On the stability of the open-string QED neutron.

We study the stability of a hypothetical QED neutron, which consists of a color-singlet system of two $d$ quarks and a $u$ quark interacting with the quantum electrodynamical (QED) interactions. As a quark cannot be isolated, the intrinsic motion of the three quarks in the lowest-energy state of the QED neutron may lie predominantly in 1+1 dimensions, as in a $d$-$u$-$d$ open string. In such an open string, the attractive $d$-$u$ and $u$-$d$ QED interactions may overcome the weaker repulsive $d$-$d$ QED interaction to bind the three quarks together. We examine the stability of the QED neutron in a phenomenological three-body model in 1+1 dimensions with an effective interaction between electric charges extracted from Schwinger's exact QED solution in 1+1 dimensions. The phenomenological model in a variational calculation yields a stable QED neutron with a mass of 44.5 MeV. The analogous QED proton with two $u$ quarks and a $d$ quark has been found to be too repulsive to be stable and does not have a bound or continuum state, onto which the QED neutron can decay via the weak interaction. Consequently, the QED neutron may be stable against the weak decay and may have a very long lifetime. Such a particle may occur at the deconfinement-to-confinement phase transition of the quark-gluon plasma and may be a signature of the deconfinement-to-confinement transition of the quark gluon plasma in high-energy heavy-ion collisions. Because of its long lifetime, self-gravitating QED neutron assemblies (and similarly QED antineutron assemblies) of various sizes may be good candidates for a part of the primordial dark matter produced during the deconfinement-to-confinement phase transition of the quark gluon plasma in the evolution of the early Universe.