Possible Evidence for Neutral Color-Singlet $q\bar q$ Quark Matter from High-Energy Pb-Emulsion Collisions

The invariant mass spectrum of $e^+e^-$ pairs produced in high-energy Pb-emulsion collisions at 160 A GeV at CERN SPS exhibits a complex structure of many resonances resting on top of a broad enhancement at invariant masses below 50 MeV, with the prominent resonance at 19 $\pm$1 MeV providing independent support for the hypothetical X17 particle. We show that this complex structure may be coherently described as signatures for the neutral color-singlet $q\bar q$ quark matter in both its deconfined and confined phases. That is, the broad enhancement may arise from thermal annihilation of QED(U(1))-deconfined quarks and antiquarks into $e^+e^-$ pairs at the phase transition temperature $T_c$(QED), theoretically estimated to be 4.75 $\pm$ 1.2 MeV from the transitional equilibrium condition. The observed 3$\pm$1 and 7$\pm$1 MeV resonances may correspond to the QED(U(1))-deconfined $d\bar d$ and $u\bar u$ Coulomb bound states near their quark rest masses, respectively, whereas the observed 19 $\pm$ 1 MeV resonance may correspond to the QED(U(1))-confined isoscalar QED meson. The approximate agreement between the theoretical and the experimental spectrum suggests that both QED(U(1))-confined and QED(U(1))-deconfined neutral color-singlet $q\bar q$ quark matter may have been produced in these high-energy Pb-emulsion collisions. We propose future experiments to confirm or refute these findings.