Production of Z′-boson resonances with large width at the LHC

E Accomando,F Coradeschi,T Cridge,J Fiaschi,F Hautmann,S Moretti,C Shepherd-Themistocleous,C Voisey

doi:10.1016/j.physletb.2020.135293

Abstract

Di-lepton searches for Beyond the Standard Model (BSM) Z′ bosons that rely on the analysis of the Breit-Wigner (BW) line shape are appropriate in the case of narrow resonances, but likely not sufficient in scenarios featuring Z′ states with large widths. Conversely, alternative experimental strategies applicable to wide Z′ resonances are much more dependent than the default bump search analyses on the modelling of QCD higher-order corrections to the production processes, for both signal and background. For heavy Z′ boson searches in the di-lepton channel at the CERN Large Hadron Collider (LHC), the transverse momentum qT of the di-lepton system peaks at qT≲10−2Mll, where Mll is the di-lepton invariant mass. We exploit this to treat the QCD corrections by using the logarithmic resummation methods in Mll/qT to all orders in the strong coupling constant αs. We carry out studies of Z′ states with large width at the LHC by employing the program reSolve, which performs QCD transverse momentum resummation up to Next-to-Next-to-Leading Logarithmic (NNLL) accuracy. We consider two benchmark BSM scenarios, based on the Sequential Standard Model (SSM) and dubbed ‘SSM wide’ and ‘SSM enhanced’. We present results for the shape and size of Z′ boson signals at the differential level, mapped in both cross section (σ) and Forward-Backward Asymmetry (AFB), and perform numerical investigations of the experimental sensitivity at the LHC Run 3 and High-Luminosity LHC (HL-LHC).

Highlights

The physics of Z bosons has been extensively studied in the literature
In conjunction with the shape difference seen at relatively low invariant masses in the AFB, these effects at considerably lower invariant masses than the resonance peak may offer a further means of probing Z physics
The AFB observable in Z physics, other than being a timehonoured diagnostic probe, has recently been established to be a discovery tool at the LHC whenever the new neutral massive gauge boson is wide, i.e., it displays a large ratio between its width Z and mass M Z

Summary

Introduction

The physics of Z bosons has been extensively studied in the literature. For an exhaustive review, e.g., see Ref. [1]. An example approach is to parameterise a functional form using simulation and constrain the overall amplitude using a low-mass control region assumed to be free from significant new physics content. This provides a background estimate in the signal region of interest. Since AFB is a ratio quantity some systematics will cancel and it is expected that QCD higher-order corrections will be lower than in the case of cross sections (and so their residual systematics) This was established to be the case for the PDF error in Ref.

Benchmark models

Numerical results

Findings

Conclusions