Resolving the $R_{\mathrm{p}\mathrm{A}}$ and $v_2$ puzzle of $D^0$ mesons in $p-$Pb collisions at the LHC

It has been a challenge to understand the experimental data on both the nuclear modification factor and elliptic flow of $D^0$ mesons in $p-$Pb collisions at LHC energies. In this work, we study these collisions with an improved multi-phase transport model. By including the Cronin effect (i.e., transverse momentum broadening) and independent fragmentation for charm quarks, we provide the first simultaneous description of the $D^0$ meson $R_{\mathrm{p}\mathrm{A}}$ and $v_2$ data at $p_{\mathrm{T}}\le 8$ GeV$\mathrm{/}c$. The model also reasonably describes the $D^0$ meson $p_{\mathrm{T}}$ spectra and the low-$p_{\mathrm{T}}$ charged hadron spectra, $R_{\mathrm{p}\mathrm{A}}$ and $v_2$. Our results show that both parton interactions and the Cronin effect are important for the $D^0$ meson $R_{\mathrm{p}\mathrm{A}}$, while parton interactions are mostly responsible for the $D^0$ meson $v_2$. It is thus essential to include the Cronin effect for the simultaneous description of the $D^0$ meson $R_{\mathrm{p}\mathrm{A}}$ and $v_2$. This work implies that the Cronin effect could also be important for heavy hadrons in large systems.