Polarization of $Λ$ and $\overlineΛ$ hyperons along the beam direction in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The polarization of the $\Lambda$ and $\overline\Lambda$ hyperons along the beam ($z$) direction, $P_{\rm z}$, has been measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02TeV recorded with ALICE at the Large Hadron Collider (LHC). The largest contribution to $P_{\rm z}$ comes from elliptic flow induced vorticity and can be characterized by the second Fourier sine coefficient $P_{\rm z,s2} = \langle P_{\rm z} \sin(2\varphi - 2 \Psi_{\rm 2}) \rangle$, where $\varphi$ is the hyperon azimuthal emission angle, and $\Psi_{\rm 2}$ is the elliptic flow plane angle. We report the measurement of $P_{\rm z,\,{\rm s2}}$ for different collision centralities, and in the 30-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The $P_{\rm z,\,{\rm s2}}$ is positive similarly as measured by the STAR Collaboration in Au-Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV, with somewhat smaller amplitude in the semi-central collisions. This is the first experimental evidence of a non-zero hyperon $P_{\rm z}$ in Pb-Pb collisions at the LHC. The comparison of the measured $P_{\rm z,\,{\rm s2}}$ 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.

 

Submitted to: PRL
e-Print: arXiv:2107.11183 | PDF | inSPIRE
CERN-EP-2021-148
Figure group

Figure 1

Fit to the invariant mass dependence of the $\mean{\cos(\theta^*_{\rm p}) \sin(2\varphi - 2\psitwo)}$ for $\overline\Lambda$ before event-plane resolution correction using Eq.~\ref{eq:Qfit} in the 30--40\% centrality class. See text for details.

Figure 2

Centrality dependence of $\mean{P_{\rm z} \sin(2\varphi - 2\psitwo)}$ averaged for \lam and $\overline\Lambda$ in Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV and it's comparison with the RHIC results for Au--Au collisions at $\sqrt{s_{\rm NN}} =$ 200 GeV. The model calculations~ for \lam and strange quark for Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV using the approach described in Ref.~ are shown by dashed-dotted lines.

Figure 3

Transverse momentum dependence of $\mean{P_{\rm z} \sin(2\varphi - 2\psitwo)}$ averaged for \lam and $\overline\Lambda$ in Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV in semi-central collisions and it's comparison with the similar RHIC results for Au--Au collisions at $\sqrt{s_{\rm NN}} =$ 200 GeV. The model calculations~ for \lam and strange quark for Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV in the 30--50\% centrality interval using the approach described in Ref.~ are shown by dashed-dotted lines.

Figure 4

The rapidity dependence of $\mean{P_{\rm z} \sin(2\varphi - 2\psitwo)}$ averaged for \lam and $\overline\Lambda$ in Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV in semi-central collisions. The model calculations~ for \lam and strange quark for Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 5.02 in the 30--50\% centrality interval using the approach described in Ref.~ are shown by dashed-dotted lines.