Multiplicities and $p_T$ spectra in ultrarelativistic heavy ion collisions from a next-to-leading order improved perturbative QCD + saturation + hydrodynamics model

We bring the EKRT framework, which combines perturbative QCD (pQCD) minijet production with gluon saturation and hydrodynamics, to next-to-leading order (NLO) in pQCD as rigorously as possible. We chart the model uncertainties, and study the viability and predictive power of the model in the light of the RHIC and LHC measurements in central $A+A$ collisions. In particular, we introduce a new set of measurement functions to define the infrared- and collinear-safe minijet transverse energy, $E_T$, in terms of which we formulate the saturation. We update the framework with the EPS09 NLO nuclear parton distributions (nPDFs), and study the propagation of the nPDF uncertainties into the computed $E_T$, saturation scales and the final-state multiplicities. The key parameters, which need to be fixed using the measurements, are identified, and their correlation is discussed. We convert the saturated minijet $E_T$ into QCD-matter initial conditions for longitudinally boost-invariant ideal hydrodynamics. We compute the charged-particle multiplicities and identified bulk hadron $p_T$ spectra in 5% most central Au+Au collisions at RHIC and Pb+Pb at the LHC. We obtain an encouragingly good agreement with the experimental data, simultaneously at RHIC and LHC, showing that the approach has a definite predictive power.