Measurement of transverse polarization of $Λ$ and $\barΛ$ hyperons inside jets in $pp$ collisions at $\sqrt{s}=200$ GeV

TL;DR

This paper delivers the first measurement of transverse $Λ$ and $\overline{Λ}$ polarization inside jets in unpolarized $pp$ collisions at $\sqrt{s}=200$ GeV, directly isolating the polarizing fragmentation function (PFF). Using STAR data and mixed-event corrections, the analysis reveals a pronounced jet-$p_T$ dependence, with $P_{Λ}$ crossing from negative to positive, while $P_{\overline{Λ}}$ stays largely negative, and explores $z$ and $j_T$ dependencies to constrain gluon fragmentation. Model comparisons, including three BELLE-informed DGMZ scenarios, show reasonable agreement at low $p_T$ but notable discrepancies at higher $p_T$ and $j_T$, highlighting the crucial role of the gluon PFF and potential TMD evolution effects. The results provide the first constraints on the gluon PFF in $pp$ collisions and demonstrate the value of unpolarized jet measurements for testing PFF universality, with implications for future EIC and multi-process studies.

Abstract

A surprisingly large transverse polarization of $Λ$ hyperons in unpolarized hadron-nucleon/nucleus collisions has been observed for 50 years, and the origin of this polarization remains an important open question. Recently, theoretical frameworks have been advanced in understanding this puzzle with the polarizing fragmentation function (PFF). We report the first measurement of $Λ$ and $\overlineΛ$ transverse polarization inside jets in unpolarized proton-proton collisions, which is directly attributed to the PFF. The polarization is measured as a function of the jet transverse momentum ($p_{\text{T}}$), the fraction of the jet momentum carried by $Λ$($\overlineΛ$) hyperons, and the transverse momentum of $Λ(\overlineΛ)$ hyperons relative to the jet axis. $Λ$ polarization shows a clear dependence on the jet $p_{\text{T}}$, while $\overlineΛ$ polarization mostly remains negative. Covering a wide jet-energy range, these data provide the first constraints on the gluon PFF and allow tests of TMD evolution and its universality.

Figures (4)

• Figure 1: Schematic of $\Lambda(\overline{\Lambda})$ inside a jet in $pp$ collisions. The polarization direction $\hat{\boldsymbol{S}}$ is defined by the jet and $\Lambda$ momenta.
• Figure 2: Transverse polarization of $\Lambda$ and $\overline{\Lambda}$ hyperons as a function of jet $p_{\text{T}}$ in unpolarized $pp$ collisions at $\sqrt{s}$ = 200 GeV at RHIC. The red and blue dashed lines are linear fits to the data points. 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 $p_{\text{T}}$ ranges [$6.2, 8.5$] GeV$/c$ (left), [$8.5, 11.9$] GeV$/c$ (middle) and $p_{\text{T}}^{jet} > 11.9$ GeV$/c$ (right). Statistical uncertainties are shown as vertical bars. Systematic uncertainties are shown as boxes. The red and blue curves show model calculations for $\Lambda$ and $\overline{\Lambda}$ respectively from Ref. DAlesio:2024ope. The average polarization in each jet $p_{\text{T}}$ range is also shown with total uncertainties.
• Figure 4: Transverse polarization of $\Lambda$ and $\overline{\Lambda}$ hyperons as a function of $j_{\text{T}}$ for different jet $p_{\text{T}}$ ranges of [$6.2, 8.5$] GeV$/c$ (left), [$8.5, 11.9$] GeV$/c$ (middle) and $p_{\text{T}}^{jet} > 11.9$ GeV$/c$ (right). Statistical uncertainties are shown as vertical bars. Systematic uncertainties are shown as boxes. The red and blue curves show model calculations for $\Lambda$ and $\overline{\Lambda}$ respectively from Ref. DAlesio:2024ope.