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Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example of the simultaneous fit to the mass spectrum and $\langle \cos\theta^{*}\sin(2\phi-2\Psi_2) \rangle^{S+B}$ for the multiplicity range 185 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 250. Vertical bars show the statistical uncertainties.
png pdf	Figure 1-a: Example of the simultaneous fit to the mass spectrum and $\langle \cos\theta^{*}\sin(2\phi-2\Psi_2) \rangle^{S+B}$ for the multiplicity range 185 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 250. Vertical bars show the statistical uncertainties.
png pdf	Figure 1-b: Example of the simultaneous fit to the mass spectrum and $\langle \cos\theta^{*}\sin(2\phi-2\Psi_2) \rangle^{S+B}$ for the multiplicity range 185 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 250. Vertical bars show the statistical uncertainties.
png pdf	Figure 2: The second-order Fourier sine coefficients of $\Lambda$ , $\overline{\Lambda}$ and $\Lambda$ + $\overline{\Lambda}$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV. Results from EPOS LHC simulations are shown as hollow markers. Vertical bars present statistical uncertainties in both the measurements and the simulations. Shaded areas show systematic uncertainties in the measurements. The $N_\mathrm{trk}^\mathrm{offline}$ values of $\Lambda$ + $\overline{\Lambda}$ results are shifted for better visibility.
png pdf	Figure 3: The second-order Fourier sine coefficients of $\Lambda$ + $\overline{\Lambda}$ polarization along the beam direction as functions of $p_{\mathrm{T}}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV with 3 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 60, 60 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 120 and 185 $\leq N_\mathrm{trk}^\mathrm{offline} <$ 250. Vertical bars show statistical uncertainties. Shaded areas show systematic uncertainties.
png pdf	Figure 4: The second-order Fourier sine coefficients of $\mathrm{K^0_S}$ , $\Lambda$ , $\overline{\Lambda}$ and $\Lambda$ + $\overline{\Lambda}$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV. Vertical bars show statistical uncertainties. Shaded areas show systematic uncertainties. The $N_\mathrm{trk}^\mathrm{offline}$ values of $\Lambda$ + $\overline{\Lambda}$ results are shifted for better visibility.
png pdf	Figure 5: The second-order Fourier sine coefficients of $\Lambda$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV extracted with tracker and HF event planes. Vertical bars show statistical uncertainties. Shaded areas show systematic uncertainties.
png pdf	Figure 6: The second-order Fourier sine coefficients of $\overline{\Lambda}$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV extracted with tracker and HF event planes. Vertical bars show statistical uncertainties. Shaded areas show systematic uncertainties.
png pdf	Figure 7: The second-order Fourier sine coefficients of $\Lambda$ + $\overline{\Lambda}$ polarization along the beam direction as functions of $N_\mathrm{trk}^\mathrm{offline}$ in pPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=$ 8.16 TeV extracted with tracker and HF event planes. Vertical bars show statistical uncertainties. Shaded areas show systematic uncertainties.

Tables

png pdf

Table 1:
The average multiplicity before (and after) corrections,

$\langle N_\mathrm{trk}^\mathrm{offline}\rangle$ (

$\langle N_\text{trk}^\text{corrected} \rangle$ ) with track

$p_{\mathrm{T}} >$ 0.4 GeV and

$|\eta_{\mathrm{lab}}| <$ 2.4 in each multiplicity interval of pPb collisions containing at least one reconstructed

$\Lambda$ or

$\overline{\Lambda}$ candidate. The uncertainties reported for

$\langle N_\text{trk}^\text{corrected} \rangle$ are systematic uncertainties, as the statistical uncertainties have been found to be negligible.

Summary

In summary, the first measurements of the second-order sine Fourier coefficients

$P_{z,s2}$ of hyperon polarization along the beam direction in proton-lead (pPb) collisions at

$\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}$ = 8.16 TeV as functions of multiplicity and transverse momentum are presented. Significant positive

$P_{z,s2}$ values are observed for

$\Lambda$ \ and

$\overline{\Lambda}$ \ particles over the entire multiplicity range from 3 to 250. An increasing trend with transverse momentum is observed for

$P_{z,s2}$ , while the values decrease with increasing particle multiplicity. The

$p_{\mathrm{T}}$ -dependence of

$P_{z,s2}$ measurements tends to follow the trend of the second-order collective flow in pPb\ collisions, while the multiplicity dependence does not. The observed signal might indicate that besides the complex vorticity structures due to expansion of the fluid-like QGP medium, alternative physics mechanisms such as the Polarizing Fragmentation Functions effects may contribute to the hyperon polarization along the beam direction in pPb\ collisions.

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