Search

We study the polarization of positively charged $W$'s in the scattering of massive electroweak bosons at hadron colliders. We rely on the separation of weak boson polarizations in the gauge-invariant, doubly-resonant part of the amplitude in Monte Carlo simulations. Polarizations depend on the reference frame in which they are defined. We discuss the change in polarization fractions and in kinematic distributions arising from defining polarization vectors in two different reference frames which have been employed in recent experimental analyses., Comment: added reference to CMS measurement, typos corrected, matches PLB published version

This document summarises the talks and discussions happened during the VBSCan Mid-Term Scientific Meeting workshop. The VBSCan COST action is dedicated to the coordinated study of vector boson scattering (VBS) from the phenomenological and experimental point of view, for the best exploitation of the data that will be delivered by existing and future particle colliders., Comment: Editors: I.Brivio, C.Charlot, R.Covarelli, R.L.Delgado, K.Lohwasser, M.Pellen, M.Slawinska, G.Ortona, K.Ozdemir, C.Petridou, I.Puljak, M.Zaro. Proceedings for the VBSCan Mid-Term Scientific Meeting of the VBSCan COST action

Isolating the scattering of longitudinal weak bosons at the LHC is an important tool to probe the ElectroWeak Symmetry Breaking mechanism. Separating polarizations of $W$ and $Z$ bosons is complicated, because of non resonant contributions and interference effects. Additional care is necessary when considering $Z$ bosons, due to the $\gamma/Z$ mixing in the coupling to charged leptons. We propose a method to define polarized signals in $Z\!Z$ and $W^+\!Z$ scattering at the LHC, which relies on the separation of weak boson polarizations at the amplitude level in Monte Carlo simulations. After validation in the absence of lepton cuts, we investigate how polarized distributions are affected by a realistic set of kinematic cuts (and neutrino reconstruction, when needed). The total and differential polarized cross sections computed at the amplitude level are well defined, and their sum reproduces the full results, up to non negligible but computable interference effects which should be included in experimental analyses. We show that polarized cross sections computed using the reweighting method are inaccurate, particularly at large energies. We also present two procedures which address the model independent extraction of polarized components from LHC data, using Standard Model angular distribution templates., Comment: version published in JHEP

Vector-boson scattering processes are of great importance for the current run-II and future runs of the Large Hadron Collider. The presence of triple and quartic gauge couplings in the process gives access to the gauge sector of the Standard Model (SM) and possible new-physics contributions there. To test any new-physics hypothesis, sound knowledge of the SM contributions is necessary, with a precision which at least matches the experimental uncertainties of existing and forthcoming measurements. In this article we present a detailed study of the vector-boson scattering process with two positively-charged leptons and missing transverse momentum in the final state. In particular, we first carry out a systematic comparison of the various approximations that are usually performed for this kind of process against the complete calculation, at LO and NLO QCD accuracy. Such a study is performed both in the usual fiducial region used by experimental collaborations and in a more inclusive phase space, where the differences among the various approximations lead to more sizeable effects. Afterwards, we turn to predictions matched to parton showers, at LO and NLO: we show that on the one hand, the inclusion of NLO QCD corrections leads to more stable predictions, but on the other hand the details of the matching and of the parton-shower programs cause differences which are considerably larger than those observed at fixed order, even in the experimental fiducial region. We conclude with recommendations for experimental studies of vector-boson scattering processes., Comment: Data and input files are available at: https://github.com/VBSCanAction/VBSComparison/ . 28 pages including figures and references. Matches the journal version

Measuring the scattering of longitudinally-polarized vector bosons represents a fundamental test of ElectroWeak Symmetry Breaking. In addition to the challenges provided by low rates and large backgrounds, there are conceptual issues which need to be clarified for the definition of a suitable signal. Since vector bosons are unstable and can only be observed through their decay products, the polarization states interfere among themselves. Moreover, already at tree level, there are diagrams which cannot be interpreted as production times decay of EW bosons but are necessary for gauge invariance. We discuss a possible way to define a cross section for polarized $W$'s, dropping all non resonant diagrams, and projecting on shell the resonant ones, thus, preserving gauge invariance. In most cases, the sum of polarized distributions reproduces the full results. In the absence of cuts, the ratios of the polarized cross sections to the full one agree with the results of a standard projection on Legendre polynomials. While the latter cannot be employed in a realistic environment, a comparison of the data with the shapes of the angular distributions for polarized vector bosons allows the extraction of the polarization fractions in the presence of selection cuts on the charged leptons., Comment: Agrees with the published version JHEP03(2018)170. Corrected reference to published version. Everything else as in v2. Additional details on the projection procedure. More detailed discussion of the cancellations among the various sets of diagrams in the high energy and high transverse momentum region

Interference effects play an important role in Electroweak Physics. They are responsible for the restoration of unitarity al large energies. When, as is often the case, higher order corrections are only available for some particular subamplitude, interferences need to be carefully computed in order to obtain the best theoretical prediction. In the new proposal to estimate the total Higgs width from the off shell cross section, the interference between the Higgs signal and the background is essential. It has been recently pointed out in gluon fusion that whenever more than one neutral, CP even, scalars are present in the spectrum large cancellations can occur. We extend these studies to Vector Boson Scattering, examining interference effects in the Higgs sector in the Standard Model and its one Higgs Singlet extension.