arXiv 1609.025362016-09-08hep-phhep-ex

在LHC的W生产：探测硬QCD的轻子角分布和参考系

W production at LHC: lepton angular distributions and reference frames for probing hard QCD

E. Richter-Was, Z. Was

备注：: 27 pages 21 figures. arXiv admin note: text overlap with arXiv:1605.05450
DOI：: 10.1140/epjc/s10052-017-4649-4

摘要：

标准模型在强电区和弱电区的精度检验是大型强子对撞机实验的物理目标之一。由于质子-质子过程的性质，基于轻子方向和能量测量的可观测量提供了最精确的特征。本文主要研究轻子对静止系中轻子从W到lnu衰变的角分布。中间态的矢量性质要求分布具有很好的精度，最多可以用二阶球面多项式来描述。我们认为，相反，一般的信念往往在文献中表达的，全套的角度系数可以通过实验测量，尽管在中微子逃逸检测的最终状态的存在。因此，关于Z/γ到l^+ l^- Drell-Yan过程的现象学，没有原则上的区别。我们还表明，与适当选择的坐标框架，只有一个系数在这个多项式分解仍然是相当大的，即使在存在一个或多个高p_T喷流。帧的必要随机选择依赖于独立于任何耦合常数的概率。通过这种方式，电弱效应（由W与费米子耦合的V-A性质决定）可以更好地与强相互作用分离。这种分离便于测量结果的解释。

英文摘要：

Precision tests of the Standard Model in the Strong and Electroweak sectors play an important role, among the physics goals of LHC experiments. Because of the nature of proton-proton processes, observables based on the measurement of the direction and energy of leptons provide the most precise signatures. In the present paper, we concentrate on the angular distribution of leptons from W to l nu decays in the lepton-pair rest-frame. The vector nature of the intermediate state imposes that distributions are to a good precision described by spherical polynomials of at most second order. We argue, that contrary to general belief often expressed in the literature, the full set of angular coefficients can be measured experimentally, despite the presence in the final state of neutrino escaping detection. There is thus no principle difference with respect to the phenomenology of the Z/gamma to l^+ l^- Drell-Yan process. We show also, that with the proper choice of the coordinate frames, only one coefficient in this polynomial decomposition remains sizable, even in the presence of one or more high p_T jets. The necessary stochastic choice of the frames relies on probabilities independent from any coupling constants. In this way, electroweak effects (dominated by the V-A nature of W couplings to fermions) can be better separated from the ones of strong interactions. The separation is convenient for the measurements interpretation.

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