Title: A new method to access heavy meson lightcone distribution amplitudes from first-principle

Abstract: We present a method to compute lightcone distribution amplitudes (LCDAs) of heavy meson within heavy quark effective theory (HQET). Our method utilizes quasi distribution amplitudes (quasi-DAs) with a large momentum component $P^z$. We point out that by sequentially integrating out $P^z$ and $m_H$, one can disentangle different dynamical scales. Integrating out $P^z$ allows to connect quasi-DAs to QCD LCDAs, and then integrating out $m_H$ enables to relate QCD LCDAs to HQET LCDAs. To verify this proposal, we make use of lattice QCD simulation on a lattice ensemble with spacing $a = 0.05187$\,fm. The preliminary findings for HQET LCDAs qualitatively align with phenomenological models. Using a recent model for HQET LCDAs, we also fit the first inverse moment $\lambda_B^{-1}$ and the result is consistent with the experimentally constrain from $B \to \gamma \ell\nu_\ell$. This agreement demonstrates the promise of our method in providing first-principle predictions for heavy meson LCDAs.