Probing vorticity structure in heavy-ion collisions by local Λ polarization

We study the local structure of the vorticity field and the Λ polarization in Au+Au collisions in the energy range √sNN=7.7--2000.28em0exGeV and Pb+Pb collisions at √sNN=27600.28em0exGeV using a multiphase transport model. We focus on the vorticity field arising from the nonuniform expansion of the fireball, which gives the circular structure of the transverse vorticity ω_⊥=(ωₓ,ωy) around the z direction as well as the quadrupole pattern of the longitudinal vorticity ωz in the transverse plane. As a consequence, the three components of the polarization vector P=(Pₓ,Py,Pz) for Λ hyperons show harmonic behaviors as sgn(Y)sinφₚ, -sgn(Y)cosφₚ, and -sin(2φₚ), where φₚ and Y are the azimuthal angle and rapidity in momentum space. These patterns of the local Λ polarization are expected to be tested in future experiments.