Observation of Multi-TeV Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array

Data from the Tibet III air shower array (with energies around 3 TeV) and from the Tibet II array (with energies around 10 TeV) have been searched for diffuse gamma rays from the Galactic plane. These arrays have an angular resolution of about 09. The sky regions searched are the inner Galaxy, 20? ? l ? 55?, and outer Galaxy, 140? ? l ? 225?, and |b| ? 2? or ? 5?. No significant Galactic-plane gamma-ray excess was observed. The 99% confidence level upper limits for gamma-ray intensity obtained are (for |b| ? 2?) 1.1 ? 10-15 cm-2 s-1 sr-1 MeV-1 at 3 TeV and 4.1 ? 10-17 cm-2 s-1 sr-1 MeV-1 at 10 TeV for the inner Galaxy, and 3.6 ? 10-16 cm-2 s-1 sr-1 MeV-1 at 3 TeV and 1.3 ? 10-17 cm-2 s-1 sr-1 MeV-1 at 10 TeV for the outer Galaxy, assuming a differential spectral index of 2.4. The upper limits are significant in the multi-TeV region when compared to those from Cerenkov telescopes in the lower energy region and other air shower arrays in the higher energy region; however, the results are not sufficient to rule out the inverse Compton model with a source electron spectral index of 2.0.