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

The longitudinal spin transfer, $D_{LL}$, from high energy polarized protons to $\Lambda$ and $\overline{\Lambda }$ hyperons has been measured for the first time in proton-proton collisions at $\sqrt{s}=200\mathrm{GeV}$ with the STAR detector at the Relativistic Heavy Ion Collider. The measurements cover pseudorapidity, $\eta$, in the range $|\eta |<1.2$ and transverse momenta, $p_{\mathrm{T}}$, up to $4\mathrm{GeV}/c$. The longitudinal spin transfer is found to be $D_{LL}=-0.03\pm 0.13(\mathrm{stat})\pm 0.04(\mathrm{syst})$ for inclusive $\Lambda$ and $D_{LL}=-0.12\pm 0.08(\mathrm{stat})\pm 0.03(\mathrm{syst})$ for inclusive $\overline{\Lambda }$ hyperons with $⟨\eta ⟩=0.5$ and $⟨p_{\mathrm{T}}⟩=3.7\mathrm{GeV}/c$. The dependence on $\eta$ and $p_{\mathrm{T}}$ is presented.

Figure 1

(a) The invariant mass distribution of $\Lambda$ (filled circles) and $\overline{\Lambda }$ (open circles) candidates from reconstructed $p+{\pi }^{-}$ and $\overline{p}+{\pi }^{+}$ track pairs in 2005 MB data after topological selections. (b) The invariant mass distribution versus $\cos {\theta }^{*}$ for $\Lambda$.

Figure 2

The spin transfer $D_{LL}$ versus $\cos {\theta }^{*}$ for (a) $\Lambda$ and (b) $\overline{\Lambda }$ hyperons, and (c) the spin asymmetry ${\delta }_{LL}$ for the control sample of $K_S^0$ mesons versus $\cos {\theta }^{*}$. The filled circles show the results for positive pseudorapidities $\eta$ with respect to the polarized beam and the open circles show the results for negative $\eta$. Only statistical uncertainties are shown. The data points with negative $\eta$ have been shifted slightly in $\cos {\theta }^{*}$ for clarity. The indicated values of ${\chi }^2$ and the spin transfer are for the data with positive and negative $\eta$, respectively.

Figure 3

The spin transfer $D_{LL}$ to (a) $\Lambda$ and (b) $\overline{\Lambda }$ hyperons produced at positive pseudorapidity with respect to the polarized proton beam from MB, JP, and HT data versus hyperon transverse momenta $p_{\mathrm{T}}$. The sizes of the statistical and systematic uncertainties are indicated by the vertical bars and bands, respectively. For clarity, the HT data points have been shifted slightly in $p_{\mathrm{T}}$. The dotted vertical lines indicate the $p_{\mathrm{T}}$ intervals in the analysis of HT and JP data.

Figure 4

Comparison of $\Lambda$ and $\overline{\Lambda }$ spin transfer $D_{LL}$ in polarized proton-proton collisions at $\sqrt{s}=200\mathrm{GeV}$ for (a) positive and (b) negative $\eta$ versus $p_{\mathrm{T}}$. The vertical bars and bands indicate the sizes of the statistical and systematic uncertainties, respectively. The $\overline{\Lambda }$ data points have been shifted slightly in $p_{\mathrm{T}}$ for clarity. The dotted vertical lines indicate the $p_{\mathrm{T}}$ intervals in the analysis of HT and JP data. The horizontal lines show model predictions evaluated at $\eta$ and largest $p_{\mathrm{T}}$ of the data.