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About 50 years ago, it was discovered that L hyperons are produced polarized in collisions of unpolarized protons on beryllium. The origin of the polarization remains a mystery, but majority of results indicate the importance of final state effects. A recently proposed technique is a measurement of L hyperon pair spin-spin correlations, which should help understand if the L polarization has any contribution from the early stage of the p+p collisions. We present results from the first experimental measurement of the L hyperon pair spin-spin correlations, using the p+p collisions at sqrt(s) = 200 GeV collected by the STAR experiment in 2012. The pairs are reconstructed at |y| < 1 with 0.5 < p_T < 5.0 GeV/c for two different pair rapidity gaps: |Delta y| < 0.5 and 0.5 < |Delta y| < 2.0. The polarization is found to be consistent with 0 for all pairs, except for L-Lbar pairs with rapidity gap |Delta y| < 0.5, with P_LL =(0.11 +- 0.03_stat +- 0.01_sys). This result indicates that the Λ polarization is likely generated exclusively by the final state effects.

Conclusions:

The STAR experiment has carried out the first experimental search for the L hyperon pair spin-spin correlations. The analysis was performed using p+p collisions at sqrt(s) = 200 GeV collected by the STAR experiment in 2012. The L and Lbar hyperons are reconstructed at mid-rapidity (|Delta y| < 1) with transverse momentum in range of 0.5 < p_T < 5.0 GeV/c. The spin-spin correlation is extracted for two different rapidity gaps between the hyperons: |Delta y| < 0.5 and 0.5 < |Delta y| < 2.0. The expected spin-spin correlation from PYTHIA is consistent with 0 for all three Lambda hyperon pair combinations, in both studied rapidity gap bins. The experimental values of the spin-spin correlation is consistent with 0 for all three hyperon pairs with the large rapidity gap (0.5 < |Delta y| < 2.0). The same applies for LL and LbarLbar hyperon pairs with the small rapidity gap (|Delta y| < 0.5 ). The spin-spin correlation for LLbar pairs with the small rapidity gap, on the other hand, shows polarization signal of P_LL =(0.11 +- 0.03_stat +- 0.01_sys).

The observation of no significant long-range spin-spin correlation of the L hyperon pairs indicates that there is no substantial contribution of initial state effects to the L hyperon spin-spin correlation in p+p collisions at sqrt(s) = 200 GeV, although statistical precision is limited. The upcoming data from the year 2024 should allow to improve the statistical precision and perform the measurement with larger rapidity gap, thanks to the STAR forward upgrade.

At the same time, the non-zero short range spin-spin correlation for the LLbar pairs supports the importance of the final state effects, especially effects like fragmentation and hadronization, which is consistent with previous L hyperon polarization measurements. In addition, this result provides additional information on how large fraction of the L hyperon spin is carried by the s quarks, as majority of the LLbar pairs with small rapidity gap likely originate from a single gluon splitting. In that case, it will be possible to develop models comparing the spin-spin correlation of the s-sbar quark pair and the observed final state short-range LLbar pair.

Figures:

Figure 0: TBA – Cartoon explaining difference between short-range and long-range spin-spin correlation of Λ hyperons.

Figure 1: 2D invariant mass of pi-p pairs, paired with pi+pbar pairs. The true L-Lbar pairs are visible as the main peak in the middle of the plot. The two ridges then correspond to a real L or Lbar matched with combinatorial background. The main peak and both ridges are on top of a continuum background that originates from background pi-p pairs paired with background pi+pbar pairs.

Figure 2: dN/dcos(theta*) distributions of decay (anti-)protons for L-Lbar, L-L, and Lbar-Lbar hyperon pairs. The hyperon pairs are measured at mid-rapidity (|y| < 1) and the distance in rapidity between the two hyperons is required to be |Delta y| < 0.5 (a)) and 0.5 < |Delta y| < 2.0 (b)). The fits to the data are used to extract the corresponding polarization values.

Figure 3: Polarization of L-Lbar, L-L, and Lbar-Lbar hyperon pairs with rapidity gap |Delta y| < 0.5 (a)) and 0.5 < |Delta y| < 2.0 (b)). The data (red circles) are compared to prediction from PYTHIA 8 (purple squares).

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