Measuring Parton Energy Loss at RHIC compared to LHC

The method of measuring ${\widehat{x}}_h={\widehat{p}}_{Ta}\mathrm{/}{\widehat{p}}_{Tt}$, the ratio of the away-parton transverse momentum, ${\widehat{p}}_{T_a}$, to the trigger-parton transverse momentum, ${\widehat{p}}_{T_t}$, using two-particle correlations at RHIC, will be reviewed. This measurement is simply related to the two new variables introduced at LHC for the di-jet fractional transverse momentum imbalance: ATLAS $A_J=(\hat{p}{Tt}-\hat{p}{Ta})/(\hat{p}{Tt}+\hat{p}{Ta})= (1-\hat{x}h)/(1+\hat{x}h)$;andCMS$\mean{(\hat{p}{Tt}-\hat{p}{Ta})/\hat{p}_{Tt}}= \mean{1-\hat{x}_h}$. Results from two-particle correlations at RHIC for ${\widehat{x}}_h$ in p-p and A+A collisions will be reviewed and new results will be presented and compared to LHC results. The importance of comparing any effect in A+A collisions to the same effect effect in p-p collisions will be illustrated and emphasized.