Precision Two-Photon Exchange Effects in Lepton–Proton Scattering within Chiral Effective Theory

by Rakshanda Goswami (IIT Guwahati)

Thursday 5 Feb 2026, 11:00 → 13:00 Europe/Amsterdam

Colloquium Room (Nikhef)

Lepton–proton scattering has long served as a primary tool for probing the internal structure of the proton. Recent developments, most notably the proton charge-radius puzzle and the associated form-factor discrepancy, have renewed interest in this process and underscored the need for increasingly precise theoretical descriptions. A key ingredient in this context is the two-photon exchange (TPE) mechanism, a subleading correction to the dominant one-photon exchange process and a major source of systematic uncertainty in the extraction of proton observables. In this talk, I present a low-energy evaluation of TPE corrections with an elastic proton intermediate state within the framework of SU(2) Heavy Baryon Chiral Perturbation Theory (HBχPT), a model-independent effective field theory of QCD. The analysis is carried out for kinematics relevant to the forthcoming MUSE experiment at the Paul Scherrer Institute. To assess the reliability of commonly employed approximations, I first compute the TPE contribution up to next-to next-to-leading order (NNLO) using the widely adopted soft-photon approximation (SPA). I then perform an exact analytical calculation without invoking any approximation scheme and systematically compare the two approaches. Special emphasis is placed on the lepton–antilepton charge asymmetry, evaluated consistently up-to and-including next-to-leading order (NLO), as well as incorporating the dominant NNLO hadronic corrections to the OPE, within both the SPA-based and exact frameworks. This comparison allows us to quantify the accuracy and limitations of the SPA and to assess its impact on precision studies of lepton–proton scattring.