Measurement of the $p(e,e'\pi^+)n$ reaction close to threshold and at low $Q^2$

TL;DR

This study measures the $p(e,e'\pi^+)n$ reaction near threshold at low $Q^2$, providing new experimental data to test theoretical models of pion production dynamics.

Contribution

First measurement of the $p(e,e'\pi^+)n$ reaction at low $Q^2$ near threshold using a new spectrometer, with separation of cross section components and comparison to theoretical models.

Findings

Experimental cross sections compared with models show discrepancies.

Transverse and longitudinal components were successfully separated.

Results indicate incomplete understanding of pion production dynamics.

Abstract

The cross section of the $p(e,e'\pi^+)n$ reaction has been measured for five kinematic settings at an invariant mass of $W = 1094$ MeV and for a four-momentum transfer of $Q^2 = 0.078$ (GeV/$c$)$^2$. The measurement has been performed at MAMI using a new short-orbit spectrometer (SOS) of the A1 collaboration, intended for detection of low-energy pions. The transverse and longitudinal cross section terms were separated using the Rosenbluth method and the transverse-longitudinal interference term has been determined from the left-right asymmetry. The experimental cross section terms are compared with the calculations of three models: DMT2001, MAID2007 and $\chi$MAID. The results show that we do not yet understand the dynamics of the fundamental pion.