  The cross section for dijet production in pp collisions at sqrt(s) = 7 TeV is
presented as a function of xi, a variable that approximates the fractional
momentum loss of the scattered proton in single-diffractive events. The
analysis is based on an integrated luminosity of 2.7 inverse nanobarns
collected with the CMS detector at the LHC at low instantaneous luminosities,
and uses events with jet transverse momentum of at least 20 GeV. The dijet
cross section results are compared to the predictions of diffractive and
nondiffractive models. The low-xi data show a significant contribution from
diffractive dijet production, observed for the first time at the LHC. The
associated rapidity gap survival probability is estimated.
