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

TL;DR

This work computes exclusive γγ fusion cross sections for the production of about 50 even-spin resonances and exotic QED atoms in ultraperipheral hadron collisions across RHIC, LHC, FCC, and proton-air interactions. Using an improved EPA with realistic photon fluxes and Low's formula, it delivers cross sections and yields for quarkonium, exotic hadrons, leptonium, and hadronium, including toponium, and provides detailed uncertainty propagation via the two-photon widths. It also analyzes the impact of these resonances as backgrounds to light-by-light scattering at low diphoton masses and outlines experimental prospects, particularly for ALICE and LHCb at the LHC and for future FCC-hh and cosmic-ray studies. The results broaden the accessible spectroscopy of C-even states, test QED bound-state dynamics, and quantify resonant backgrounds relevant for precision LbL measurements in UPCs.

Abstract

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 RHIC, LHC and FCC colliders, 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 $π^0$ 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 ($γγ\toγγ$) over $m_{γγ} = 0.1$--15 GeV in PbPb UPCs at the LHC.

Figures (13)

• Figure 1: Schematic diagrams of the photon-photon production of a pair of opposite-charge particles $\rm X^+X^-$ followed by the formation of an $\rm (X^+X^-)$ onium-like bound state in p-p (left), proton-nucleus (center), and nucleus-nucleus (right) collisions. The $\rm (X^+X^-)$ states considered in this work are even-spin states formed by pairs of quarks ($\rm X= q$) bound by their QCD interaction, or of leptons ($\rm X = e, \mu, \tau$) or hadrons ($\rm X = mesons$, or baryons) bound by their QED interaction.
• Figure 2: Effective photon-photon luminosities as a function of $m_\mathrm{X}$, $\mathrm{d}{\mathcal{L}}_{\gamma\gamma}/\mathrm{d}m_\mathrm{X}$ given by Eqs. (\ref{['eq:gagalumi']})--(\ref{['eq:Psurv']}), for ultraperipheral Au-Au (200 GeV), Pb-Pb (5.5 TeV), p-Pb (8.8 TeV), and p-p (14 TeV) collisions at RHIC and LHC (left), and Pb-Pb (39 TeV), p-Pb (62.8 TeV), and p-p (100 TeV) at FCC, and p-air (400 TeV) for cosmic rays at the GZK cutoff (right), obtained with the gamma-UPC code with $\gamma$ photon fluxes derived from the corresponding ion charged form factors.
• Figure 3: Cross sections for the $\gamma\gamma$ production of even-spin light mesons (with $m_\mathrm{X}\lesssim 1$ GeV and known $\gamma\gamma$ widths) as a function of nucleon-nucleon c.m. energy $\!\sqrt{s_{_\text{NN}}}$, in Pb-Pb or Au-Au (solid curves), p-Pb (dashed curves), and p-p (dotted curves) UPCs. The curves are $\ln ^3(s_{_\text{NN}})$ fits to guide the eye.
• Figure 4: Cross sections for the $\gamma\gamma$ production of even-spin light mesons (with $m_\mathrm{X}= 1$--1.5 GeV and known $\gamma\gamma$ widths) as a function of nucleon-nucleon c.m. energy $\!\sqrt{s_{_\text{NN}}}$, in Pb-Pb or Au-Au (solid curves), p-Pb (dashed curves), and p-p (dotted curves) UPCs. The curves are $\ln ^3(s_{_\text{NN}})$ fits to guide the eye.
• Figure 5: Cross sections for the $\gamma\gamma$ production of light mesons (with $m_\mathrm{X}= 1.5$--2.0 GeV and known $\gamma\gamma$ widths) as a function of nucleon-nucleon c.m. energy $\!\sqrt{s_{_\text{NN}}}$, in Pb-Pb or Au-Au (solid curves), p-Pb (dashed curves), and p-p (dotted curves) UPCs. The curves are $\ln ^3(s_{_\text{NN}})$ fits to guide the eye.