Contribution of two-pion annihilation to dilepton emission from proton-nucleon collisions

The Dilepton Spectrometer collaboration has reported a bump in the $e^{+}$ $e^{\mathrm{-}}$ mass distribution at around 2$m_{\pi }$ in 4.9 GeV proton-Be collisions. Previous calculations have shown that this bump does not have its origin in nucleon-nucleon bremsstrahlung nor in the Dalitz decay of the Δ resonance. In this paper we estimate the contribution from two-pion annihilation based on relativistic kinetic theory, on the experimentally measured cross sections for the production of up to five pions, and on a phase space generator for the pion momentum distributions. The calculation fails to reproduce the data.. The origin of the bump remains unexplained.