Observation of strangeness enhancement with charmed mesons in high-multiplicity $p\mathrm{P}\mathrm{b}$ collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=8.16$TeV

The production of prompt $D_s^{+}$ and $D^{+}$ mesons is measured by the LHCb experiment in proton-lead ($p\mathrm{P}\mathrm{b}$) collisions in both the forward ($1.5<y^{\ast }<4.0$) and backward ($-5.0<y^{\ast }<-2.5$) rapidity regions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{\mathrm{N}\mathrm{N}}}=8.16$TeV. The nuclear modification factors of both $D_s^{+}$ and $D^{+}$ mesons are determined as a function of transverse momentum, $p_{\mathrm{T}}$, and rapidity. In addition, the $D_s^{+}$ to $D^{+}$ cross-section ratio is measured as a function of the charged particle multiplicity in the event. An enhanced $D_s^{+}$ to $D^{+}$ production in high-multiplicity events is observed for the whole measured $p_{\mathrm{T}}$ range, in particular at low $p_{\mathrm{T}}$ 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 $p\mathrm{P}\mathrm{b}$ 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.