Spectra of Free Diquark in the Bethe-Salpeter Approach
  In this work, we employ the Bethe-Salpeter (B-S) equation to investigate the spectra of free diquarks and their B-S wave functions. We find that the B-S approach can be consistently applied to study the diqaurks with two heavy quarks or one heavy and one light quarks, but for two light-quark systems, the results are not reliable. There are a few free parameters in the whole scenario which can only be fixed phenomenologically. Thus, to determine them, one has to study baryons which are composed of quarks and diquarks. 
Yan-Ming Yu, Hong-Wei Ke, Yi-Bing Ding, Xin-Heng Guo, Hong-Ying Jin,   Xue-Qian Li, Peng-Nian Shen and Guo-Li Wang