  Recent observations suggest that the first stages of planet formation likely
take place in the Class 0/I phase of Young Stellar Object evolution, when the
star and the disk are still embedded in an infalling envelope. In this study we
perform grain coagulation calculations to investigate the very first stage of
planet formation, the collisional growth of dust grains, in Class 0/I disks. We
find that the slow increase in grain mass by high-velocity collision with much
smaller grains ("sweep-up") allows $\sim 50 M_\oplus$ of grains to grow well
beyond the fragmentation barrier into $\sim$kg pebbles by the end of Class 0/I
(0.1 Myr). We analyze the linear growth and saturation of sweep-up to
understand our results quantitatively, and test whether the sweep-up outcome is
sensitive to disk parameters and details of the grain coagulation model. The
sweep-up pebble population could be important for planet formation, because
they are less well-coupled to the gas (compared to the main population below
the fragmentation barrier) and therefore more favorable to known mechanisms of
dust clump formation (which initiate planetesimal formation). It also contains
enough mass to form all planet cores, based on observational estimates of the
planet mass budget. Our findings motivate future studies of grain growth and
planetesimal formation in Class 0/I disks, including the subsequent evolution
of this sweep-up population.
