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DTSTAMP:20261008T211730Z
SUMMARY:QFT-gravity Seminar: Luke Lippstreu
DESCRIPTION:Unitarity and the Forward Direction\n\nAbstract: Most theorems 
 of scattering theory assume that particles stop interacting at late times\
 , an assumption violated by four-dimensional theories containing photons a
 nd gravitons. I will describe how theories with long-range forces have gen
 uinely new analytic properties: no identity term in the S-matrix\, a new d
 istributional structure in the forward direction\, and a modified optical 
 theorem. As an application\, I will turn to a puzzle in the S-matrix boots
 trap: applied to long-range forces\, standard techniques give bounds on co
 uplings that depend on an arbitrary infrared scale. In a solvable model wh
 ose exact bound is known independently\, accounting for these novel analyt
 ic properties\, together with distorted-wave perturbation theory\, removes
  the infrared scale and gives bounds that converge rapidly to the exact on
 e. The same framework resums infinite families of Feynman diagrams\, inclu
 ding diagrams involving multiple polylogarithms and elliptic integrals\, i
 nto compact expressions. This suggests that a better treatment of long-ran
 ge dynamics may also simplify multi-loop amplitudes.
URL:https://www.imperial.ac.uk/events/214268/qft-gravity-seminar-luke-lipps
 treu/
DTSTART;TZID=Europe/London:20261016T120000
DTEND;TZID=Europe/London:20261016T130000
LOCATION:H503\, Huxley Building\, South Kensington Campus\, Imperial Colleg
 e London\, London\, SW7 2AZ\, United Kingdom
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