Searching for charged Higgs boson in polarized top quark

The charged Higgs boson is quite common in many new physics models. In this study we examine the potential of observing a heavy charged Higgs boson in its decay mode of top quark and bottom quark in the type-II two-Higgs-doublet model. In this model, the chirality structure of the coupling of a charged Higgs boson to the top and bottom quark is very sensitive to the value of tanβ. As the polarization of the top quark can be measured experimentally from the top-quark decay products, one could make use of the top-quark polarization to determine the value of tanβ. We perform a detailed analysis of measuring top-quark polarization in the production channels gb→tH^- and gb̄→t̄H⁺. We calculate the helicity amplitudes of the charged Higgs boson production and decay. Our calculation shows that the top quark from the charged Higgs boson decay provides a good probe for measuring tanβ, especially for the intermediate tanβ region. On the contrary, the top quark produced in association with the charged Higgs boson cannot be used to measure tanβ because its polarization is highly contaminated by the t-channel kinematics.