The mass-concentration relation in lensing clusters: the role of
  statistical biases and selection effects
  The relation between mass and concentration of galaxy clusters traces their
formation and evolution. Massive lensing clusters were observed to be
over-concentrated and following a steep scaling in tension with predictions
from the concordance $\Lambda$CDM paradigm. We critically revise the relation
in the CLASH, the SGAS, the LOCUSS, and the high-redshift samples of weak
lensing clusters. Measurements of mass and concentration are anti-correlated,
which can bias the observed relation towards steeper values. We corrected for
this bias and compared the measured relation to theoretical predictions
accounting for halo triaxiality, adiabatic contraction of the halo, presence of
a dominant BCG and, mostly, selection effects in the observed sample. The
normalisation, the slope and the scatter of the expected relation are strongly
sample-dependent. For the considered samples, the predicted slope is much
steeper than that of the underlying relation characterising dark-matter only
clusters. We found that the correction for statistical and selection biases in
observed relations mostly solve the tension with the $\Lambda$CDM model.
