The measurement of the flux, spectral energy and spatial distributions of diffuse Galactic gamma-ray emission probes the density distribution, propagation, and spectral features of cosmic rays in different locations of our Galaxy. Theoretical models for diffuse gamma ray emission are usually based on the assumption that the cosmic-ray flux and spectrum measured at Earth is representative of the typical cosmic-ray flux and spectrum present throughout the Galaxy and on our knowledge of matter and radiation fields. The global excess of diffuse GeV gamma rays over such theoretical predictions previously observed by the EGRET experiment could not be confirmed by Fermi measurements.

The significant enhancement of diffuse gamma ray emission at TeV energies with respect to such models that was reported by the Milagro experiment for the Cygnus region (measured emission = 8x predicted) and the inner Galaxy (5x) is still not unambiguously explained and needs further study. Two main factors will help to resolve this data-model discrepancy:

  1. an improved (dedicated) theoretical modeling of diffuse TeV gamma-ray emission
  2. an improved measurement of extended emissions in terms of sensitivity, energy and angular resolution.

The latter will in particular help with the disentanglement of diffuse emission from the emission of already resolved and unresolved source. This will be provided by the HAWC observatory, an all-sky high-altitude water Cherenkov detector in Mexico.

The instrument will begin taking data with a subset of detector components this fall. Operations with one third of the experiment will begin in September 2013, and the complete detector is expected to be online in September 2014. We will review and interpret previous diffuse gamma-ray emission results and present estimations of HAWC sensitivities to diffuse Multi-TeV gamma-ray emission at different stages of the experiment.