Abstract
.We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging 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 rightarrow 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 sides.
Highlights
Dielectron production in nucleon-nucleon collisions at kinetic beam energies below the η meson threshold production offers a unique possibility to study bremsstrahlung radiation with time-like virtual photons
The nucleon contribution provides information on the elastic time-like electromagnetic form factors in a region of four-momentum transfer squared 0 < q2 4m2p, where mp is the proton mass, which is inaccessible to measurements in e+e− or pp annihilation
Particle identification is provided by a hadron blind Ring Imaging Cherenkov (RICH) detector, centered around the target, two time-of-flight walls based on plastic scintillators covering polar angles θ > 45◦ (TOF) and θ < 45◦ (TOFino), respectively, and a Pre-Shower detector placed behind the TOFino
Summary
Dielectron production in nucleon-nucleon collisions at kinetic beam energies below the η meson threshold production offers a unique possibility to study bremsstrahlung radiation with time-like virtual photons. The cross sections can differ between the models substantially (up to a factor 2–4) in some phase space regions and need to be constrained further by experimental data Another approach, often used in microscopic transport model calculations to account for the nucleon-nucleon bremsstrahlung, is the soft photon approximation [8,9,10]. Its presence enhances the dielectron yield at large invariant masses Such a contribution can be interpreted as a formation of a ρ-like final state via annihilation of the exchanged charged pion with a pion from the nucleon meson cloud.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
