Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

TL;DR

This study measures electron pair production in elementary and light nucleus collisions, revealing a significant excess in neutron-proton reactions and suggesting current models do not fully explain the production mechanisms.

Contribution

First measurement of electron pairs in n+p reactions at 1.25 GeV/u, highlighting differences from p+p and implications for modeling light nucleus collisions.

Findings

Electron pair yield in n+p is ten times higher than in p+p for invariant mass > 0.15 GeV/c2.

Electron spectra in C+C are consistent with a superposition of elementary n+p and p+p reactions.

Current models do not fully account for the observed electron pair production mechanisms.

Abstract

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.