A $W^{\pm }$ polarization analyzer from Deep Neural Networks

Abstract

In this paper, we train a Convolutional Neural Network to classify longitudinally and transversely polarized hadronic $W^{\pm }$ using the images of boosted $W^{\pm }$ jets as input. The images capture angular and energy information from the jet constituents that is faithful to properties of the original quark/anti-quark $W^{\pm }$ decay products without the need for invasive substructure cuts. We find that the difference between the polarizations is too subtle for the network to be used as an event-by-event tagger. However, given an ensemble of $W^{\pm }$ events with unknown polarization, the average network output from that ensemble can be used to extract the longitudinal fraction $f_L$. We test the network on Standard Model $pp\to W^{\pm }Z$ events and on $pp\to W^{\pm }Z$ in the presence of dimension-6 operators that perturb the polarization composition.