QCD with Large Number of Quarks: Effects of the Instanton -- Anti-instanton Pairs

We calculate the contribution of the instanton -- anti-instanton ($I\bar I$) pairs to the vacuum energy of QCD-like theories with $N_f$ light fermions using the saddle point method. We find a qualitative change of the behavior: for $N_f \ge 6$ it starts to oscillate with $N_f$. Similar behaviour was known for quantum mechanical systems interacting with fermions. We discuss the possible consequences of this phenomenon, and its relation to the mechanism of chiral symmetry breaking in these theories. We also discuss the asymptotics of the perturbative series associated with the $I\bar I$ contribution, comparing our results with those in literature.