Transverse momentum fluctuations and their correlation with elliptic flow in nuclear collisions

We propose observables v0 and v0(pT) which quantify the relative fluctuations in the total transverse momentum at fixed multiplicity. We first study the factorization of the fixed multiplicity momentum-dependent two-particle correlation function into a product of v0(pTa) and v0(pTb) within realistic hydrodynamic simulations. Then we present computations of v0(pT) for different particle types. We determine the relation between the integrated v0 and previously measured observables and compare results from a hybrid hydrodynamics-based model to experimental data. The effects of bulk viscosity and an initial pre-equilibrium stage on the results are quantified. We find that v0 is strongly correlated with the initial state entropy per elliptic area, S/A. Using this result, we explain how the observed correlations between the elliptic flow and the transverse momentum (both in simulations and experiment) reflect the initial state correlations between 1/A and ellipticity É2 at fixed multiplicity. We argue that the systematic experimental study of v0, with the same sophistication as used for the other vn, can contribute significantly to our understanding of quark gluon plasma properties.