  The aim of this thesis is to calculate field-theoretically as rigorously as
possible the initial state of partonic matter produced in ultrarelativistic
heavy-ion collisions at CERN-LHC and BNL-RHIC colliders. The computed minijet
initial conditions are then used in the initialization of the relativistic
hydrodynamical modeling of these collisions.
  In the theoretical introduction part the computation of parton production
cross section at next-to-leading order (NLO) perturbative QCD (pQCD) is
discussed. Furthermore, the full analytical calculation for the squared
quark-quark scattering matrix element including the systematic ultraviolet
renormalization is presented. Finally, the subtraction method allowing for the
cancellation of the infrared and collinear singularities in the partonic QCD
cross section at NLO is discussed.
  In the more phenomenological part of the thesis the original EKRT model,
which combines collinearly factorized leading-order pQCD minijet production
with gluon saturation, is introduced. Next, the minijet production is
generalized rigorously to NLO. In particular, a new set of measurement
functions is introduced to define the produced infrared- and collinear-safe
minijet transverse energy, in terms of which the saturation is now formulated.
Finally, the framework is updated with the latest knowledge of nuclear parton
distribution functions.
  Using the NLO-improved EKRT model with hydrodynamics we obtained a good
agreement with the measured centrality dependence of the
low-transverse-momentum bulk observables, simultaneously at the LHC and RHIC.
In particular, aiming at a determination of the QCD matter properties from
these measurements, we were able to constrain the temperature dependence of the
QCD matter shear viscosity, which is an important result.
