# Analysis of the exclusive final state npe$^+$e$^-$ in quasi-free np reaction

## TL;DR

This study investigates dielectron production in quasi-free neutron-proton collisions, revealing an excess of $e^+e^-$ pairs above the $ ext{pi}^0$ mass and evaluating bremsstrahlung mechanisms through experimental data and models.

## Contribution

It provides new experimental data on dielectron production in neutron-proton collisions and compares different bremsstrahlung models to explain the excess of $e^+e^-$ pairs.

## Key findings

- Confirmed excess of $e^+e^-$ pairs above $ ext{pi}^0$ mass
- Differential cross sections constrain bremsstrahlung models
- Results suggest different production mechanisms in neutron-proton collisions

## Abstract

We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25 GeV/u inpinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of $e^+e^-$ pairs above the $\pi^{0}$ mass in the exclusive channel $dp \to npe^{+}e^{-}(p_{spect})$ as compared to the exclusive channel $ppe^{+}e^{-}$ measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double-$\Delta$ excitation combined with intermediate $\rho$-meson production. Differential cross sections as a function of the $e^+e^-$ invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical side.