Photoproduction of charged final states in ultra-peripheral collisions and electroproduction at an electron-ion collider

Abstract

Ultra-peripheral collisions (UPCs) of relativistic ions are an important tool for studying photoproduction at high energies. Vector meson photoproduction is an important tool for nuclear structure measurements and other applications. A future electron-ion collider (EIC) will allow additional studies, using virtual photons with a wide range of $Q^2$. We propose a significant expansion of the UPC and EIC photoproduction physics programs to include charged final states which may be produced via Reggeon exchange. We consider two examples: $a_2^{+}(1320)$, which is a conventional $q\overline{q}$ meson, and the exotic $Z_c^{+}(4430)$ state (modeled here as a tetraquark). The $Z_c^{+}(4430)$ cross-section depends on its internal structure, so photoproduction can test whether the $Z_c^{+}(4430)$ is a tetraquark or other exotic object. We calculate the rates and kinematic distributions for $\gamma p\to X^{+}n$ in $pA$ UPCs and $ep$ collisions at an EIC and in UPCs. The rates are large enough for detailed studies of these final states. Because the cross-section for Reggeon exchange is largest near threshold, the final state rapidity distribution depends on the beam energies. At high-energy colliders like the proposed LHeC or $pA$ collisions at the LHC, the final states are produced at far forward rapidities. For lower energy colliders, the systems are produced closer to mid-rapidity, within reach of central detectors.

5 pages with 3 figures

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• Photoproduction of pentaquarks states at the LHC
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• EIC Physics from An All-Silicon Tracking Detector
• Backward-Angle ($u$-channel) Production at an Electron-Ion Collider
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• New opportunities at the photon energy frontier
• Exclusive Charmonium Production at the Electron-ion collider in China
• Production of the bottomonium-like states $Z_b$ states at $e$-$h$ and ultraperipheral $h$-$h$ collisions
• Elastic and inelastic $J/\psi$ photoproduction in $p$-$p$ collisions at LHC energies: the feature of Weizsäcker-Williams approximation