Speaker
Description
W and Z boson polarization is a direct probe of electroweak symmetry breaking, as the longitudinal states are physical remnants of the Higgs mechanism and their production fraction is uniquely sensitive to anomalous gauge couplings and new physics.
While leptonic decay channels of W/Z provide clean polarization measurements through the lepton angular distribution, they suffer from limited branching fractions, neutrino reconstruction ambiguities, and restricted kinematic reach. The hadronic channel — dominant in branching fraction and fully reconstructible in the boosted regime — has remained experimentally inaccessible for polarization studies due to the overwhelming QCD multijet background, which will be a more promiment effect in HL-LHC.
Due to the large QCD background it is inevitable that we must need a tagger to get sufficient signal purity. However, on the other hand the main challenge with pure polarized state classification taggers (beside QCD background) is that they suffer from a fundamental limitation: a tagger trained to discriminate longitudinal from transverse polarization states necessarily learns the same angular correlations one wishes to measure, creating a circular dependency that biases any subsequent shape analysis. We argue instead that a better approach is a QCD-vs-signal discriminant that is explicitly decorrelated from the polarization-sensitive observable (calling X), preserving the angular information in the data for unbiased extraction.
We present a three-dimensional extension of the Designed Decorrelated Tagger (DDT) [1], which simultaneously decorrelates an existing tagger score from the jet soft drop mass and also a cos θ* proxy observable (X) constructed in the large-R jet rest frame. This construction ensures a flat QCD selection efficiency at a fixed working point, making possible the use of mass sidebands for data-driven background estimation in different regions of X. We implement and test multiple X definitions based on subjet momenta obtained from declustering the jet.
Using the public JetClass [2] dataset and existing taggers, we demonstrate that the 3D DDT approach on these taggers successfully makes the decorrelation for the QCD background, and paves the way to create a workable analysis strategy blueprint for future hadronic W/Z polarization measurements.
1: JHEP 05 (2016) 156
2: https://zenodo.org/records/6619768