Evidence of spin-orbital angular momentum interactions in relativistic heavy-ion collisions

The first evidence of spin alignment of vector mesons ($K^{*0}$ and $\phi$) 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 ($\sqrt{s_{\rm NN}}$) 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 ($p_{\rm T} <~$ 2 GeV/$c$) for $K^{*0}$ and $\phi$ at a level of 3$\sigma$ and 2$\sigma$, respectively. No significant spin alignment is observed for the $K^0_S$ meson (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.

 

Phys. Rev. Lett. 125 (2020) 012301
HEP Data
e-Print: arXiv:1910.14408 | PDF | inSPIRE
CERN-EP-2019-251
Figure group

Figure 1

(Color online) 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 √s = 13 TeV and Pb–Pb collisions at √sNN = 2.76 TeV. Panels (a) and (b) show results for K∗0 and φ with respect to EP and PP. Panel (c) is the results for vector mesons in pp collisions with respect to PP

Figure 2

(Color online) Transverse momentum dependence of ρ00 for K∗0, φ , and K0 S mesons at |y| < 0.5 in Pb–Pb collisions at √sNN = 2.76 TeV and minimum bias pp collisions at √s = 13 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 φ (right column). The statistical and systematic uncertainties are shown as bars and boxes, respectively

Figure 3

Measurements of \rh as a function of $\langle N_{\mathrm {part}} \rangle$ for \kst and \ph mesons at low and high \pt in \pb collisions. The statistical and systematic uncertainties are shown as bars and boxes, respectively. Some data points are shifted horizontally for better visibility.

Figure 4

(Color online) ρ00 values from data in 10–50% Pb–Pb collisions at 0.8 < pT < 1.2 GeV/c with respect to various planes compared with expectations from model simulations with and without added elliptic flow (v2). The statistical and systematic uncertainties are shown as bars and boxes, respectively.