Polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ Hyperons along the Beam Direction in Pb-Pb Collisions at ${\sqrt{s}}_{NN}=5.02\mathrm{TeV}$

The polarization of the $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons along the beam ($z$) direction, $P_z$, has been measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to $P_z$ comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient $P_{z,s2}=⟨P_z\sin (2\phi -2{\mathrm{\Psi }}_2)⟩$, where $\phi$ is the hyperon azimuthal emission angle and ${\mathrm{\Psi }}_2$ is the elliptic flow plane angle. We report the measurement of $P_{z,s2}$ for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The $P_{z,s2}$ is positive similarly as measured by the STAR Collaboration in Au-Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon $P_z$ in Pb-Pb collisions at the LHC. The comparison of the measured $P_{z,s2}$ with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.

Figure 1

Fit to the invariant mass dependence of the $⟨\cos {\theta }_p^{*}\sin (2\phi -2{\mathrm{\Psi }}_2^{\mathrm{EP}})⟩$ for $\overline{\mathrm{\Lambda }}$ before event-plane resolution correction using Eq. (8) in the 30%–40% centrality class. See the text for details.

Figure 2

Centrality dependence of $P_{z,s2}$ averaged for $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ and its comparison with the RHIC results for Au-Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The model calculations [38] for $\mathrm{\Lambda }$ and strange quark for Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ using the approach described in Ref. [23] are shown by dash-dotted lines.

Figure 3

Transverse momentum dependence of $P_{z,s2}$ averaged for $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in semicentral collisions and its comparison with the similar RHIC results for Au-Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The model calculations [38] for $\mathrm{\Lambda }$ and strange quark for Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in the 30%–50% centrality interval using the approach described in Ref. [23] are shown by dash-dotted lines.

Figure 4

The rapidity dependence of $P_{z,s2}$ averaged for $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ in semicentral collisions. The model calculations [38] for $\mathrm{\Lambda }$ and strange quark for Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ in the 30%–50% centrality interval using the approach described in Ref. [23] are shown by dash-dotted lines.