First measurement of D$^{*+}$ vector spin alignment in Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 5.02}$ TeV

TL;DR

This study investigates heavy-flavor spin polarization in ultrarelativistic heavy-ion collisions by measuring the prompt $D^{*+}$ spin alignment relative to the axis perpendicular to the reaction plane in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Using the decay chain $D^{*+}\to D^0\pi^+$ with $D^0\to K^-\pi^+$, the analysis extracts the spin-density matrix element $\rho_{00}$ from the angular distribution $dN/d\cos\vartheta^*$, correcting for acceptance, event-plane resolution, and the mixture of prompt/non-prompt sources via a multivariate (BDT) approach. The result shows $\rho_{00}$ consistent with $\tfrac{1}{3}$ for central collisions, while in midcentral (30--50%) events at $0.3<|y|<0.8$ and $15<p_{\mathrm{T}}<30$ GeV/$c$, $\rho_{00}$ exceeds $\tfrac{1}{3}$ with a significance of $3.1\sigma$, suggesting a possible early-time polarization of charm quarks. These findings provide crucial input for theoretical models of charm-quark polarization and hadronization under strong vorticity and magnetic-field effects and motivate further Run 3 measurements of charm-hadron spin alignment.

Abstract

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 $ρ_{00}$ 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 $ρ_{00}>1/3$ has been found for $p_{\rm T}>15$ GeV/$c$ and $0.3<|y|<0.8$ with a significance of $3.1σ$. The measured spin alignment of prompt D$^{*+}$ mesons is compared with the one of inclusive J$/ψ$ mesons measured at forward rapidity ($2.5 < y < 4$).

Figures (5)

• Figure 1: Invariant-mass distributions $\Delta M$ of $\mathrm{D^{*+}}$ candidates with $4<p_\mathrm{T}\xspace<6~\mathrm{GeV}\xspace/c$ and $0.0<|\cos{\vartheta^*}\xspace|<0.2$ (left panel), and with $15<p_\mathrm{T}\xspace<30~\mathrm{GeV}\xspace/c$ and $0.8<|\cos{\vartheta^*}\xspace|<1.0$ (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 $\mathrm{D^{*+}}$ candidates with $4<p_\mathrm{T}\xspace<6~\mathrm{GeV}\xspace/c$ and with $15<p_\mathrm{T}\xspace<30~\mathrm{GeV}\xspace/c$ 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. \ref{['eq:dndcost']}. 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 $\mathrm{D^{*+}}$ mesons with $4<p_\mathrm{T}\xspace<6~\mathrm{GeV}\xspace/c$ and $|y|<0.8$ in the 30--50% centrality class. Right: measured fraction of prompt $\mathrm{D^{*+}}$ mesons as a function of $p_\mathrm{T}$ 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 $\mathrm{D^{*+}}$ mesons as a function of $p_\mathrm{T}$ in the rapidity interval $0.3 < |y| < 0.8$ for central (0--10%) and midcentral (30--50%) Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02~\mathrm{TeV}\xspace$. Right: $\rho_{00}$ of prompt $\mathrm{D^{*+}}$ mesons as a function of $p_\mathrm{T}$ in the two rapidity intervals for midcentral (30--50%) Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02~\mathrm{TeV}\xspace$. 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 $\mathrm{D^{*+}}$ mesons as a function of $p_\mathrm{T}$ in the rapidity interval $0.3 < |y| < 0.8$ compared to that of inclusive $\mathrm{J}/\uppsi$ mesons in the rapidity interval $2.5<y<4$ measured in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02~\mathrm{TeV}\xspace$ in the 30--50% centrality class. The bottom panel shows the deviation of the measurements from the null hypothesis ($\rho_{00}\xspace = 1/3$) in units of the total uncertainties $\sigma_{\rho_{00}\xspace}$.