  We investigate the formation of metal-poor globular clusters (GCs) at the
center of two dark matter halos with $M_{\rm halo} \sim4\times10^7 M_\odot$ at
$z>10$ using cosmological radiation-hydrodynamics simulations. We find that
very compact ($\lesssim$ 1 pc) and massive ($\sim6\times10^5 M_\odot$) clusters
form rapidly when pristine gas collapses isothermally with the aid of efficient
Ly$\alpha$ emission during the transition from molecular-cooling halos to
atomic-cooling halos. Because the local free-fall time of dense star-forming
gas is very short ($\ll 1\,{\rm Myr}$), a large fraction of the collapsed gas
is turned into stars before stellar feedback processes blow out the gas and
shut down star formation. Although the early stage of star formation is limited
to a small region of the central star-forming disk, we find that the disk
quickly fragments due to metal enrichment from supernovae. Sub-clusters formed
in the fragmented clouds eventually merge with the main cluster at the center.
The simulated clusters closely resemble the local GCs in mass and size but show
a metallicity spread that is much wider than found in the local GCs. We discuss
a role of pre-enrichment by Pop III and II stars as a potential solution to the
latter issue. Although not without shortcomings, it is encouraging that a naive
blind (not tuned) cosmological simulation presents a possible channel for the
formation of at least some massive GCs.
