6) Paper proposal

Title:

Transverse spin transfer of Lambda and Anti-Lambda Hyperons in Polarized proton-proton collisonns at \sqrt{s}=200 GeV at RHIC

PAs: Jincheng Mei，Qinghua Xu

Proposed Target Journal: Phys. Rev. D

Abstract:

The transverse spin transfer from polarized protons to Λ and Λ̄ hyperons is expected to provide sensitivity to the transversity distribution of the nucleon and to the transversely polarized fragmen- tation functions. We report the first measurement of the transverse spin transfer to Λ and Λ̄ along the polarization direction of the fragmenting quark, D_TT, in transversely polarized proton-proton collisions at sqrt{s} = 200 GeV with the STAR detector at RHIC. The data correspond to an integrated luminosity of 18 pb−1 and cover the pseudorapidity range |η| < 1.2 and transverse momenta p_T up to8 GeV/c. The dependence on p_T and η are presented. The D_TT results are found to be comparable with a model prediction, and are also consistent with zero within uncertainties.

Figures:

FIG. 1: The invariant mass distribution for Lambda (open circles) and anti-Lambda (filled circles) candidates for trigger combined sample after selections with 1 <$p_{\mathrm{T}}$ < 8 GeV/c in this analysis.

FIG. 2: The invariant mass distribution versus cos\theta^{*} for Lambda candidates in the jet near-side with 1< p_T < 8 GeV/c in this analysis as an example.

FIG. 3: The spin transfer $D _{TT}$ versus cos for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons, and c) the spin asymmetry $\delta_{TT}$ for the control sample of $K_S^0$ mesons versus cos/theta in the $p_T$ bin of (2,3) GeV/c for triggered combined sample. The red circles show the results for positive pseudo-rapidity $\eta$ with respect to the polarized beam and the blue squares show the results for negative $\eta$. Only statistical uncertainties are shown.

FIG. 4: The spin transfer $D_\mathrm{TT}$ for $\Lambda$ and $\bar{\Lambda}$ versus $p_\mathrm{T}$ in polarized proton-proton collisions at $\sqrt{s}=200\,\mathrm{GeV}$ at STAR, in comparison with model predictions for (a) positive $\eta$ and (b) negative $\eta$. The vertical bars and bands indicate the sizes of the statistical and systematic uncertainties, respectively. The $\bar{\Lambda}$ results have been offset to slightly larger $p_T$ values for clarity.

Tables:

TABLE I: Summary of selection cuts and the Λ and Λ̄ candidate counts and the residual background fractions in each pTbin. Here “DCA” denotes distance of closest approach, and N(σ) quantitatively measures the distance of a particle track to a certain particle band in dE/dx vs. rigidity space[28]. \ver{l} is representative of the vector from PV to Λ decay point and p⃗ is the reconstructed momentum of Λ.

Summary:
In summary, we report the first measurement on the transverse spin transfer, D_TT, to Λ and Λ ̄ in transversely polarized proton-proton collisions at \sqrt{s} = 200 GeV at RHIC. The data correspond to an integrated luminosity of 18 pb⁻¹ taken at STAR experiment in the year of 2012, which cover mid-rapidity, |η| < 1.2 and p_T up to 8GeV/c. The DTT value and precision at the highest pT bin, where the effects are expected to be largest, are found to be D_TT = 0.031 ± 0.033(stat.) ± 0.008(sys.) for Λ and D_TT = −0.034 ± 0.040(stat.) ± 0.009(sys.) for Λ ̄ at ⟨η⟩ = 0.5 and ⟨p_T⟩ = 6.7 GeV/c. The results for D_TTare found to be consistent with zero for Λ and Λ̄ within uncertainties, and are also consistent with model predictions.