On the Incidence of Strong Gravitational Lensing by Clusters in the Las   Campanas Distant Cluster Survey
  The observed incidence of strongly lensing clusters exceeds the predictions of a Lambda-CDM model by about a factor of 10. We revisit the observational side of this discrepancy by measuring the incidence of strong lensing in a subsample of clusters drawn from the Las Campanas Distant Cluster Survey (LCDCS). Among clusters with 0.5<= z <= 0.7, the redshift range in which we focus our search, we find two strongly lensed systems within an effective search area of 69 sq. deg. There is at least one other strongly lensed system in the LCDCS outside of this redshift range, where we are less complete. Over all redshifts, the Lambda-CDM model produces one large arc every 146 sq. degrees. Assuming Poisson statistics, the probability of finding 3 or more strongly lensing clusters in 69 sq. degrees is 0.012. The lensing incidence within the LCDCS is in agreement with that derived from an X-ray selected sample and what has been preliminarily presented from an independent optical cluster survey. The origin of the disagreement between theory and observations, which remains at least at the order of magnitude scale for the Lambda-CDM model, lies either in the concordance cosmological model, in the characteristics of the resulting cluster potentials, or in the adopted source population. 
Dennis Zaritsky (U. Arizona) and Anthony H. Gonzalez (U. Florida)