  We investigate the potential for enhancing search sensitivity for signals
having charm quarks in the final state, using the sizable bottom-mistagging
rate for charm quarks at the LHC. Provided that the relevant background
processes contain light quarks instead of charm quarks, the application of
b-tagging on charm quark-initiated jets enables us to reject more background
events than signal ones due to the relatively small mistagging rate for light
quarks. The basic idea is tested with two rare top decay processes: i) t -> c h
-> c b bbar and ii) t -> b H+ -> b bbar c where h and H+ denote the Standard
Model-like higgs boson and a charged higgs boson, respectively. The major
background source is a hadronic top quark decay such as t -> b W+ -> b sbar c.
We test our method with Monte Carlo simulation at the LHC 14TeV, and find that
the signal-over-background ratio can be increased by a factor of O(6-7) with a
suitably designed (heavy) flavor tagging algorithm and scheme.
