Dilepton production in proton-proton and quasifree proton-neutron reactions at 1.25 GeV

TL;DR

This study models dilepton production in proton-proton and proton-neutron reactions at 1.25 GeV, successfully reproducing proton-proton data and exploring the sensitivity of proton-neutron reactions to pion form factors.

Contribution

It provides a detailed effective Lagrangian model for dilepton production, including Delta isobar contributions, and compares predictions with recent experimental data.

Findings

Model reproduces pp reaction data well, with Delta dominance.

Underpredicts pn reaction cross sections in 0.40-0.55 GeV/c^2 mass range.

Pion electromagnetic form factor significantly influences pn reaction predictions.

Abstract

We investigate the pp --> pp e+e- and quasifree pn --> pn e+e- reactions within an effective Lagrangian model at the laboratory kinetic energy of 1.25 GeV for which experimental data have recently been reported by the HADES Collaboration. The model uses a meson-exchange approximation to describe the initial nucleon-nucleon (NN) scattering. Contributions to the reaction amplitudes are included from the NN bremsstrahlung as well as from the excitation, propagation and radiative decay of the Delta(1230) isobar state. It is found that the HADES data on the e+e- invariant mass distribution in the pp --> pp e+e- reaction are reproduced excellently by our model where the Delta isobar term dominates the spectrum. However, the quasifree pn --> pn e+e- cross sections are underpredicted in the invariant mass region of 0.40 - 0.55 GeV/c^2 even after including contributions of the eta Dalitz decay and the subthreshold production and decay of the rho^0 meson via the baryonic resonance N*(1520). In the case of the quasifree pn --> pn e+e- reaction, a strong sensitivity to the pion electromagnetic form factor is observed which helps to bring the calculated cross sections closer to the data in the higher dilepton mass region.