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arXiv 0810.0934hep-phhep-ex
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LHC的顶级喷气机

Top Jets at the LHC

Leandro G. Almeida, Seung J. Lee, Gilad Perez, Ilmo Sung, Joseph Virzi

备注：
46 pages, 23 figures
发表信息：
Phys.Rev.D79:074012,2009
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摘要：

我们研究了在LHC上重建高p_T强子衰变顶夸克。识别高能顶夸克的主要挑战是衰变产物变得越来越准直。这降低了利用顶部衰变链的拓扑结构的常规方法的功效。我们专注于顶夸克的衰变产物被重建为一个单一的喷流，一个“顶喷流”的情况下。最基本的“顶标签”方法的基础上，射流质量测量被认为是详细的。为了分析top-tagging方法，QCD喷流背景的理论和实验方面进行了检查。基于QCD因子分解，我们导出了QCD喷注质量谱形状的一个简单解析近似。我们观察到一个很好的协议与蒙特卡罗模拟。我们以标准模型中高p_T t \bar{t}产生为例，表明我们的理论QCD喷流质量分布可以通过边带分析有效地表征背景.我们表明，在25 fb^{-1}的数据下，我们的方法允许我们分辨出来自QCD背景的p_T $\ge$ 1 TeV的顶喷流，以及100 fb^{-1}的约1.5 TeV的顶喷流，而不依赖于b标记。为了进一步提高重要性，我们考虑射流形状（在0807.0234 [hep-ph]中分析），它解决了锥形射流内部能量流的子结构。本文还提出了一种利用b夸克p_T测量顶夸克极化的方法。我们的方法的主要优点是：（i）质量分布是由第一原理计算驱动，而不是仅仅依赖于蒙特卡罗模拟;（ii）对于高p_T喷流（p_T $\ge$ 1 TeV），红外安全喷流形状变量是强大的探测器分辨率的影响。我们的分析是适用于其他助推质量粒子，如电弱规范玻色子和希格斯玻色子。

英文摘要：

We study the reconstruction of high p_T hadronically-decaying top quarks at the LHC. The main challenge in identifying energetic top quarks is that the decay products become increasingly collimated. This reduces the efficacy of conventional methods that exploit the topology of the top decay chain. We focus on the cases where the decay products of the top quark are reconstructed as a single jet, a "top-jet". The most basic "top-tag" method based on jet mass measurement is considered in detail. To analyze the top-tagging method, theoretical and experimental aspects of the QCD jet background are examined. Based on QCD factorization, we derive a simple analytic approximation for the shape of the QCD jet mass spectrum. We observe a good agreement with the Monte Carlo simulation. We consider high-p_T t\bar{t} production in the Standard Model as an example, and show that our theoretical QCD jet mass distributions can efficiently characterize the background via sideband analyses. We show that with 25 fb^{-1} of data, our approach allows us to resolve top-jets with p_T $\ge$ 1 TeV from the QCD background, and about 1.5 TeV top-jets with 100 fb^{-1}, without relying on b-tagging. To further improve the significance we consider jet shapes (analyzed in 0807.0234 [hep-ph]), which resolve the substructure of energy flow inside cone jets. A method of measuring top quark polarization by using the b-quark p_T is also presented. The main advantages of our approach are: (i) the mass distributions are driven by first principle calculations, instead of relying solely on Monte Carlo simulation; (ii) for high p_T jets (p_T $\ge$1 TeV), IR-safe jet shape variables are robust against detector resolution effects. Our analysis is applicable to other boosted massive particles such as the electroweak gauge bosons and the Higgs.

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