  Star-forming galaxies produce gamma-rays primarily via pion production,
resulting from inelastic collisions between cosmic ray protons and the
interstellar medium (ISM). The dense ISM and high star formation rates of
luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) imply that
they should be strong gamma-ray emitters, but so far only two LIRGs have been
detected. Theoretical models for their emission depend on the unknown fraction
of cosmic ray protons that escape these galaxies before interacting. We analyze
Fermi-LAT data for 82 of the brightest IRAS LIRGs and ULIRGs. We examine each
system individually and carry out a stacking analysis to constrain their
gamma-ray fluxes. We report the detection of the nearest ULIRG Arp 220
(~4.6sigma). We observe a gamma-ray flux (0.8--100 GeV) of 2.4e-10 phot cm^-2
s^-1 with photon index of 2.23 (8.2e10^41 ergs s^-1 at 77 Mpc) We also derive
upper limits for the stacked LIRGs and ULIRGs. The gamma-ray luminosity of
Arp~220 and the stacked upper limits agree with calorimetric predictions for
dense star-forming galaxies. With the detection of Arp 220, we extend the
gamma-ray--IR luminosity correlation to the high luminosity regime with
log(L_[0.1-100 GeV]) = 1.25log(L_[8-1000]) + 26.7 as well as the
gamma-ray--radio continuum luminosity correlation with log(L_[0.1-100 GeV]) =
1.22log(L_[1.4 GHz]) + 13.3. The current survey of Fermi-LAT is on the verge of
detecting more LIRGs/ULIRGs in the local universe, and we expect even more
detections with deeper Fermi-LAT observations or the next generation of
gamma-ray detectors.
