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[Abridged] In contemporary literature, the calculation of modifications tothe inflationary scalar power spectrum due to the loops from the higher orderinteraction terms in the Hamiltonian have led to a discussion regarding thevalidity of perturbation theory. Recently, there have been efforts to examinethe contributions to the scalar power spectrum due to the loops arising fromthe cubic order terms in the action describing the perturbations, specificallyin inflationary scenarios that permit an epoch of ultra slow roll (USR). Aphase of USR inflation leads to significant observational consequences, such asthe copious production of primordial black holes. In this work, we study theloop contributions to the scalar power spectrum in a scenario of USR inflationarising due to the quartic order terms in the action describing the scalarperturbations. We compute the loop contributions to the scalar power spectrumdue to the dominant term in the action at the quartic order. We consider ascenario wherein a phase of USR is sandwiched between two stages of slow rollinflation and analyze the behavior of the loop contributions in terms of theparameters involved. We examine the late, intermediate and early epochs of USRduring inflation. In the inflationary scenario involving a late phase of USR,for reasonable choices of the parameters, we show that the loop corrections arenegligible for the entire range of wave numbers. In the intermediate case, thecontributions from the loops prove to be scale invariant over large scales, andwe find that these contributions can amount to 30% of the leading order powerspectrum. In the case wherein USR sets in early, we find that the loopcontributions could be negative and can dominate the power spectrum at theleading order, which indicates a breakdown of the perturbative expansion. Weconclude with a brief summary and outlook.

Loop contributions to the scalar power spectrum due to quartic order action in ultra slow roll inflation