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17 December 2015, 16:00, BLTP Blokhintsev hall (4th floor)

14.12.2015
Seminar QUANTUM FIELD THEORY


Yu-tin Huang (National Taiwan University) "Unitarity constraint on low energy effective theories"

Abstract
Unitarity of the S-matrix, in the form of consistent factorisation, imposes strong constraint on the low energy effective theory even without the knowledge of its UV completion. Famous examples include the positivity constraint on the coefficient of the leading higher dimensional operator. By combining with supersymmetry, we will show that such constraint allows us to fix the coefficients of a large number of operators for the coulomb branch effective action of N=4 SYM. We also consider another application, the pertrubative unitarization of gravity scattering. Assuming that the high energy fixed angle scattering is tamed by the inclusion of new massive degrees of freedom while it is weakly coupled, unitarity forces us to have at least an infinite tower of massive particles with each mass squared given by an integer multiplied by a universal scale.