This letter reports the first measurement of spin alignment, with respect to the helicity axis, for ${\rm D^{*+}}$ 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 ($|y|<~0.8$) as a function of transverse momentum ($p_{\rm T}$) in proton-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 ${\rm D^{*+}} \to {\rm D}^0 (\to {\rm K}^{-}\pi^{+}) \pi^+$ decay products, in the ${\rm D^{*+}}$ rest frame, with respect to the ${\rm D^{*+}}$ momentum direction in the pp centre of mass frame. The $\rho_{00}$ value for prompt ${\rm D^{*+}}$ mesons is consistent with $1/3$, which implies no spin alignment. However, for non-prompt ${\rm D^{*+}}$ mesons an evidence of $\rho_{00}$ larger than $1/3$ is found. The measured value of the spin density element is $\rho_{00}=0.455\pm0.022(\text{stat.})\pm0.035(\text{syst.})$ in the $5 <~ p_{\rm T} <~ 20$ GeV/$c$ interval, which is consistent with a PYTHIA 8 Monte Carlo simulation coupled with the EVTGEN package, which implements the helicity conservation in the decay of ${\rm D^{*+}}$ meson from beauty mesons. In non-central heavy-ion collisions, the spin of the ${\rm D^{*+}}$ 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 ${\rm D^{*+}}$ mesons due to the helicity conservation coupled to the collective anisotropic expansion may mimic the signal of global spin alignment in heavy-ion collisions.
Phys. Lett. B 846 (2023) 137920
HEP Data
e-Print: arXiv:2212.06588 | PDF | inSPIRE
CERN-EP-2022-283
Figure group
Figure 2
Invariant-mass distributions $\Delta M$ for the charm-enhanced (left panel) and the beauty-enhanced (right panel) samples in pp collisions at $\s=13 \TeV$, selected with the high-multiplicity trigger. The distributions are obtained at midrapidity ($|y|<0.8$) in $5<\pt<7 \GeV/c$ and $0.0<\cost<0.2$. |  |
Figure 3
Acceptance and efficiency corrected angular distributions of the decay product in the rest frame of the \Dstar meson with respect to the helicity axis. The left panel shows the angular distribution for the charm-enhanced \Dstar sample in pp collisions at $\s=13 \TeV$, selected with the high-multiplicity trigger and the right panel shows the same but for the beauty-enhanced \Dstar sample. The distributions are obtained at midrapidity ($|y|<0.8$) in $5<\pt<7 \GeV/c$ and are fitted with Eq. \ref{eqn1} to extract the \rzz values. Only statistical uncertainties are shown. |  |
Figure 4
The spin density matrix element (\rzz) as a function of \fnonprompt in $5<\pt<7 \GeV/c$ for charm- and beauty-enhanced samples obtained for minimum-bias and high-multiplicity triggered pp collisions at $\s=13 \TeV$. Only statistical uncertainties are shown. The blue line represents the linear fit to the data with the blue band indicating its $1\sigma$ confidence interval. |  |
Figure 5
The spin density matrix element (\rzz) for prompt and non-prompt \Dstar mesons as a function of \pt in pp collisions at $\s=13 \TeV$. Measurements are carried out with respect to the helicity axis at $|y|<0.8$. Statistical and systematic uncertainties are represented by bars and boxes, respectively. The measurements are compared with the estimations from the QCD-based MC event generator \pythia + \evtgen . Model estimations are shown by colour bands where the width of the band corresponds to the statistical uncertainty in model calculations. |  |
Figure 6
Model simulation based on the MC event generator \pythia + \evtgen for the spin density matrix element (\rzz) for \Dstar mesons with respect to the direction perpendicular to the reaction plane as a function of \fnonprompt. Model simulations are carried out for \Dstar mesons at midrapidity ($|y|<0.8$) with two input values of $v_{2}$. |  |