Published by Institute of Physics Publishing for SISSA Received: July 7, 2006 Accepted: August 24, 2006 Published: October 4, 2006 Causality, analyticity and an IR obstruction to UV completion Allan Adams,a Nima Arkani-Hamed,a Sergei Dubovsky,abc Alberto Nicolisa and Riccardo Rattazzib∗ JHEP10(2006)014 aJefferson Physical Laboratory, Harvard University, Cambridge, MA 02138, U.S.A. bCERN Theory Division, CH-1211 Geneva 23, Switzerland cInstitute for Nuclear Research of the Russian Academy of Sciences, 60th October Anniversary Prospect, 7a, 117312 Moscow, Russia E-mail:
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[email protected] Abstract: We argue that certain apparently consistent low-energy effective field theories described by local, Lorentz-invariant Lagrangians, secretly exhibit macroscopic non-locality and cannot be embedded in any UV theory whose S-matrix satisfies canonical analyticity constraints. The obstruction involves the signs of a set of leading irrelevant operators, which must be strictly positive to ensure UV analyticity. An IR manifestation of this restriction is that the “wrong” signs lead to superluminal fluctuations around non-trivial backgrounds, making it impossible to define local, causal evolution, and implying a surpris- ing IR breakdown of the effective theory. Such effective theories can not arise in quantum field theories or weakly coupled string theories, whose S-matrices satisfy the usual analyt- icity properties. This conclusion applies to the DGP brane-world model modifying gravity in the IR, giving a simple explanation for the difficulty of embedding this model into con- trolled stringy backgrounds, and to models of electroweak symmetry breaking that predict negative anomalous quartic couplings for the W and Z.