Jet quenching and neutral pion production in Au+Au collisions at RHIC

In the jet quenching model, we have analysed the PHENIX data on nuclear modification factor of $\pi^0$, in Au+Au collisions at $\sqrt{s}$=200 GeV, and extracted the initial gluon density of the medium produced. In jet quenching, partons lose energy in the medium, depending on the medium density as well as on the in-medium path length. Systematic analysis indicate that in most central (0-10% centrality) collisions, medium density is very large $dN_g/dy \sim$ 2150. Medium density decreases exponentially as the collision centrality decreases and in very peripheral (70-92% centrality) collisions, $dN_g/dy \sim$ 3. Initial energy density of the medium also decreases smoothly from $\epsilon_0 \sim$ 20 $GeV/fm^3$ in most central collisions to $\epsilon_0 \sim$ 3 $GeV/fm^3$ in most peripheral collisions. Very large $dN_g/dy$ or $\epsilon_0$ indicate very dense matter formation in central Au+Au collisions.