Boosting to identify: pseudoscalar searches with di-leptonic tops

The quest for new heavy states is a critical component of the LHC physics program. In this letter, we study the search for pseudoscalar bosons produced in association with a $t\bar{t}$ pair. We consider the final state $t\bar{t} A \to t\bar{t} b\bar{b}$ with di-leptonic top pair signature, and reconstruct the boosted $A \to b\bar{b}$ candidate with jet substructure techniques, achieving a remarkable sensitivity over a broad range of pseudoscalar masses and Yukawa couplings. We apply this strategy to a Type-I Two-Higgs-Doublet Model, demonstrating its ability to probe a realistic, UV-complete extended Higgs sector. In particular, we find that the $13~TeV$ LHC with $300~fb^{-1}$ of data can constrain the region $\tan\beta>1.5$ at 95$\%$ CL for a light pseudoscalar with $m_A = 50$ GeV. Moreover, the whole mass range $20~GeV<m_A<210~GeV$ can be ruled out for $\tan\beta \leq 1$. Finally, we show that it is also possible to directly probe the CP-structure of the heavy scalar, and hence to distinguish a CP-odd (A) from a CP-even (H) 2HDM resonance.