  We employ X-ray stacking techniques to examine the contribution from X-ray
undetected, mid-infrared-selected sources to the unresolved, hard (6-8 keV)
cosmic X-ray background (CXB). We use the publicly available, 24 micron Spitzer
Space Telescope MIPS catalogs from the Great Observatories Origins Deep Survey
(GOODS) - North and South fields, which are centered on the 2 Ms Chandra Deep
Field-North and the 1 Ms Chandra Deep Field-South, to identify bright (S_24 >
80 microJy) mid-infrared sources that may be powered by heavily obscured AGNs.
We measure a significant stacked X-ray signal in all of the X-ray bands
examined, including, for the first time, a significant (3.2 sigma) 6-8 keV
stacked X-ray signal from an X-ray undetected source population. We find that
the X-ray-undetected MIPS sources make up about 2% (or less) of the total CXB
below 6 keV, but about 6% in the 6-8 keV band. The 0.5-8 keV stacked X-ray
spectrum is consistent with a hard power-law (Gamma = 1.44 +/- 0.07), with the
spectrum hardening at higher X-ray energies. Our findings show that these
bright MIPS sources do contain obscured AGNs, but are not the primary source of
the unresolved 50% of 6-8 keV CXB. Our study rules out obscured, luminous QSOs
as a significant source of the remaining unresolved CXB and suggests that it
most likely arises from a large population of obscured, high-redshift (z > 1),
Seyfert-luminosity AGNs.
