Probing EFT breakdown in the tails of $$W^+ W^-$$ observables

Abstract We test clipping effective field theory (EFT) simulations as a method of ensuring EFT validity. The procedure imposes that, at the level of the simulation, the invariant mass of a $$W^+W^-$$ W + W - pair $$M_{WW}$$ M WW is less than the new physics scale $$\\Lambda $$ Λ . We compare this to two other methods, comparison bin by bin of dimension-6 and dimension-8 squared contributions and implementing a cut on data. We find that setting $$M_{WW} < \\Lambda $$ M WW < Λ is not strict enough to ensure that the hierarchy of EFT operators is respected for dimension-6 and dimension-8 contributions. We also show that, even when using a stricter cut on $$M_{WW}$$ M WW , due to different correlations between $$M_{WW}$$ M WW and $$M_{e\\mu }$$ M e μ at different EFT orders, the bins in $$M_{e\\mu }$$ M e μ (the invariant mass of the leptons originating from W decays) used in an EFT fit may not truly be in the regime of EFT validity when performing a dimension-6 fit with $$M_{WW} < \\Lambda $$ M WW < Λ . We also explore the correlations of three transverse mass observables: $$M_{T1}, M_{T2}$$ M T 1 , M T 2 and $$M_{T3}$$ M T 3 , finding that $$M_{T1}$$ M T 1 and $$M_{T3}$$ M