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

by Rakshanda Goswami (Department of Physics, IIT Guwahati, India)

Tuesday 3 Feb 2026, 15:15 → 17:15 Europe/Berlin

Nußallee 14-16/0.023 (HISKP) - Lecture Hall (HISKP)

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 the-
oretical descriptions. A key ingredient in this context is the two-photon ex-
change (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 the-
ory of QCD. The analysis is carried out for kinematics relevant to the forthcom-
ing 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 ap-
proaches.
Special emphasis is placed on the lepton–antilepton charge asymmetry, eval-
uated 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 quan-
tify the accuracy and limitations of the SPA and to assess its impact on precision
studies of lepton–proton scattring.