Invariant mass b tag.
Can be called after QIPBTAG or QVSRCH, to further increase b purity.
For each event-half QBMTAG loops over tracks in order of decreasing inconsistency with primary vertex, and stops when their combined invariant mass exceeds 1.8 GeV. (i.e. The mass of a typical charmed hadron). It then returns a tag, CLMASS, indicating the probablity that the ``last track" used came from primary vertex.
For a very high purity tag, CLMASS should be combined with the QIPTBAG or QVSRCH tags. The optimal linear combination (together with suggested cuts) is:
0.7*CLMASS - 0.3*log10(QIPBTAG) > 2.4
or 0.7*CLMASS + 0.3*QVSRCH > 8.3