We present significant evidence of halo assembly bias for SDSS redMaPPer galaxy clusters in the redshift range [ 0.1 , 0.33 ] .
By dividing the 8,648 clusters into two subsamples based on the average member galaxy separation from the cluster center , we first show that the two subsamples have very similar halo mass of M _ { 200 m } \simeq 1.9 \times 10 ^ { 14 } ~ { } h ^ { -1 } M _ { \odot } based on the weak lensing signals at small radii R \lower 3.01 pt \hbox { $ \buildrel < \over { \sim } $ } 10 ~ { } h ^ { -1 } { Mpc } .
However , their halo bias inferred from both the large-scale weak lensing and the projected auto-correlation functions differs by a factor of \sim 1.5 , which is a signature of assembly bias .
The same bias hypothesis for the two subsamples is excluded at 2.5 \sigma in the weak lensing and 4.4 \sigma in the auto-correlation data , respectively .
This result could bring a significant impact on both galaxy evolution and precision cosmology .