Production of four-prong final states in photon-photon collisions

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Abstract

Results are presented on the exclusive production of four-prong final states in photon-photon collisions from the TPC/Two-Gamma detector at the SLAC ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ storage ring PEP. Measurement of dE/dx and momentum in the time-projection chamber (TPC) provides identification of the final states 2${\ensuremath{\pi}}^{+}$2${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$, ${K}^{+}$${K}^{\mathrm{\ensuremath{-}}}$${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$, and 2${K}^{+}$2${K}^{\mathrm{\ensuremath{-}}}$. For two quasireal incident photons, both the 2${\ensuremath{\pi}}^{+}$2${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ and ${K}^{+}$${K}^{\mathrm{\ensuremath{-}}}$${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ cross sections show a steep rise from threshold to a peak value, followed by a decrease at higher mass. Cross sections for the production of the final states ${\ensuremath{\rho}}^{0}$${\ensuremath{\rho}}^{0}$, ${\ensuremath{\rho}}^{0}$${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$, and \ensuremath{\varphi}${\ensuremath{\pi}}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ are presented, together with upper limits for \ensuremath{\varphi}${\ensuremath{\rho}}^{0}$, \ensuremath{\varphi}\ensuremath{\varphi}, and ${K}^{\mathrm{*}0}$K\ifmmode\bar\else\textasciimacron\fi{} $^{\mathrm{*}0}$. The ${\ensuremath{\rho}}^{0}$${\ensuremath{\rho}}^{0}$ contribution dominates the four-pion cross section at low masses, but falls to nearly zero above 2 GeV. Such behavior is inconsistent with expectations from vector dominance but can be accommodated by four-quark resonance models or by t-channel factorization. Angular distributions for the part of the data dominated by ${\ensuremath{\rho}}^{0}$${\ensuremath{\rho}}^{0}$ final states are consistent with the production of ${J}^{P}$${=2}^{+}$ or ${0}^{+}$ resonances but also with isotropic (nonresonant) production. When one of the virtual photons has mass (${m}_{\ensuremath{\gamma}}^{2}$=-${Q}^{2}$\ensuremath{\ne}0), the four-pion cross section is still dominated by ${\ensuremath{\rho}}^{0}$${\ensuremath{\rho}}^{0}$ at low final-state masses ${W}_{\ensuremath{\gamma}\ensuremath{\gamma}}$ and by 2${\ensuremath{\pi}}^{+}$2${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ at higher mass. Further, the dependence of the cross section on ${Q}^{2}$ becomes increasingly flat as ${W}_{\ensuremath{\gamma}\ensuremath{\gamma}}$ increases.

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