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

Abstract

The invariant mass spectrum of $e^{+}{e}^{-}$ pairs produced in high-energy Pb-emulsion collisions at 160 A GeV at CERN SPS exhibits a highly complex structure of a broad enhancement at 11$\pm$1 MeV with a full width of about 10 MeV, and additional many narrow resonances within the experimental bin width of 2 MeV, including a prominent narrow resonance at 19 $\pm$1 MeV that provides an independent support for the hypothetical X17 particle. We show that the highly complex spectrum may be coherently described as the signatures for the neutral color-singlet $q\bar{q}$ quark matter in both its deconfined phase as well as its confined phase. That is, the broad enhancement at 11$\pm$1 MeV 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), which can be theoretically estimated to be 4.75 $\pm$ 1.04 MeV from the transitional equilibrium condition. The observed narrow resonances at 3$\pm$1 and 7$\pm$1 MeV may correspond to the QED(U(1))-deconfined $u\bar{u}$ and $d\bar{d}$ Coulomb bound states near their quark rest masses, respectively, whereas the observed narrow resonance at 19 $\pm$ 1 MeV may correspond to the QED(U(1))-confined isoscalar QED meson. The approximate agreement between the theoretical and the experimental spectrum suggests tentatively 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 collision, pending confirmation of the Pb-emulsion collision data. We propose future experiments to confirm or refute these findings.

Comments: 30 pages, 4 figures