# Two-photon exchange corrections to $\gamma^*N\Delta$ form factors for $Q^2\leq 4 $ (Gev/$c$)$^2$

## TL;DR

This paper calculates two-photon exchange corrections to gamma-star N Delta transition form factors at moderate to high Q^2, finding small effects on some form factors but significant corrections on others, impacting multipole analyses.

## Contribution

It provides the first hadronic model estimate of TPE effects on gamma-star N Delta form factors, including comparisons with partonic calculations at high Q^2.

## Key findings

- TPE effects on G*_M are negligible.
- TPE effects on G*_E and G*_C are up to 15% at Q^2 ~ 4 GeV^2.
- Significant TPE corrections suggest their inclusion in high Q^2 multipole analyses.

## Abstract

We evaluate the corrections of the two-photon exchange (TPE) process on the $\gamma^* N\Delta$ transition form factors. The contributions of the TPE process to the $eN\rightarrow e\Delta(1232)\rightarrow eN\pi$ are calculated in a hadronic model with the inclusion of only the elastic nucleon intermediate states, to estimate its effects on the multipoles $M^{(3/2)}_{1^+}, E^{(3/2)}_{1^+}, S^{(3/2)}_{1^+}$ at the $\Delta$ peak. We find that TPE effects on $G^*_M$ is very small. $G^*_E,$ and $ G^*_C$ are also little affected at small $Q^2 $. For $G^*_E$, the TPE effects reach about $3 - 8\%$ near $Q^2\sim 4$ GeV$^2$, depending on the model, MAID or SAID, used to emulate the data. For $G^*_C$, the TPE effects decrease rapidly with increasing $\epsilon$ while growing with increasing $Q^2$ to reach $\sim 6 - 15\%$ with $Q^2 \sim 4$ GeV$^2$ at $\epsilon = 0.2$. Sizeable TPE corrections to $G^*_E$ and $G^*_C$ found here points to the need of including TPE effects in the multipole analysis in the region of high $Q^2$ and small $\epsilon$. The TPE corrections to $R_{EM}$ and $R_{SM}$ obtained in our hadronic calculation are compared with those obtained in a partonic calculation for moderate momentum transfer of $2<Q^2<4$ GeV$^2$.