Polarised-boson pairs at the LHC with NLOPS accuracy

Measurements of vector boson polarisation in vector boson production processes offer a powerful probe of the electroweak symmetry breaking mechanism, scrutinising the Standard Model and new physics scenarios alike. Since massive vector bosons can only be observed as intermediate particles, polarised cross section templates from simulation are necessary to extract their polarisation from measurable unpolarised distributions. In this work we present an extension of the Sherpa Monte-Carlo event generator allowing the simulation of polarised cross sections for vector boson production processes. Based on the narrow-width approximation, polarised cross sections of all possible polarisation combinations for an arbitrary number of intermediate vector bosons can be simulated in a single simulation run. In addition, it is possible to directly predict the interference between different intermediate polarisation states, and various differing polarisation definitions can be studied simultaneously. Besides the simulation of polarised cross sections at fixed LO and LO+PS accuracy as well as in multijet-merged calculations, we also present parton-shower-matched polarised cross sections with approximate NLO QCD corrections in the vector boson production processes. We demonstrate that the differences of this approximation to full NLO QCD predictions are small and it thus opens up the possibility for fully-simulated calculations at the hadron level including polarisation information and higher-order QCD effects for the first time.

Polarised cross sections for vector boson production with Sherpa

Hoppe,

Schönherr,

Siegert

2024

Triple-gauge couplings in LHC diboson production: a SMEFT view from every angle

El Faham,

Pelliccioli,

Vryonidou

2024

This study explores fully leptonic WZ and WW production at the LHC within the SMEFT framework at NLO in QCD, focusing on both CP-even and CP-odd triple-gauge-coupling dimension-six operators. We investigate the off-shell processes, contrasting our findings in inclusive setups with those in the presence of realistic fiducial selections. Alongside the conventional kinematic observables, we examine polarisation-sensitive observables and angular coefficients. Moreover, we assess potential SMEFT effects on asymmetry observables. Through a sensitivity analysis, we identify critical LHC observables that are particularly sensitive to SMEFT-induced modifications, thereby shedding light on potential avenues for new physics searches in diboson production at the LHC.

NLO EW and QCD corrections to polarised same-sign WW scattering at the LHC

Denner,

Haitz,

Pelliccioli

2024

We present the first calculation of same-sign WW scattering at the LHC in the fully leptonic decay channel including the modelling of polarisation for intermediate electroweak bosons and radiative corrections up to NLO EW+QCD accuracy. The predictions rely on a pole expansion and on the split of polarisation states at matrix-element level. Doubly-polarised and unpolarised signals are investigated together with full off-shell results. A detailed phenomenlogical analysis is carried out focusing on differential observables that discriminate between polarisation states, paving the way for refined polarisation-oriented analyses of vector-boson scattering with Run-3 LHC data.