Background: In the context of the investigation of the quark gluon plasma produced in heavy-ion collisions,
hadrons containing heavy (charm or beauty) quarks play a special role for the characterization of the hot and dense
medium created in the interaction. The measurement of the production of charm and beauty hadrons in proton–
proton collisions, besides providing the necessary reference for the studies in heavy-ion reactions, constitutes
an important test of perturbative quantum chromodynamics (pQCD) calculations. Heavy-flavor production in
proton–nucleus collisions is sensitive to the various effects related to the presence of nuclei in the colliding
system, commonly denoted cold-nuclear-matter effects. Most of these effects are expected to modify open-charm
production at low transverse momenta (pT) and, so far, no measurement of D-meson production down to zero
transverse momentum was available at mid-rapidity at the energies attained at the CERN Large Hadron Collider
(LH
C).
Purpose: The measurements of the production cross sections of promptly produced charmed mesons in p-Pb
collisions at the LHC down to pT = 0 and the comparison to the results from pp interactions are aimed at the
assessment of cold-nuclear-matter effects on open-charm production, which is crucial for the interpretation of
the results from Pb-Pb collisions.
Methods: The prompt charmed mesons D0, D+, D∗+, and D+
s were measured at mid-rapidity in p-Pb collisions at
a center-of-mass energy per nucleon pair √sNN = 5.02 TeV with the ALICE detector at the LHC. D mesons were
reconstructed from their decays D0 → K−π+, D+ → K−π+π+, D∗+ → D0π+, D+
s → φπ+ → K−K+π+,
and their charge conjugates, using an analysis method based on the selection of decay topologies displaced from
the interaction vertex. In addition, the prompt D0 production cross section was measured in pp collisions at
√s = 7 TeV and p-Pb collisions at √sNN = 5.02 TeV down to pT = 0 using an analysis technique that is based
on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex.
Results: The production cross section in pp collisions is described within uncertainties by different
implementations of pQCD calculations down to pT = 0. This allowed also a determination of the total cc¯
production cross section in pp collisions, which is more precise than previous ALICE measurements because it
is not affected by uncertainties owing to the extrapolation to pT = 0. The nuclear modification factor RpPb(pT),
defined as the ratio of the pT-differential D meson cross section in p-Pb collisions and that in pp collisions
scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be
compatible with unity within uncertainties. The results are compared to theoretical calculations that include
cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks
with an expanding deconfined medium.
Conclusions: These measurements add experimental evidence that the modification of the D-meson transverse
momentum distributions observed in Pb–Pb collisions with respect to pp interactions is due to strong final-state
effects induced by the interactions of the charm quarks with the hot and dense partonic medium created in
ultrarelativistic heavy-ion collisions. The current precision of the measurement does not allow us to draw
conclusions on the role of the different cold-nuclear-matter effects and on the possible presence of additional
hot-medium effects in p-Pb collisions. However, the analysis technique without decay-vertex reconstruction,
applied on future larger data samples, should provide access to the physics-rich range down to pT = 0.
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