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arXiv 1301.3756nucl-exhep-exnucl-th
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$\sqrt{s_{\rm NN}} = 2.76$ TeV上Pb-Pb碰撞平衡函数的电荷关联

Charge correlations using the balance function in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV

ALICE Collaboration

备注：
26 pages, 7 captioned figures, 2 tables, authors from page 20, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/476
发表信息：
Phys. Lett. B 723 (2013) 267-279
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摘要：

在高能重离子碰撞中，发射粒子之间的相关性可以作为一种探针来深入了解电荷产生机制。在这封信中，我们报告了在大型强子对撞机上用ALICE探测器在$\sqrt{s_{\rm NN}}=2.76$ TeV的Pb-Pb碰撞中使用相对赝快度（$Δη$）和方位角（$Δφ$）中的电荷平衡函数进行研究的第一个结果。平衡函数的宽度随着两个投影中中心性的增加而减小（即，对于更多的中心碰撞）。这种中心依赖性是不再现HIJING，而AMPT，一个模型，其中包括字符串和部分子再散射，表现出定性协议与测量的相关性在$Δφ$，但未能描述的相关性在$Δη$。一个热爆炸波模型，结合当地的电荷守恒和调整，以描述$p_{\rm T}$光谱和v $_2$测量报告的ALICE，被用来拟合的平衡函数的宽度的中心依赖性，并提取在冻结出平衡电荷的平均分离。在较低的能量与测量结果的比较显示与$\sqrt{s_{\rm NN}}$的顺序：所有中心的平衡功能变得更窄，随着能量的增加。这与LHC能量下较大径向流的影响一致，但也与平衡电荷的后期创建方案一致。然而，相对减少的平衡功能宽度在$Δη$和$Δφ$与中心从最高的SPS到LHC的能量表现出只有很小的差异。这一观察结果不能仅仅在大部分电荷是在重离子碰撞演化的后期阶段产生的框架内加以解释。

英文摘要：

In high-energy heavy-ion collisions, the correlations between the emitted particles can be used as a probe to gain insight into the charge creation mechanisms. In this Letter, we report the first results of such studies using the electric charge balance function in the relative pseudorapidity ($Δη$) and azimuthal angle ($Δφ$) in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV with the ALICE detector at the Large Hadron Collider. The width of the balance function decreases with growing centrality (i.e. for more central collisions) in both projections. This centrality dependence is not reproduced by HIJING, while AMPT, a model which incorporates strings and parton rescattering, exhibits qualitative agreement with the measured correlations in $Δφ$ but fails to describe the correlations in $Δη$. A thermal blast-wave model incorporating local charge conservation and tuned to describe the $p_{\rm T}$ spectra and v$_2$ measurements reported by ALICE, is used to fit the centrality dependence of the width of the balance function and to extract the average separation of balancing charges at freeze-out. The comparison of our results with measurements at lower energies reveals an ordering with $\sqrt{s_{\rm NN}}$: the balance functions become narrower with increasing energy for all centralities. This is consistent with the effect of larger radial flow at the LHC energies but also with the late stage creation scenario of balancing charges. However, the relative decrease of the balance function widths in $Δη$ and $Δφ$ with centrality from the highest SPS to the LHC energy exhibits only small differences. This observation cannot be interpreted solely within the framework where the majority of the charge is produced at a later stage in the evolution of the heavy--ion collision.

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