Figures from paper submitted to Journal of High Energy Physics

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 ($2.5 <~ y <~ 4$).

JHEP 10 (2025) 094
e-Print: arXiv:2504.00714 | PDF | inSPIRE