The spin alignment of vector mesons with light front quarks

The global spin alignment of the vector meson has been observed in relativistic heavy ion collisions, but is still on hot debates in the theoretical community. Here we propose to apply the light front framework to explain this phenomenon since the light front form explicitly describes the hadron spin including both the quark spin and the orbital angular momentum. After applying the light front spinor, we find that the spin alignment in the polarization of vector mesons with $\rho_{00}>1/3$ can be naturally manifested and in particular, the obtained spin alignment for $\phi$ meson is in good agreement with the experimental data. This implies that to explain the spin alignment it is important to properly include the contribution from the gluon interactions that are presented in terms of the orbital angular momentum of the hadron bound state.Comment: 7 pages, 2 figure

Collective flow and the fluid behavior in p/d/3^3He+Au collisions at sNN=200\sqrt{s_{NN}} = 200 GeV

By varying the intrinsic initial geometry, the p/d/$^3$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) provide a unique opportunity to understand the collective behavior in the small systems. In this paper, we employ the hybrid model iEBE-VISHNU with TRENTO initial conditions to study the collective flow and the fluid behavior in p/d/$^3$He+Au collisions. With fine-tuned parameters, iEBE-VISHNU can describe the $v_2(p_T)$ and $v_3(p_T)$ data from the PHENIX and STAR collaborations. However, for these parameter sets tuned to fit the STAR data, the hydrodynamic simulations have already beyond their limits with the average Knudsen number $\langle K_n \rangle$ obviously larger than one. Our calculations demonstrate that, for a meaningful evaluation of the fluid behavior in the small systems, model simulations should also pay attention to the validity range of hydrodynamics

Shear-induced spin polarization and “strange memory” in heavy-ion collisions

We discuss the theory of the spin polarizations induced by hydrodynamic gradients, which includes a newly discovered shear-induced polarization (SIP) term. In the phenomenological study using a hydrodynamic model, we discover that the local polarization contributed by SIP is substantial and has the “same sign” as the experimental measurements. Also, we find that the “sign” property of the local polarization observed in experiments seems to be related to “memory” effects on the polarizations of strange quarks in quark-gluon plasma