We study effects of CP violation in the associated production of a charged Higgs boson and a top quark at the LHC, $pp to tH^pm + X$. We calculate the CP violating asymmetry between the total cross section for $H^+$ and $H^-$ production at next-to-leading order in the MSSM, and perform a detailed numerical analysis. In the production the asymmetry is of the order of 20%. The asymmetry in the production and any subsequent decay of an on-shell charged Higgs boson is to a good approximation the sum of the asymmetry in the production and the asymmetry in the decay. We consider subsequent decays of $H^pm $ to $t b$, $ u_tau tau^pm$ and $W h^0$. In the case with $H^pm to t b$ decay, mainly due to CP violating box graphs with gluino, the asymmetry can go up to $sim$ 12%.
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