Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

The first evidence of spin alignment of vector mesons ($K^{*0}$ and $\varphi$) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ${\rho }_{00}$ is measured at midrapidity ($|y|<0.5$) in Pb-Pb collisions at a center-of-mass energy ($\sqrt{s_{\mathrm{NN}}}$) of 2.76 TeV with the ALICE detector. ${\rho }_{00}$ values are found to be less than $1/3$ ($1/3$ implies no spin alignment) at low transverse momentum ($p_T<2\mathrm{GeV}/c$) for $K^{*0}$ and $\varphi$ at a level of $3\sigma$ and $2\sigma$, respectively. No significant spin alignment is observed for the $K_S^0$ meson ($\text{spin}=0$) in Pb-Pb collisions and for the vector mesons in $pp$ collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.

Figure 1

Angular distribution of the decay daughter in the rest frame of the meson with respect to the quantization axis at $|y|<0.5$ for $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$. Panels (a) and (b) show results for $K^{*0}$ and $\varphi$ with respect to EP and PP. Panel (c) is the results for vector mesons in $pp$ collisions with respect to PP.

Figure 2

Transverse momentum dependence of ${\rho }_{00}$ for $K^{*0}$, $\varphi$, and $K_S^0$ mesons at $|y|<0.5$ in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ and minimum bias $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$. Results are shown for spin alignment with respect to the event plane [panels (a),(b)], production plane [(c),(d)], and random event plane [(e),(f)] for $K^{*0}$ (left column) and $\varphi$ (right column). The statistical and systematic uncertainties are shown as bars and boxes, respectively.

Figure 3

Measurements of ${\rho }_{00}$ as a function of $⟨N_{\text{part}}⟩$ for $K^{*0}$ and $\varphi$ mesons at low and high $p_T$ in Pb-Pb collisions. The statistical and systematic uncertainties are shown as bars and boxes, respectively. Some data points are shifted horizontally for better visibility.