  We present a spectroscopic search for halo field stars that originally formed
in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561
red giants with typical halo metallicities (-1.8 < [Fe/H] < -1.0), we identify
16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating
depleted carbon and enhanced nitrogen abundances relative to the rest of the
data set. Since globular clusters are the only environment known in which stars
form with this pattern of atypical light-element abundances, we claim that
these stars are second-generation globular cluster stars that have been lost to
the halo field via normal cluster mass-loss processes. Extrapolating from
theoretical models of two-generation globular cluster formation, this result
suggests that globular clusters contributed significant numbers of stars to the
construction of the Galactic halo: we calculate that a minimum of 17% of the
present-day mass of the stellar halo was originally formed in globular
clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric
distance, suggesting that the inner-halo population may be the primary
repository of these stars.
