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arXiv 1909.01281nucl-exhep-ex
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LHC上Pb-Pb碰撞中$Λ$和$\overlineΛ$超子的全局极化

Global polarization of $Λ$ and $\overlineΛ$ hyperons in Pb-Pb collisions at the LHC

ALICE Collaboration

摘要:

用大型强子对撞机(LHC)的ALICE记录了$\sqrt{s_{\rm{NN}}}$ = 2.76和5.02 TeV的Pb-Pb碰撞中$Λ$和$\overlineΛ$超子的全局极化.计算结果与碰撞中心度和超子横动量($p_{\rm{T}}$)的关系在中心度5- 50%,$0.5 < p_{\rm{T}} <5$ GeV/ $c$和快度$|y|<0.5$范围内有差异.在能量为2.76和5.02 TeV的Pb-Pb碰撞中,超子的全局极化平均值为0,$\langle P_{\rm{H}}\rangle$(%)$\approx$ - 0.01 $\pm$ 0.05(stat.)$\pm$ 0.03(系统)在碰撞中心度范围15- 50%中,其中预期最大的信号。结果是兼容的期望的基础上外推的测量在较低的碰撞能量在RHIC,流体动力学模型计算,和经验估计的基础上碰撞能量的依赖性的定向流,所有这些预测的全球极化值在大型强子对撞机能量的顺序为0.01%。

英文摘要:

The global polarization of the $Λ$ and $\overlineΛ$ hyperons is measured for Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 and 5.02 TeV recorded with the ALICE at the LHC. The results are reported differentially as a function of collision centrality and hyperon's transverse momentum ($p_{\rm{T}}$) for the range of centrality 5-50%, $0.5 < p_{\rm{T}} <5$ GeV/$c$, and rapidity $|y|<0.5$. The hyperon global polarization averaged for Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 and 5.02 TeV is found to be consistent with zero, $\langle P_{\rm{H}}\rangle$ (%) $\approx$ - 0.01 $\pm$ 0.05 (stat.) $\pm$ 0.03 (syst.) in the collision centrality range 15-50%, where the largest signal is expected. The results are compatible with expectations based on an extrapolation from measurements at lower collision energies at RHIC, hydrodynamical model calculations, and empirical estimates based on collision energy dependence of directed flow, all of which predict the global polarization values at LHC energies of the order of 0.01%.

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