  We use a newly assembled large sample of 3,545 star-forming galaxies with
secure spectroscopic, grism, and photometric redshifts at z=1.5-2.5 to
constrain the relationship between UV slope (beta) and dust attenuation
(L(IR)/L(UV)=IRX). Our sample benefits from the combination of deep Hubble
WFC3/UVIS photometry from the Hubble Deep UV (HDUV) Legacy survey and existing
photometric data compiled in the 3D-HST survey, and extends the range of UV
luminosity and beta probed in previous UV-selected samples. IRX is measured
using stacks of deep Herschel/PACS 100 and 160 micron data, and the results are
compared with predictions of the IRX-beta relation for different assumptions of
the stellar population model and obscuration curve. We find that z=1.5-2.5
galaxies have an IRX-beta relation that is consistent with the predictions for
an SMC extinction curve if we invoke sub-solar metallicity models that are
currently favored for high-redshift galaxies, while the commonly assumed
starburst attenuation curve over-predicts the IRX at a given beta by a factor
of ~3. The IRX of high-mass (M*>10^9.75 Msun) galaxies is a factor of >4 larger
than that of low-mass galaxies, lending support for the use of stellar mass as
a proxy for attenuation. The commonly observed trend of fainter galaxies having
bluer beta may simply reflect bluer intrinsic UV slopes for such galaxies,
rather than lower obscurations. The IRX-beta for young/low-mass galaxies
implies a dust curve that is steeper than the SMC, suggesting a lower
attenuation at a given beta relative to older/more massive galaxies. The lower
attenuations and higher ionizing photon output implied by low metallicity
stellar population models point to Lyman continuum production efficiencies,
xi_ion, that may be elevated by a factor of ~2 relative to the canonical value
for L* galaxies, aiding in their ability to keep the universe ionized at z~2.
[Abridged]
