Abstract
The measurement of polarization fractions of massive gauge bosons at the LHC provides an important check of the Standard Model and in particular of the Electroweak Symmetry Breaking mechanism. Owing to the unstable character of W and Z bosons, devising a theoretical definition for polarized signals is not straightforward and always subject to some ambiguity. Focusing on W-boson pair production at the LHC in the fully leptonic channel, we propose to compute polarized cross-sections and distributions based on the gauge-invariant doubly-resonant part of the amplitude. We include NLO QCD corrections to the leading quark-induced partonic process and also consider the loop- induced gluon-initiated process contributing to the same final state. We present results for both an inclusive setup and a realistic fiducial region, with special focus on variables that are suited for the discrimination of polarized cross-sections and on quantities that can be measured experimentally.
Highlights
JHEP09(2020)164 run of the LHC will enable polarization measurements even in processes with rather small cross-sections, like vector-boson scattering [9, 10]
The definition of polarized signals presented in this work relies on the double-pole approximation (DPA) [22,23,24,25,26,27], which is expected to be more accurate than the narrow-width approximation or a decay chain
We present phenomenological results for polarized signals in W-pair production. We have investigated both singly-polarized and doubly-polarized configurations, since the experimental interest lies both in the single-boson polarization fractions and in the extraction of the doubly-longitudinal cross-section
Summary
A precise definition of polarized signals can be given only for on-shell particles. Since W and Z bosons are unstable particles, selecting their polarization states is always afflicted with some ambiguity. Beyond simple decay-chain techniques, a better solution is provided by the MadSpin method [19, 20] that preserves LO spin correlations and reintroduces an off-shell-ness of weak-boson propagators Another approach, which has proven to be accurate in vectorboson scattering and other multi-boson signatures, is given by the DPA [22,23,24,25,26,27] or pole approximations in general [41, 42]: given a resonant amplitude, the numerator of the resonant diagrams is projected on shell to restore gauge invariance, while the Breit-Wigner modulation is kept with off-shell kinematics. We leave the treatment of NLO EW corrections to future work
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