CMS-PAS-HIN-24-002

CMS-PAS-HIN-24-002
Azimuthal dependence of hyperon polarization along the beam direction in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 8.16 TeV
CMS Collaboration
3 June 2024

Abstract: The polarization of the $\Lambda$ and $\overline{\Lambda}$ hyperons along the beam ( $z$ ) direction, $P_z$ , has been measured for the first time in pPb collisions at the LHC with $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV data collected by the CMS experiment. Assuming the largest contributions to $P_z$ come from collective flow, this polarization can be characterized by the Fourier sine coefficients $P_{z,sn} = \langle P_{z}\sin(n\phi - n\Psi_{n}) \rangle$ , where $\phi$ is the hyperon azimuthal emission angle and $\Psi_{n}$ is the $n$ -th order flow plane angle. The second order Fourier sine coefficient $P_{z,s2}$ is reported for $\Lambda$ and $\overline{\Lambda}$ particles as a function of transverse momentum and event multiplicity. A significant positive $P_{z,s2}$ signal is observed for 0.8 $< p_{\mathrm{T}} <$ 6.0 GeV/ $c$ over the entire multiplicity range (from 3 to 250 charged particles). The measured $P_{z,s2}$ values increase as a function of $p_{\mathrm{T}}$ and decrease as a function of multiplicity. These results can provide new constraints on the potential creation of quark-gluon plasma droplets and the spin polarization mechanism in pp and pA systems.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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