Transverse spin transfer to $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons in polarized proton-proton collisions at $\sqrt{s}=200\mathrm{GeV}$

The transverse spin transfer from polarized protons to $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons is expected to provide sensitivity to the transversity distribution of the nucleon and to the transversely polarized fragmentation functions. We report the first measurement of the transverse spin transfer to $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ along the polarization direction of the fragmenting quark, $D_{\mathrm{TT}}$, in transversely polarized proton-proton collisions at $\sqrt{s}=200\mathrm{GeV}$ with the STAR detector at RHIC. The data correspond to an integrated luminosity of $18{\mathrm{pb}}^{-1}$ and cover the pseudorapidity range $|\eta |<1.2$ and transverse momenta $p_{\mathrm{T}}$ up to $8\mathrm{GeV}/c$. The dependence on $p_{\mathrm{T}}$ and $\eta$ are presented. The $D_{\mathrm{TT}}$ results are found to be comparable with a model prediction and are also consistent with zero within uncertainties.

Figure 1

Invariant mass distribution for $\mathrm{\Lambda }$ (open circles) and $\overline{\mathrm{\Lambda }}$ (filled circles) candidates with $1<p_{\mathrm{T}}<8\mathrm{GeV}/c$ after selection in this analysis.

Figure 2

Invariant mass distribution versus $\cos {\theta }^{*}$ for $\mathrm{\Lambda }$ candidates with $1<p_{\mathrm{T}}<8\mathrm{GeV}/c$ in this analysis as an example.

Figure 3

Spin transfer $D_{\mathrm{TT}}^{\text{raw}}$ versus $\cos {\theta }^{*}$ for (a) $\mathrm{\Lambda }$ and (b) $\overline{\mathrm{\Lambda }}$ hyperons, and (c) the spin asymmetry ${\delta }_{\mathrm{TT}}$ for the control sample of $K_S^0$ mesons versus $\cos {\theta }^{*}$ in the $p_{\mathrm{T}}$ bin of $(2,3)\mathrm{GeV}/c$. The red circles show the results for positive pseudorapidity $\eta$ with respect to the polarized beam, and the blue squares show the results for negative $\eta$. Only statistical uncertainties are shown.

Figure 4

Spin transfer $D_{\mathrm{TT}}$ for $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ versus $p_{\mathrm{T}}$ in polarized proton-proton collisions at $\sqrt{s}=200\mathrm{GeV}$ at STAR, in comparison with model predictions [15, 16] for (a) positive $\eta$ and (b) negative $\eta$. The vertical bars and hollow rectangles indicate the sizes of the statistical and systematic uncertainties, respectively. The $\overline{\mathrm{\Lambda }}$ results have been offset to slightly larger $p_{\mathrm{T}}$ values for clarity.