# Small System Phenomenology at RHIC and LHC

We discuss aspects of energy loss, thermodynamics, and running coupling in small collision systems at RHIC and LHC. The small system size correction to the opacity expansion energy loss increases the destructive LPM interference linearly with energy, thus predicting a decrease in the suppression of high momentum particles above and beyond that just due to foreshortened path lengths. Massless scalar thermal field theory in a box predicts significant changes to the pressure, entropy density, etc., which mimic the temperature-dependent behavior of thermal QCD; thus some part of the signal of QGP formation taken from thermodynamics might actually be due to geometry. Despite the breaking of conformal invariance by placing fields in a box, the energy momentum tensor remains traceless unless we include running coupling corrections. A toy model for the finite system size corrections to the running coupling significantly reduces the trace anomaly, suggesting that current hydrodynamics simulations significantly under-predict eta/s in small systems. We discuss a new regularization scheme, denominator regularization, which allows a first principles derivation of the small system size correction to the running coupling in field theories and to critical exponents in condensed matter physics.