Vacuum structure of a modified MIT Bag
  An alternative to introducing and subsequently renormalizing classical parameters in the expression for the vacuum energy of the MIT bag for quarks is proposed in the massless case by appealing to the QCD trace anomaly and scale separation due to asymptotic freedom. The explicit inclusion of gluons implies an unrealistically low separation scale. 
R. Hofmann and T. Gutsche, M. Schumann and R.D. Viollier