Resolving the RpA and v₂ puzzle of D⁰ mesons in $p-$Pb collisions

It has been difficult to reconcile the experimental data on the D⁰ meson nuclear modification factor and elliptic flow in $p-$Pb collisions at LHC energies. Here we study these observables with the string melting version of a multi-phase transport model, which has been improved with the implementation of the Cronin effect (or transverse momentum broadening) and independent fragmentation for charm quarks. Using a strong Cronin effect allows us to provide the first simultaneous description of the D⁰ meson RpA and v₂ data at pT ≤ 8 GeV$/c$. The model also provides a reasonable description of the D⁰ meson pT spectra and the low-pT (below ~ 2 GeV$/c$) charged hadron spectra in $p+p$ and $p-$Pb collisions as well as RpA and v₂ in $p-$Pb collisions. We find that both parton scatterings and the Cronin effect are important for the D⁰ meson RpA, while parton scatterings are mostly responsible for the D⁰ meson v₂. Our results indicate that it is crucial to include the Cronin effect for the simultaneous description of the D⁰ meson RpA and v₂. Since the Cronin effect is expected to grow with the system size, this work implies that the Cronin effect could also be important for heavy hadrons in large systems.