Experimental results related to charged particle and $\pi^{ 0}$ suppression obtained at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven for Au-Au (Cu-Cu) collisions and at the Large Hadron Collider (LHC) at CERN for Pb-Pb (Xe-Xe) collisions are compiled in terms of the usual nuclear modification factors, $R_{ AA} $ and $R_{ CP}$, and of the newly introduced $R^{ N}_{ AA}$ and $R^{ N}_{ CP}$ as a function of $\langle N_{ part} \rangle$ and $\langle dN_{ ch}/d\eta\rangle$. The studies are focused on a $p_T$ range in the region of maximum suppression evidenced in the experiments. The core contribution to $R_{ AA}$ is presented. Considerations on the missing suppression in high charged particle multiplicity events for pp collisions at 7 TeV are presented. The trends of $R_{ CP}$ and $R^N_{ CP}$ for charged particles and $R_{ AA}^{ \pi^0}$ and $(R_{ AA}^N)^{ \pi^0}$ as a function of $\sqrt{ s_{ NN}}$, measured at RHIC in Au-Au collisions and at LHC in Pb-Pb collisions, show a suppression that becomes larger from $\sqrt{ s_{ NN}}$ = 39 GeV up to $\sqrt{ s_{ NN}}$=200 GeV, followed by a saturation up to the highest energy of $\sqrt{ s_{ NN}}$ =5.02 TeV in Pb-Pb collisions. A clear change in the dependence of $(1-R_{ AA}^{ \pi^0})/\langle dN/dy \rangle$ for the most central collisions as a function of collision energy is evidenced in the region of $\sqrt{ s_{ NN}}$ =62.4 - 130 GeV.
Categories: nucl-ex hep-ph