First measurement of prompt and non-prompt D⁎+ vector meson spin alignment in pp collisions at s=13 TeV

This letter reports the first measurement of spin alignment, with respect to the helicity axis, for D*(+) vector mesons and their charge conjugates from charm-quark hadronisation (prompt) and from beauty-meson decays (non-prompt) in hadron collisions. The measurements were performed at midrapidity (vertical bar y vertical bar < 0.8) as a function of transverse momentum (p(T)) in proton-proton (pp) collisions collected by ALICE at the centre-of-mass energy root s= 13TeV. The diagonal spin density matrix element rho(00) of D*(+) mesons was measured from the angular distribution of the D*(+) -> D-0(-> K- pi(+))pi(+) decay products, in the D*(+) rest frame, with respect to the D*(+) momentum direction in the pp centre of mass frame. The rho(00) value for prompt D*(+) mesons is consistent with 1/3, which implies no spin alignment. However, for nonprompt D*(+) mesons an evidence of rho(00) larger than 1/3 is found. The measured value of the spin density element is rho(00)= 0.455 +/- 0.022(stat.) +/- 0.035(syst.) in the 5 < p(T) < 20GeV/c interval, which is consistent with a Pythia 8 Monte Carlo simulation coupled with theEvtGenpackage, which implements the helicity conservation in the decay of D*+ meson from beauty mesons. In non-central heavy-ion collisions, the spin of the D*(+) mesons may be globally aligned with the direction of the initial angular momentum and magnetic field. Based on the results for pp collisions reported in this letter it is shown that alignment of non-prompt D*(+) mesons due to the helicity conservation coupled to the collective anisotropic expansion may mimic the signal of global spin alignment in heavy-ion collisions. (c) 2023 European Organization for Nuclear Research. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).