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SUMMARY:What is the iε for the S-matrix? - Sebastian Mizera (IAS)
DTSTART:20220224T160000Z
DTEND:20220224T170000Z
UID:TALK168545@talks.cam.ac.uk
CONTACT:Chiung Hwang
DESCRIPTION:Can the S-matrix be complexified in a way consistent with caus
 ality? Since the 60's\, the affirmative answer to this question has been w
 ell-understood for 2→2 scattering of the lightest particle in theories w
 ith a mass gap\, where the S-matrix is analytic everywhere except at norma
 l-threshold branch cuts. We ask whether this picture extends to realistic 
 theories\, such as the Standard Model\, that include massless fields\, UV/
 IR divergences\, and unstable particles. Especially in the presence of lig
 ht states running in the loops\, the traditional iε prescription for appr
 oaching branch cuts breaks down\, because causality requirements for the i
 ndividual Feynman diagrams can be mutually incompatible. To fix this issue
 \, we propose an iε-like prescription for deforming branch cuts in the sp
 ace of Mandelstam invariants without modifying the analytic properties. Th
 is procedure results in a small complex strip around the real slice of the
  kinematic space\, where the S-matrix remains causal.
LOCATION:Online (Zoom)
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