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The many different theoretical studies of energy loss of a quark or gluontraversing a medium have one thing in common: the transport coefficient of agluon in the medium, $\hat{q}$, which is defined as the mean 4-momentumtransfer$^2$, $\left<q^2\right>$, by a gluon to the medium per gluon mean freepath, $\lambda_{\rm mfp}$. In the original BDMPSZ formalism, the energy loss ofan outgoing parton, $-dE/dx$, per unit length ($x$) of a medium with totallength $L$, due to coherent gluon bremsstrahlung, is proportional to the$\left< q^2\right>$ and takes the form: ${-dE/dx }\simeq \alpha_s\left<{q^2(L)}\right>=\alpha_s\, \mu^2\, L/\lambda_{\rm mfp} =\alpha_s\,\hat{q}\, L\ $ , where $\mu$, is the mean momentum transfer per collision.Thus, the total energy loss in the medium goes like $L^2$. Additionally, theaccumulated momentum$^2$, $\left<{k_{\perp}^2}\right>$, transverse to a gluontraversing a length $L$ in the medium is well approximated by$\left<{k_{\perp}^2}\right>\approx\left<{q^2(L)}\right>=\hat{q}\, L$.  A simple estimate shows that the$\left<{k_{\perp}^2}\right>\approx\hat{q}\,L$ should be observable at RHIC at$\sqrt{s_{NN}}=200$ GeV via the broadening of di-hadron azimuthal correlationsresulting in an azimuthal width $\sim\sqrt{2}$ larger in Au$+$Au than in $p+p$collisions . Measurements relevant to this issue will be discussed as well asrecent STAR jet results presented at QM2014. Other topics to be discussedinclude the danger of using forward energy to define centrality in $p(d)+$Acollisions for high $p_T$ measurements, the danger of not using comparison$p+p$ data at the same $\sqrt{s}$ in the same detector for $R_{AA}$ or latelyfor $R_{pA}$ measurements.

Is $\hat{q}$ a physical quantity or just a parameter? and other unanswered questions in High-$p_T$ Physics