Observation of strangeness enhancement with charmed mesons in high-multiplicity pPb collisions at sNN =8.16 TeV

The production of prompt Ds+ and D+ mesons is measured by the LHCb experiment in proton-lead (pPb) collisions in both the forward (1.5<y∗<4.0) and backward (-5.0<y∗<-2.5) rapidity regions at a nucleon-nucleon center-of-mass energy of sNN=8.16 TeV. The nuclear modification factors of both Ds+ and D+ mesons are determined as a function of transverse momentum, pT, and rapidity. In addition, the Ds+ to D+ cross section ratio is measured as a function of the primary charged particle multiplicity in the event. An enhanced Ds+ to D+ production in high-multiplicity events is observed for the whole measured pT range, in particular at low pT and backward rapidity, where the significance exceeds six standard deviations. This constitutes the first observation of strangeness enhancement in charm quark hadronization in high-multiplicity pPb collisions. The results are also qualitatively consistent with the presence of quark coalescence as an additional charm quark hadronization mechanism in high-multiplicity proton-lead collisions.