The first measurement of prompt D$^{*+}$-meson spin alignment in ultrarelativistic heavy-ion collisions with respect to the direction orthogonal to the reaction plane is presented. The spin alignment is quantified by measuring the element $\rho_{00}$ of the diagonal spin-density matrix for prompt D$^{*+}$ mesons with $4<~p_{\rm T}<~30$ GeV/$c$ in two rapidity intervals, $|y|<~0.3$ and $0.3<~|y|<~0.8$, in central (0-10%) and midcentral (30-50%) Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Evidence of spin alignment $\rho_{00}>1/3$ has been found for $p_{\rm T}>15$ GeV/$c$ and $0.3<~|y|<~0.8$ with a significance of $3.1\sigma$. The measured spin alignment of prompt D$^{*+}$ mesons is compared with the one of inclusive J/$\psi$ mesons measured at forward rapidity ($2.5 <~ y <~ 4$).
Submitted to: JHEP
e-Print: arXiv:2504.00714 | PDF | inSPIRE
CERN-EP-2025-072
Figure group
Figure 2
Acceptance and efficiency corrected angular distributions of the decaying pion in the rest frame of the \Dstar candidates with $4\pt6 \GeV/c$ and with $15\pt30 \GeV/c$ in the rapidity interval $0.3|y|0.8$ for Pb--Pb collisions in the 30--50\% centrality class. The distributions are fitted with the function from Eq. ???. Only statistical uncertainties are shown. |  |
Figure 3
Left: raw yield as a function of the BDT selection and extracted contributions from prompt and non-prompt \Dstar mesons with $4\pt6 \GeV/c$ and $|y|0.8$ in the 30--50\% centrality class. Right: measured fraction of prompt \Dstar mesons as a function of \pt in the 0--10\% and 30--50\% centrality classes. Only the statistical uncertainties are reported. |  |
Figure 4
Left: the spin density matrix element \rzz of prompt \Dstar mesons as a function of \pt in the rapidity interval $0.3 |y| 0.8$ for central (0--10\%) and midcentral (30--50\%) Pb--Pb collisions at $\snn=5.02 \TeV$. Right: \rzz of prompt \Dstar mesons as a function of \pt in the two rapidity intervals for midcentral (30--50\%) Pb--Pb collisions at $\snn=5.02 \TeV$. The measurements are carried out with respect to the direction perpendicular to the reaction plane. Statistical and systematic uncertainties are represented by bars and boxes, respectively. |  |
Figure 5
The spin density matrix element \rzz of prompt \Dstar mesons as a function of \pt in the rapidity interval $0.3 |y| 0.8$ compared to that of inclusive \Jpsi mesons in the rapidity interval $2.5< y>4$ measured in Pb--Pb collisions at $\snn=5.02 \TeV$ in the 30--50\% centrality class. The bottom panel shows the deviation of the measurements from the null hypothesis ($\rzz = 1/3$) in units of the total uncertainties $\sigma_{\rzz}$. < /y> |  |