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 1
Invariant-mass distributions $\Delta M$ of \Dstar candidates with $4 < p_{\rm T} < 6$ GeV/$c$ and $0.0 < |\cos \theta^{*}|< 0.2$ (left panel), and with $15 < p_{\rm T} < 30$ GeV/$c$ and $0.8 < |\cos \theta^{*}|< 1.0$ (right panel) in the rapidity interval $0.3< |y| < 0.8$ for the 30$-$50% centrality class. |  |
Figure 2
Acceptance and efficiency corrected angular distributions of the decaying pion in the rest frame of the D$^*$ candidates with $4 < p_{\rm T} < 6$ GeV/$c$ and with $15 < p_{\rm T} < 30$ GeV/$c$ in the rapidity interval $0.3 < |y| < 0.8$ for Pb$-$Pb collisions in the 30$-$50% centrality class. The distributions are fitted with the function from Eq.1. Only statistical uncertainties are shown. |  |
Figure 3
Left: raw yield as a function of the BDT selection and extracted contributions from prompt and non-prompt D$^*$ mesons with $4 < p_{\rm T} < 6$ GeV/$c$ and $|y| < 0.8$ in the 30$-$50% centrality class. Right: measured fraction of prompt D$^*$ mesons as a function of $p_{\rm T}$ in the 0$-$10% and 30$-$50% centrality classes. Only the statistical uncertainties are reported. |  |
Figure 4
Left: the spin density matrix element $\rho_{00}$ of prompt D$^*$ mesons as a function of $p_{\rm T}$ in the rapidity interval $0.3 < |y|< 0.8$ for central (0$-$10%) and midcentral (30$-$50%) Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Right: $\rho_{00}$ of prompt D$^*$ mesons as a function of $p_{\rm T}$ in the two rapidity intervals for midcentral (30$-$50%) Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ 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 $\rho_{00}$ of prompt D$^*$ mesons as a function of $p_{\rm T}$ in the rapidity interval $0.3 < |y| < 0.8$ compared to that of inclusive J$/\psi$ mesons in the rapidity interval $2.5< y< 4$ measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV in the 30$-$50% centrality class. The bottom panel shows the deviation of the measurements from the null hypothesis ($\rho_{00} = 1/3$) in units of the total uncertainties $\sigma_{\rho_{00}}$. |  |