  We have measured the spectrum of the Cosmic X-ray Background (CXB) in the 2-8
keV range with the high throughput EPIC/MOS instrument onboard XMM-Newton. A
large sample of high galactic latitude observations was used, covering a total
solid angle of 5.5 square degrees. Our study is based on a very careful
characterization and subtraction of the instrumental background, which is
crucial for a robust measurement of the faintest diffuse source of the X-ray
sky. The CXB spectrum is consistent with a power law having a photon index
Gamma=1.41+/-0.06 and a normalization of 2.46+/-0.09 photons/(cm^2 s sr keV) at
3 keV (~11.6 photons/(cm^2 s sr keV) at 1 keV), corresponding to a 2-10 keV
flux of (2.24+/-0.16)x10^(-11) erg/(cm^2 s deg) (90% confidence level,
including the absolute flux calibration uncertainty). Our results are in
excellent agreement with two of the most recent CXB measurements, performed
with BeppoSAX LECS/MECS data (Vecchi et al. 1999) and with an independent
analysis of XMM-Newton EPIC/MOS data (Lumb et al. 2002), providing a very
strong constrain to the absolute sky surface brightness in this energy range,
so far affected by a ~40% uncertainty. Our measurement immediately implies that
the fraction of CXB resolved by the recent deep X-ray observations in the 2-10
keV band is of 80+/-7% (1sigma), suggesting the existence of a new population
of faint sources, largely undetected within the current sensitivity limits of
the deepest X-ray surveys.
