6) Paper proposal


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

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 pband cover the pseudorapidity range |η| < 1.2 and transverse momenta p_up to8 GeV/c. The dependence on p_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.


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.


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.

In summary, we report the first measurement on the transverse spin transfer, DTT, 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 pb1 taken at STAR experiment in the year of 2012, which cover mid-rapidity, |η| < 1.2 and pT up to 8GeV/cThe DTT value and precision at the highest pbin, where the effects are expected to be largest, are found to be DTT = 0.031 ± 0.033(stat.) ± 0.008(sys.) for Λ and DTT = 0.034 ± 0.040(stat.) ± 0.009(sys.) for Λ ̄ at η= 0.5 and pT= 6.7 GeV/cThe results for DTTare found to be consistent with zero for Λ and Λ̄ within uncertainties, and are also consistent with model predictions.
Paper draft and review:  
   Paper draft history
   Latest paper draft version:  paperDraft modified with PRD referee 

 PWGC review
   Collaboration review

Analysis Note:

   Analysis Note Draft

Support Materials:

   Web links:
  • Lambda reconstruction status
          1, Statistics of Lambda and anti-Lambda Reconstruction, Inclusive
 Statistics of Lambda and anti-Lambda Reconstruction, Jet near-side
  • MC production and data comparison
          1, MC production summary: hard_pT weight
MC and data comparison for inclusive hyperons
MC and data comparison for jet near-side hyperons
  • Trigger Bias

          1, Trigger bias parameters (fz_shift, feed-down fraction, fragmenting parton flavor fraction, subprocess fraction) plots 
Uncertainty to D_TT from trigger bias


Main Analysis Code: