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The authors study the consequence of unitarity, causality and crossing on the forward 2->2 scattering amplitudes in quantum field theories. Out of an infinite set of energy dependent quantities, they identify an optimal finite subset of constraints, suitable to bound effective field theories at any finite order in the energy expansion. The study may help determine the acceptable range of validity of effective field theories in both couplings and energy.
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Bellazzini et al. (2021) studied this question.
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