Fundamental Physics Seminar Series - Analytic Methods for Loop-Level Cosmological Correlators

Abstract

Heavy particles with masses of order the Hubble scale may have been produced during inflation and left characteristic imprints on primordial correlators, providing a unique window into ultra-high-energy physics through cosmological collider physics, where cosmological correlators with massive exchanges serve as the central observables. While tree-level correlators have been extensively studied, loop-level correlators remain much less understood, especially in situations that break de Sitter (dS) boosts and therefore evade the simplifying constraints of symmetry.

In this talk, the speaker will present recent progress on loop-level cosmological correlators using two complementary approaches. First, he will introduce the method of spectral dispersion, a new approach that reconstructs correlators from their on-shell data up to local counterterms by combining dS spectral decomposition with dispersion relations. He will then apply this method to one-loop bubble diagrams, showing how it makes it possible to recover simple analytic expressions directly from the oscillatory nonlocal signals and naturally leads to irreducible results that are free of UV divergences and independent of renormalization schemes. Second, he will discuss direct computations based on the partial Mellin-Barnes (PMB) representation, a framework that extends naturally beyond bubble topology. Using bubble and triangle diagrams as examples, the speaker will show how it cleanly separates signal and background contributions, clarifies the origin of ultraviolet divergences, and provides a transparent treatment of renormalization. It may also offer a useful route toward more general dS-boost-breaking processes. Taken together, these results contribute to a more systematic analytic understanding of loop-level correlators in de Sitter space and point toward further progress on more general loop topologies and boost-breaking processes.


About the Speaker

Mr. QIN Zhehan is a PhD candidate in the Department of Physics at Tsinghua University. He received his bachelor’s degrees in Physics and in Mathematics and Applied Mathematics from Tsinghua University in 2021. His research is in theoretical particle physics and cosmology, with a particular focus on cosmological correlators, cosmological collider physics, and quantum field theory in curved spacetime. He develops analytic methods for cosmological observables, studies their analytic structure, and seeks to understand the proper effective field theory description in de Sitter spacetime.


About the Center for Fundamental Physics

For more information, please refer to https://cfp.hkust.edu.hk/.


For Attendees' Attention

Seating is on a first come, first served basis.