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arXiv 2005.11128nucl-exhep-ex
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在LHC上首次测量核碰撞中的夸克偶素极化

First measurement of quarkonium polarization in nuclear collisions at the LHC

ALICE Collaboration

备注：
21 pages, 3 captioned figures, 3 tables, authors from page 16, published version, figures at http://alice-publications.web.cern.ch/node/6223
发表信息：
Phys. Lett. B 815 (2021) 136146
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摘要：

用ALICE探测器测量了LHC上$\sqrt{s_{\rm{NN}}}=5.02$ TeV的Pb-Pb碰撞中产生的J/ $ψ$和$Υ(1{\rm S})$的极化。通过在快度区$2.5<y<4$中衰变为μ子对来重建夸克偶素，并测量μ子的极角和方位角分布，进行了研究。在螺旋度和Collins-Soper参考系中，分别在J/ $ψ$和$Υ(1{\rm S})$的横向动量间隔$2<p_{\rm T}<10$ GeV/ $c$和$p_{\rm T}<15$ GeV/ $c$中测量了偏振参数$λ_θ$、$λ_ϕ$和$λ_{θϕ}$。J/ $ψ$的偏振参数被发现是兼容的零，在一个最大的约两个标准偏差在低$p_{\rm T}$，为两个参考帧和整个$p_{\rm T}$范围内。的值进行了比较与PP碰撞在$\sqrt{s}=7$和8 TeV在一个类似的运动学区域的ALICE和LHCb实验获得的相应结果。虽然有更大的不确定性，极化参数为$Υ(1{\rm S})$生产的铅铅碰撞也是一致的零。

英文摘要：

The polarization of inclusive J/$ψ$ and $Υ(1{\rm S})$ produced in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region $2.5<y<4$ and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters $λ_θ$, $λ_ϕ$ and $λ_{θϕ}$ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval $2<p_{\rm T}<10$ GeV/$c$ and $p_{\rm T}<15$ GeV/$c$ for the J/$ψ$ and $Υ(1{\rm S})$, respectively. The polarization parameters for the J/$ψ$ are found to be compatible with zero, within a maximum of about two standard deviations at low $p_{\rm T}$, for both reference frames and over the whole $p_{\rm T}$ range. The values are compared with the corresponding results obtained for pp collisions at $\sqrt{s}=7$ and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for $Υ(1{\rm S})$ production in Pb-Pb collisions are also consistent with zero.

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