Exclusive photon-fusion production of even-spin resonances and exotic QED atoms in high-energy hadron collisions

The cross sections for the single exclusive production of (pseudo)scalar and (pseudo)tensor hadrons, as well as of even-spin QED bound states formed by pairs of opposite-charge leptons or hadrons, are estimated for photon-fusion processes in ultraperipheral collisions (UPCs) of proton-proton, proton-nucleus, and nucleus-nucleus at the Relativistic Heavy-Ion Collider, Large Hadron Collider (LHC) and Future Circular Collider, as well as in proton-air interactions at the highest energies reached by cosmic rays impinging on Earth. The UPC cross sections are computed in the equivalent photon approximation with realistic photon fluxes from the charged form factors of proton, lead, gold, and nitrogen ions. The production of four types of even-spin systems are considered: quarkonium (spin-0, 2, 4 meson bound states, from the lightest <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msup> <a:mi>π</a:mi> <a:mn>0</a:mn> </a:msup> </a:math> meson up to toponium), exotic hadrons (including candidate multiquark states), leptonium (positronium, dimuonium, and ditauonium), as well as mesonium (pionium, kaonium, D-onium, and B-onium) and baryonium (notably, protonium) QED atoms. The expected yields at the different colliders are presented for about 50 such even-spin composite resonances, for which the ALICE and LHCb experiments have potential reconstruction capabilities at the LHC. The impact of the diphoton decays of such even-spin states is also discussed as resonant backgrounds in the measurement of light-by-light scattering ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>γ</c:mi> <c:mi>γ</c:mi> <c:mo stretchy="false">→</c:mo> <c:mi>γ</c:mi> <c:mi>γ</c:mi> </c:math> ) over <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:msub> <f:mrow> <f:mi>m</f:mi> </f:mrow> <f:mrow> <f:mi>γ</f:mi> <f:mi>γ</f:mi> </f:mrow> </f:msub> <f:mo>≈</f:mo> <f:mn>0.1</f:mn> <f:mi>–</f:mi> <f:mn>15</f:mn> </f:mrow> </f:math> -GeV masses in Pb-Pb UPCs at the LHC.