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This letter reports the first measurement of spin alignment, with respect tothe helicity axis, for ${\rm D^{*+}}$ vector mesons and their charge conjugatesfrom charm-quark hadronisation (prompt) and from beauty-meson decays(non-prompt) in hadron collisions. The measurements were performed atmidrapidity ($|y|<0.8$) as a function of transverse momentum ($p_{\rm T}$) inproton-proton (pp) collisions collected by ALICE at the centre-of-mass energy$\sqrt{s} = 13$ TeV. The diagonal spin density matrix element $\rho_{00}$ of${\rm D^{*+}}$ mesons was measured from the angular distribution of the ${\rmD^{*+}} \to {\rm D}^0 (\to {\rm K}^{-}\pi^{+}) \pi^+$ decay products, in the${\rm D^{*+}}$ rest frame, with respect to the ${\rm D^{*+}}$ momentumdirection in the pp centre of mass frame. The $\rho_{00}$ value for prompt${\rm D^{*+}}$ mesons is consistent with $1/3$, which implies no spinalignment. However, for non-prompt ${\rm D^{*+}}$ mesons an evidence of$\rho_{00}$ larger than $1/3$ is found. The measured value of the spin densityelement is $\rho_{00}=0.455\pm0.022(\text{stat.})\pm0.035(\text{syst.})$ in the$5 < p_{\rm T} < 20$ GeV/$c$ interval, which is consistent with a PYTHIA 8Monte Carlo simulation coupled with the EVTGEN package, which implements thehelicity conservation in the decay of ${\rm D^{*+}}$ meson from beauty mesons.In non-central heavy-ion collisions, the spin of the ${\rm D^{*+}}$ mesons maybe globally aligned with the direction of the initial angular momentum andmagnetic field. Based on the results for pp collisions reported in this letterit is shown that alignment of non-prompt ${\rm D^{*+}}$ mesons due to thehelicity conservation coupled to the collective anisotropic expansion may mimicthe signal of global spin alignment in heavy-ion collisions.

First measurement of prompt and non-prompt ${\rm D^{*+}}$ vector meson spin alignment in pp collisions at $\sqrt{s} = 13$ TeV