1 Introduction

Two-particle angular correlations allow the mechanisms of particle production to be investigated and the event properties of ultra-relativistic hadronic collisions to be studied. In particular, the azimuthal and pseudorapidity distribution of “associated” charged particles with respect to a “trigger” D meson is sensitive to the charm-quark production, fragmentation, and hadronisation processes in proton–proton (pp) collisions and to their possible modifications in larger collision systems, like proton–nucleus (pA) or nucleus–nucleus (AA) [1]. The typical structure of the correlation function, featuring a “near-side” (NS) peak at \((\Delta \varphi ,\Delta \eta ) = (0,0)\) (where \(\Delta \varphi \) is the difference between charged-particle and D-meson azimuthal angles \(\varphi _\mathrm{ch} - \varphi _\mathrm{D}\), and \(\Delta \eta \) the difference between their pseudorapidities \(\eta _\mathrm{ch} - \eta _\mathrm{D}\)) and an “away-side” (AS) peak at \(\Delta \varphi = \pi \) extending over a wide \(\Delta \eta \) range, as well as its sensitivity to the different charm-quark production mechanisms, are described in details in [2].

In this paper, results of azimuthal correlations of prompt D mesons with charged particles at midrapidity in pp and p–Pb collisions at \(\sqrt{s_{\mathrm{NN}}} = 5.02\ \hbox {TeV}\) are presented, where “prompt” refers to D mesons produced from charm-quark fragmentation, including the decay of excited charmed resonances and excluding D mesons produced from beauty-hadron weak decays. The study of the near-side correlation peak is strongly connected to the characterisation of charm jets and of their internal structure, in terms of their particle multiplicity and angular profile. Probing the near-side peak features as a function of the charged-particle transverse momentum (\(p_\mathrm{T} \)), possibly up to values of a few GeV/c, gives not only access to the transverse-momentum distribution of the jet constituents, but can also provide insight into how the jet-momentum fraction not carried by the D meson is shared among the other particles produced by the parton fragmentation, as well as on the correlation between the \(p_\mathrm{T} \) of these particles and their radial displacement from the jet axis, which is closely related to the width of the near-side correlation peak. This study provides further and complementary information with respect to the analysis of charm jets reconstructed as a single object through a track-clustering algorithm and tagged by their charm content [3,4,5].

The azimuthal-correlation function of D mesons with charged particles is largely sensitive to the various stages of the D-meson and particle evolution, as hard-parton scattering, parton showering, fragmentation and hadronisation [6]. Its description by the available Monte Carlo event generators like PYTHIA [7, 8], HERWIG [9,10,11], and EPOS 3 [12, 13] or pQCD calculations like POWHEG [14, 15] coupled to event generators handling the parton shower, depends on several features, including the order of the hard-scattering matrix-element calculations (leading order or next-to-leading order), the modelling of the parton shower, the algorithm used for the fragmentation and hadronisation, and the description of the underlying event. The azimuthal-correlation function of D mesons with charged particles in pp collisions at \(\sqrt{s} = 7\ \hbox {TeV}\) measured by ALICE is described within uncertainties by simulations produced using PYTHIA6, PYTHIA8 and POWHEG+PYTHIA6 event generators [2]. However, more precise and differential measurements are needed to set constraints to models and be sensitive to the differences among their expectations.

The validation of Monte Carlo simulations for angular correlations of heavy-flavour particles in pp collisions is also useful for interpreting the results in nucleus–nucleus collisions, for which the measurements in pp collisions are used as reference. The temperature and energy density reached in nucleus–nucleus collisions at LHC energies are large enough to produce a quark–gluon plasma (QGP), a deconfined state of strongly-interacting matter [16,