  We perform a joint jet tomographic analysis of the data on the nuclear
modification factor $R_{AA}$ from PHENIX at RHIC and ALICE at LHC. The
computations are performed accounting for radiative and collisional parton
energy loss with running coupling constant. Our results show that the observed
slow variation of $R_{AA}$ from RHIC to LHC indicates that the QCD coupling
constant is suppressed in the quark-gluon plasma produced at LHC.
