Polarization fraction measurement in ZZ scattering using deep learning

Abstract

Measuring longitudinally polarized vector boson scattering in the ZZ channel is a promising way to investigate unitarity restoration with the Higgs mechanism and to search for possible new physics. We investigated several deep neural network structures and compared their ability to improve the measurement of the longitudinal fraction ZLZL. Using fast simulation with the Delphes framework, a clear improvement is found using a previously investigated 'particle-based' deep neural network on a preprocessed dataset and applying principle component analysis to the outputs. A significance of around 1.7 standard deviations can be achieved with the integrated luminosity of 3000 f b −1 that will be recorded at the High-Luminosity LHC. The technique developed in this article is also useful to other LHC analyses involving helicity fraction measurement. PACS numbers: HL-LHC, vector boson scatter, electroweakVector boson scattering (VBS) is a rare standard model (SM) process which plays a crucial role in electroweak symmetry breaking. The LHC and High-Luminosity LHC (HL-LHC) have enormous potential to both initially observe and study the features of rare processes such as VBS. Our knowledge of the VBS topology at hadron colliders can be combined with advanced data analysis techniques, such as deep learning, to make this pursuit even more promising.Many VBS studies have been performed based on LHC data, including measurements of W ± W ± [1, 2], W ± Z [3, 4] and Zγ [5, 6]. The topic of this paper is the channel ZZ → 4l. While this channel has the advantage of a clean final state, it suffers from a low production cross section, small branching-ratio of the Z boson to charged leptons, and two large irreducible QCD backgrounds, the production of ZZ via quark-antiquark annihiliation (qqZZ) and via a gluon box diagram (ggZZ). ZZ scattering has recently been observed by ATLAS with a significance larger than 5 standard deviations, using 139 f b −1 of LHC Run II data collected at √ s = 13 TeV [7]. A prior measurement made by CMS with 35.9f b −1 collected at √ s = 13 TeV reported an observed significance of 2.7 standard deviations [8]. Measuring the longitudinally polarized component of VBS (the LL component) is a critical next step for the field, as it is closely related to the important theoretical property of unitarity restoration, through Higgs and possible new physic [9,10]. In the context of Higgs and VBS discoveries, it becomes one of the next big targets, to test directly that Higgs regulates Higgs Goldstone scattering [11]. Prbing LL scattering can also provide alternative and model independent way of measuring Higgs coupling [12]. Fig. 1 [left] shows comparision of 4 lepton invariant mass distributions among the SM and its LL component, the SM but with Higgs and W or Z boson couplings scaled by a factor of 0.8, and its LL component. In this example, the LL component can be sensitive to Higgs couplings with massive gauge bosons, as any deviations from the SM prediction leads to large enhancements of the LL mode especially at hig...

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