08/24/2022
By Spasen Chaykov

The Kenndy College of Sciences, Department of Physics and Applied Physics, invites you to attend a Master’s thesis defense by Spasen Chaykov on “Loop corrections in Minkowski spacetime away from equilibrium."

Date: Aug. 26, 2022
Time: noon
Location: Virtual on Zoom. People interested in attending should contact spasen_chaykov@student.uml.edu
Thesis Title: Loop corrections in Minkowski spacetime away from equilibrium

Committee Chair: Nishant Agarwal, Department of Physics and Applied Physics, University of Massachusetts Lowell

Committee Members:

  • Archana Kamal, Department of Physics and Applied Physics, University of Massachusetts Lowell
  • Viktor Podolskiy, Department of Physics and Applied Physics, University of Massachusetts Lowell

Abstract

Loop corrections to unequal-time correlation functions in Minkowski spacetime exhibit secular growth due to a breakdown of time-dependent perturbation theory. In this thesis, we study the time-dependence of one- and two-point correlation functions of massless scalar quantum field theories on a Minkowski background with a cubic or quartic self-interaction. We first calculate correlations in the late-time limit and in the vacuum of the free theory; in this limit, the calculation, including UV renormalization, reduces to that in in-out perturbation theory. We find linear or logarithmic growth in time, depending on whether the interaction strength is dimension-one or dimensionless, respectively. We next develop the Weisskopf-Wigner (WW) resummation method, that proceeds by demanding unitarity within a truncated Hilbert space, to calculate resummed correlators and find that it gives an exact exponentiation of the late-time perturbative result. The resummed (unequal-time) correlator thus decays with an exponential or polynomial time-dependence, which is suggestive of `universal' behavior that depends on the dimensions of the interaction strength. Lastly, we calculate the unequal-time two-point correlator starting the field evolution at an arbitrary initial time using in-in perturbation theory. We find that in addition to usual in-out counterterms we also need initial state counterterms to cancel all UV divergences and that the late-time limits match with in-out and WW, as expected.