Abstract
The triple neutral gauge couplings are absent in the Standard Model (SM) at the tree level. They receive no contributions from dimension-6 effective operators, but can arise from effective operators of dimension-8. We study the scale of new physics associated with such dimension-8 operators that can be probed by measuring the reaction followed by decays, at future colliders including the CEPC, FCC-ee, ILC and CLIC. We demonstrate how angular distributions of the final-state mono-photon and leptons can play a key rôle in suppressing SM backgrounds. We further demonstrate that using electron/positron beam polarizations can significantly improve the signal sensitivities. We find that the dimension-8 new physics scale can be probed up to the multi-TeV region at such lepton colliders.
Highlights
At the time of writing, there is no confirmed evidence for phenomena in accelerator experiments that require new physics beyond the Standard Model (SM) [1, 2], pending clarifications of the apparent discrepancy between the SM prediction and the experimental value of the anomalous magnetic moment of the muon, and of the apparent anomalies in b-hadron decays into strange and charmed particles
We study the scale of new physics associated with such dimension-8 operators that can be probed by measuring the reaction e+e− → Zγ followed by Z → ll, ννdecays, at future e+e− colliders including the CEPC, FCC-ee, ILC and CLIC
This Standard Model Effective Field Theory (SMEFT) approach has mainly been applied with the assumption that only dimension-6 SMEFT operators [4] contribute to the experimental observables under study [5,6,7]
Summary
At the time of writing, there is no confirmed evidence for phenomena in accelerator experiments that require new physics beyond the Standard Model (SM) [1, 2], pending clarifications of the apparent discrepancy between the SM prediction and the experimental value of the anomalous magnetic moment of the muon, and of the apparent anomalies in b-hadron decays into strange and charmed particles. It is plausible to assume that the SM particles have the same dimension-4 interactions as in the SM, and seek to characterize possible deviations from SM predictions in terms of higher-dimensional effective operators constructed out of SM fields, whose contributions are suppressed by some power of an underlying new physics scale Λ ≫ 100 GeV [3]. This Standard Model Effective Field Theory (SMEFT) approach has mainly been applied with the assumption that only dimension-6 SMEFT operators [4] contribute to the experimental observables under study [5,6,7]. We analyze the scattering amplitudes for e+e− → Zγ, considering separately the transverse and longitudinal polarizations of the final-state Z bosons
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