Open string QED meson description of the X17 particle and dark matter.

As a quark and an antiquark cannot be isolated, the intrinsic motion of a composite $q \bar q$ system in its lowest-energy states lies predominantly in 1+1 dimensions, as in an open string with the quark and the antiquark at its two ends. Accordingly, we study the lowest-energy states of an open string $q\bar q$ system in QCD and QED in 1+1 dimensions. We show that $\pi^0, \eta$, and $\eta'$ can be adequately described as open string $q\bar q$ QCD mesons. By extrapolating into the $q\bar q$ QED sector in which a quark and an antiquark interact with the QED interaction, we find an open string isocalar $I(J^\pi)$=$0(0^-)$ QED meson state at 19.2 MeV and an isovector $(I(J^\pi)$=$1(0^-), I_3=0)$ QED meson state at 38.5 MeV. The predicted mass of the $I(J^\pi)$=$0(0^-)$ $q\bar q$ QED meson is close to the mass of the X17 particle observed in the decay of the excited $0(0^-)$ state of He$^4$ to make it a good candidate for the X17 particle. The decay products of $q\bar q$ QED mesons may show up as excess $e^+e^-$ pair decay products in the anomalous soft photon phenomenon in hadron productions in high-energy hadron-proton collisions and $e^+$-$e^-$ annihilations. Measurements of the invariant masses of excess $e^+e^-$ pairs and the detection of the QED meson decay gamma rays will provide tests for the existence of the open string $q\bar q$ QED mesons. An assembly of gravitating QED mesons are expected to emit electron-positron pairs and/or gamma rays and their decay energies will be modified by their gravitational binding energies. Consequently, a self-gravitating isoscalar QED meson assembly whose mass $M$ and radius $R$ satisfy $(M/M_\odot)/(R/R_\odot) \gtrsim 4.71 \times 10^5$ will not produce electron-positron pairs nor gamma rays and may be a good candidate for the primordial dark matter.