A model of stable chromomagnetic vacuum in higher-dimensional Yang-Mills   theory
  We study the effective action in Euclidean Yang-Mills theory with a compact simple gauge group in one-loop approximation assuming a covariantly constant gauge field strength as a background. For groups of higher rank and spacetimes of higher dimensions such field configurations have many independent color components taking values in Cartan subalgebra and many ``magnetic fields'' in each color component. In our previous investigation it was shown that such background is stable in dimensions higher than four provided the amplitudes of ``magnetic fields'' do not differ much from each other. In present paper we calculate exactly the relevant zeta-functions in the case of equal amplitudes of ``magnetic fields''. For the case of two ``magnetic fields'' with equal amplitudes the behavior of the effective action is studied in detail. It is shown that in dimensions $d=4,5,6,7$ $({\rm mod}\, 8)$, the perturbative vacuum is metastable, i.e., it is stable in perturbation theory but the effective action is not bounded from below, whereas in dimensions $d=9,10,11$ $({\rm mod}\, 8)$ the perturbative vacuum is absolutely stable. In dimensions $d=8$ $({\rm mod}\, 8)$ the perturbative vacuum is stable for small values of coupling constant but becomes unstable for large coupling constant leading to the formation of a non-perturbative stable vacuum with nonvanishing ``magnetic fields''. The critical value of the coupling constant and the amplitudes of the vacuum ``magnetic fields'' are evaluated exactly. 
Ivan G. Avramidi (University of Greifswald)