Resolving the $R_{\rm pA}$ and $v_2$ puzzle of $D^0$ mesons in $p-$Pb collisions

Abstract

It has been difficult to reconcile the experimental data on the 𝐷 0 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 𝐷 0 meson 𝑅 pA and 𝑣 2 data at 𝑝 T ≤ 8 GeV/𝑐. The model also provides a reasonable description of the 𝐷 0 meson 𝑝 T spectra and the low-𝑝 T (below ∼ 2 GeV/𝑐) charged hadron spectra in 𝑝 + 𝑝 and 𝑝−Pb collisions as well as 𝑅 pA and 𝑣 2 in 𝑝−Pb collisions. We find that both parton scatterings and the Cronin effect are important for the 𝐷 0 meson 𝑅 pA , while parton scatterings are mostly responsible for the 𝐷 0 meson 𝑣 2 . Our results indicate that it is crucial to include the Cronin effect for the simultaneous description of the 𝐷 0 meson 𝑅 pA and 𝑣 2 . 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.