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arXiv 1509.04776hep-exnucl-ex
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用ATLAS探测器观测$\sqrt{s}=$ 13和2.76 TeV $pp$碰撞中的长程椭圆各向异性

Observation of long-range elliptic anisotropies in $\sqrt{s}=$13 and 2.76 TeV $pp$ collisions with the ATLAS detector

ATLAS Collaboration

备注：
10 pages plus author list + cover pages (26 pages total), 4 figures, no tables, published in Physical Review Letters, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2015-09
发表信息：
Phys. Rev. Lett. 116, 172301 (2016)
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摘要：

ATLAS在LHC的$\sqrt{s}$ =13和2.76 TeV的$pp$碰撞中，使用在$|η|$ <2.5的赝快度间隔中测量的带电粒子，测量了作为相对方位角$Δϕ$和赝快度$Δη$的函数的两粒子关联。在测量的带电粒子多重性的不同间隔中评估的相关函数显示在$Δϕ\sim 0$处的多重性依赖的增强，其在广泛的$Δη$范围内延伸，这被称为“脊”。在2< $|Δη|$ <5的范围内测量每触发粒子产率$Y(Δϕ)$。对于这两种碰撞能量，$Y(Δϕ)$分布在所有多重性间隔被发现是一致的线性组合的每触发粒子产量测量碰撞小于20重建的轨道，和一个恒定的组合调制的贡献$\cos{(2Δϕ)}$。拟合的傅立叶系数$v_{2,2}$，表现出因式分解，表明脊的结果从每个事件$\cos{(2ϕ)}$调制的单粒子分布与傅立叶系数$v_2$。的$v_2$值作为多重性和横向动量的函数。它们被发现是近似恒定的多重性的函数，并有一个类似于在$p$ +Pb和Pb+Pb碰撞中测得的$p_{\mathrm{T}}$依赖。在13和2.76 TeV的数据$v_2$值是一致的不确定性。这些结果表明，在$pp$碰撞中的脊来自相同或相似的基础物理在$p$ +Pb碰撞中观察到的，并负责脊的动力学没有很强的$\sqrt{s}$依赖。

英文摘要：

ATLAS has measured two-particle correlations as a function of relative azimuthal-angle, $Δϕ$, and pseudorapidity, $Δη$, in $\sqrt{s}$=13 and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|η|$<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $Δϕ\sim 0$ that extends over a wide range of $Δη$, which has been referred to as the "ridge". Per-trigger-particle yields, $Y(Δϕ)$, are measured over 2<$|Δη|$<5. For both collision energies, the $Y(Δϕ)$ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by $\cos{(2Δϕ)}$. The fitted Fourier coefficient, $v_{2,2}$, exhibits factorization, suggesting that the ridge results from per-event $\cos{(2ϕ)}$ modulation of the single-particle distribution with Fourier coefficients $v_2$. The $v_2$ values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a $p_{\mathrm{T}}$ dependence similar to that measured in $p$+Pb and Pb+Pb collisions. The $v_2$ values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arises from the same or similar underlying physics as observed in $p$+Pb collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.

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