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tokens: ones before we can unambiguously identify the assembly bias signature in the universe. As discussed in Section 5.1 , the best candidate appears to be , followed by and the mean age (see also the proxy discussed in Lim et al. 2016 ). These in principle could be obtained from high-quality spectra with VESPA, as well as the aforementioned codes, and will be subjects of our future investigation. Inspiration could also come from the analysis of Miyatake et al. ( 2015 ), who have recently claimed a detection of strong assembly bias using galaxy clusters. Their proxy for halo formation time is , the mean projected separation of member galaxies from the cluster center. In principle, it is possible to adopt a similar proxy for galaxy scale halos, although the number of satellites is much smaller, and the membership determination is less certain. We conclude by noting that it is also imperative to better understand theoretically the origin of assembly bias across the mass spectrum of halos. If the cause at low-mass scales is the backsplash halos, then instead of identifying the best formation time indicator, one should look for ways to observationally distinguish such galactic systems, or to take such population fully into account when comparing with theoretical models. Regarding the high-mass end, since it is likely that the physical origin of assembly bias in high-mass halos is believed to be quite different than for low-mass halos (Dalal et al. 2008 ), the detection presented by Miyatake et al. ( 2015 ) is not necessarily inconsistent with the non-detection reported here. It is clear that the assembly bias phenomenon is far richer and complicated than expected, and warrants further investigations, such as exploring better proxies for halo formation time, and the scale dependence of assembly bias (Section 2.4.1 ). 6. SUMMARY As the assembly bias is a robust prediction of the CDM theory of structure formation, establishing it observationally would not only further vindicate this extremely successful theory, but also shed light on the baryonic physics of galaxy formation. In this exploratory study of the detection of assembly bias, we have shown (with the aid of weak gravitational lensing) that some previous claims of detection may be simply due to differences in halo mass of the galaxy samples, rather than a real manifestation of assembly bias (Section 3 ). We have then investigated a couple of ways of constructing galaxy samples with similar mean halo masses, thus facilitating a direct search for assembly bias in the real data (Section 4 ). We focus on the halo mass scale of , where assembly bias is expected to be large, and construct early- and late-forming halo samples by making use of a central galaxy catalog ( Y07 ), under the assumption that the SFH of central galaxies reflects the formation history of underlying halos (which is well supported by a state-of-the-art galaxy formation model; Section 5.1 ). Satellite galaxies living in massive halos