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Updated on Wed, 2024-10-16 06:10. Originally created by gty on 2024-09-13 22:04.

Paper title: Measurement of transverse polarization of $\Lambda/\overline{\Lambda}$ within jet in $pp$ collisions at $\sqrt{s}$ =200 GeV

PAs: Taoya Gao, Qinghua Xu, Yi Yu, Jinlong Zhang

Target Journal: Phys. Rev. L

Analysis note:
Please find here.

Abstracts:

We report the first measurement of $\Lambda$ and $\overline{\Lambda}$ polarization transverse to their production plane within jets in unpolarized proton-proton collisions at $\sqrt{s}$ = 200 GeV. The polarization was measured as a function of jet transverse momentum ($p_T$), the fraction of jet momentum carried by $\Lambda$/$\overline{\Lambda}$ ($z$), and the transverse momentum of $\Lambda/\overline{\Lambda}$ relative to the jet axis ($j_T$). Significant polarization was observed, with positive values for $\Lambda$ and negative values for $\overline{\Lambda}$, showing a clear dependence on jet $p_T$ and $z$. These results provide new insights into the mechanism of $\Lambda$ spontaneous polarization and offer experimental constraints for the extraction of polarizing Fragmentation Functions.

Figures:


Figure 1:The schematic of $\Lambda$ inside jet at pp collisions. The polarization direction $\vec{S}$ was defined by jet and $\Lambda$ momentum.



Figure 2:Transverse polarization of $\Lambda$ and $\overline{\Lambda}$ as a function of jet $p_T$ in unpolarized $pp$ collisions at $\sqrt{s}$ =200 GeV at STAR. Statistical uncertainties are shown as vertical bars. Systematic uncertainties are shown as boxes.



Figure 3:Transverse polarization of $\Lambda$,and $\overline{\Lambda}$ as a function of $z$ at different jet $p_T$ ranges of $6 < p_T^{jet} < 8.4$ GeV (left), $8.4 < p_T^{jet} 12$ GeV (middle) and $ p_T^{jet} > 12$ GeV (right). Statistical uncertainties are shown as vertical bars. Systematic uncertainties are shown as boxes.


Figure 4: Transverse polarization of $\Lambda$,and $\overline{\Lambda}$ as a function of $j_T$ at different jet $p_T$ ranges of $6 < p_T^{jet} < 8.4$ GeV (left), $8.4 < p_T^{jet} 12$ GeV (middle) and $ p_T^{jet} > 12$ GeV (right). Statistical uncertainties are shown as vertical bars. Systematic uncertainties are shown as boxes.


Conclusions:

  • Our analysis is the first measurement of transverse polarization of $\Lambda$ and $\overline{\Lambda}$ within jet in unpolarized $pp$ collisions at $\sqrt{s}$ =200 GeV.
  • Significant polarizations of $\Lambda$ and $\overline{\Lambda}$ are observed with clear dependence on jet $p_T$.
  • The $z$ and $j_T$ dependence of polarization are measured, and visible $z$ dependencies are observed for medium to high jet $p_T$.
  • These measurements provide important constraints on polarizing Fragmentation Functions.



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