) 2021 The Author(s). Published by Elsevier B.V.