Measurement of Large Transverse Momentum Hadrons and Constraints on   Medium Opacity Parameters
  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. 
D. Winter (for the PHENIX collaboration)