  The PHENIX experiment has measured {\piz}s in Au+Au collisions at
sqrt(s_{NN}) = 200 GeV, with good statistics for transverse momentum, pT, up to
20 GeV/c. A fivefold suppression is found, which is essentially constant for
5<pT<20 GeV/c. While the production of high pT pions in high energy p+p
collisions is well described in the framework of perturbative QCD, production
in ultra-relativistic heavy ion collisions involves additional degrees of
freedom related the opacity of the deconfined medium produced in the
collisions. The uncertainties of the latest measurement are small enough to
constrain model-dependent opacity-related parameters, such as the transport
coefficient of the medium, qhat or initial gluon density dN^g/dy at the level
of +/- 20-25% (one standard deviation), by applying a statistical method
proposed. From the dependence of the suppression level to the number of
participant nucleons, we have also confirmed that the energy loss increases as
pT of partons increases. The latest PHENIX result on high transverse momentum
hadrons are presented, and the property of the medium created in the collisions
is discussed in detail.
