  Recent data on the nuclear modification factor $R_{AA}$ of jet fragments in
2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC) indicate that the
jet-medium coupling in a Quark-Gluon Plasma (QGP) is reduced at LHC energies
and not compatible with the coupling deduced from data at the Relativistic
Hadron Collider (RHIC). We estimate the reduction factor from a combined fit to
the available data on $R_{AA}(\sqrt{s},p_T,b)$ and the elliptic flow
$v_2(\sqrt{s},p_T,b)$ at $\sqrt{s}=0.2,2.76$ ATeV over a transverse momentum
range $p_T$ 10-100 GeV and a broad impact parameter, b, range. We use a simple
analytic "polytrope" model ($dE/dx=- \kappa E^{a} x^z T^{c}$) to investigate
the dynamical jet-energy loss model dependence. Varying a=0-1 interpolates
between weakly-coupled and strongly-coupled models of jet-energy dependence
while z=0-2 covers a wide range of possible jet-path dependencies from elastic
and radiative to holographic string mechanisms. Our fit to LHC data indicates
an approximate 40% reduction of the coupling $\kappa$ from RHIC to LHC and
excludes energy-loss models characterized by a jet-energy exponent with a>1/3.
In particular, the rapid rise of $R_{AA}$ with $p_T$>10 GeV combined with the
slow variation of the asymptotic $v_2(p_T)$ at the LHC rules out popular
exponential geometric optics models (a=1). The LHC data are compatible with
$0\leq a\leq 1/3$ pQCD-like energy-loss models where the jet-medium coupling is
reduced by approximately 10% between RHIC and LHC.
