# 5) Paper proposal

Under:
Title:
Improved measurement of the longitudinal spin transfer to $\Lambda$ and $\bar{\Lambda}$
hyperons in polarized proton-proton collisions at $\sqrt{ s}$ = 200 GeV at RHIC

PAs:

Ramon Cendejas, Jian Deng, Jincheng Mei, Ernst Sichtermann, Qinghua Xu, and Jinlong Zhang

Proposed Target Journal:
PRD

Abstract:

The longitudinal spin transfer $D_{LL}$ of $\Lambda$ and $\bar{\Lambda}$ hyperons is expected to be sensitive to the helicity distribution function of strange quarks and anti-quarks, and to the longitudinally polarized fragmentation functions. We report an improved measurement on the longitudinal spin transfer of $\Lambda$ and $\bar{\Lambda}$ hyperons in proton-proton collisions at $\sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data are based on an approximately twelve times larger than that for our previously reported measurement and cover a kinematic range of $|\eta|<$ 1.2 in pseudo-rapidity and cover transverse momentum $p_T$ up to 6 GeV/c.  The dependences on $\eta$ and $p_T$ are presented and compared with model evaluations.

Figures:

Fig.1

Caption:
a) The invariant mass distribution for $\Lambda$ (red filled circles) and $\bar{\Lambda}$ (blue open circles) candidates with 3 $< p_T <$ 4 GeV/c in this analysis and b) the corresponding distribution versus the hyperon rest-frame angle $cos\theta^*$.

Fig.2

Caption:
The raw spin transfer $D _{LL}^{raw}$ versus cos for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons, and c) the spin asymmetry $\epsilon_{LL}$ for the control sample of $K_S^0$ mesons versus cos/theta  in the $p_T$ bin of (3,4) GeV/c for JP1 triggered sample. The red filled circles show the results for positive pseudorapidity $\eta$ with respect to the polarized beam and the blue open circles show the results for negative $\eta$. Only statistical uncertainties are shown.

Fig.3

Caption:
Comparison of spin transfer $D_{LL}$ for  for positive and negative $\eta$ versus $p_T$ for differently triggered samples in the present analysis, together with previously published results in the region of kinematic overlap. The results obtained with the L2JetHigh trigger have been offset to slightly larger $p_T$ values for clarity. The published results have been offset to slightly smaller $p_T$ values.

Fig.4

Caption:  Comparison of  spin transfer $D_{LL}$ with model predictions for positive $\eta$ versus $p_T$. The vertical bars and boxes indicate the sizes of the statistical and systematic uncertainties, respectively. The $\bar{\Lambda}$ results have been offset to slightly larger pT values for clarity.

Tables:
Tab 1

Caption: Summary of the selection cuts for \lla\ reconstruction for run9 Jet-Patch triggered sample and the corresponding $\Lambda$ ($\bar{\Lambda}$) counts and background fraction . Here DCA'' denotes distance of closest approach'' (to the primary vertex for single track by default) , and $\vec r$ denotes the vector from primary vertex to the V0 vertex and $\vec p$ denotes the momentum vector of V0.

Conclusion:

In summary, we report an improved measurement of the longitudinal spin transfer, $D_{LL}$, to $\Lambda$ hyperons and $\bar{\Lambda}$ anti-hyperons in longitudinally polarized proton-proton collisions at $\sqrt{s}$ = 200 GeV.  The data correspond to 19 pb$^{-1}$ with an average beam polarization of 57\% and were obtained with the STAR experiment in the year 2009.  The data cover -1.2 $<\eta<$ 1.2 and $p_T$ up to 6 GeV/c.  The longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $\pm$ 0.048 (stat) $\pm$ 0.014(sys) for $\Lambda$ hyperons and $D_{LL}$ = 0.032 $\pm$ 0.043(stat) $\pm$ 0.019 (sys) for $\bar{\Lambda}$ anti-hyperons produced with $<$$\eta$$>$ = 0.5 and $<$$p_T$$>$ = 5.9 GeV/c.  While the data do not provide conclusive evidence for a spin transfer signal, the data tend to be below a model evaluation (DSV seen. 3) based on the extreme assumption that the quark polarized fragmentation functions are flavor-independent.

Paper draft and review:

Previous Publication:

Run 2005 DLL (PRD(R)) (comparison between run 9 and published run 5 data  points .pdf

Documentation:

Presentations:

• Preliminary result release at SPIN2012, DNP2013 (comparison between preliminary and final results .png)
• Nov 10, 2016 Analysis Meeting @LBL (link)
• May 16, 2017 Collaboration Meeting @BNL (link)