Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

TL;DR

This CMS study provides evidence for elastic light-by-light scattering in ultraperipheral PbPb collisions at 5.02 TeV, quantifying a fiducial γγ→γγ cross section in good agreement with the Standard Model and establishing an R ratio relative to QED dielectron production to reduce systematics. By exploiting the mγγ spectrum, the analysis sets new exclusion limits on axion-like particles in the 5–90 GeV mass range, using a CLs framework to translate diphoton final states into constraints on ALP couplings. The work combines data-driven background control with detailed MC modeling (MadGraph, Starlight, SuperChic) and robust efficiency calibrations, offering a precise benchmark for photon-photon processes in heavy-ion UPCs and for BSM searches in this channel.

Abstract

Evidence for the light-by-light scattering process, $γγ$ $\to$ $γγ$, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390 $μ$b$^{-1}$ recorded by the CMS experiment at the LHC. Light-by-light scattering processes are selected in events with two photons exclusively produced, each with transverse energy E$_\mathrm{T}^γ$ $>$ 2 GeV, pseudorapidity $|η^γ|$ $\lt$ 2.4, diphoton invariant mass $m^{γγ}$ $\gt$ 5 GeV, diphoton transverse momentum $p_\mathrm{T}^{γγ}$ $\lt$ 1 GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 9.0 $\pm$ 0.9 (theo) events for the signal and 4.0 $\pm$ 1.2 (stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 3.7 standard deviations, and has properties consistent with those expected for the light-by-light scattering signal. The measured fiducial light-by-light scattering cross section, $σ_\mathrm{fid} (γγ$ $\to$ $γγ) =$ 120 $\pm$ 46 (stat) $\pm$ 28 (syst) $\pm$ 12 (theo) nb, is consistent with the standard model prediction. The $m^{γγ}$ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the $γγ$ $\to$ a $\to$ $γγ$ process, in the mass range $m_{\mathrm{a}} =$ 5-90 GeV.

Figures (7)

• Figure 1: Schematic diagrams of light-by-light scattering ($\gamma\gamma \to \gamma\gamma$, left), QED dielectron ($\gamma\gamma \to { }\xspace{ }\xspace$, centre), and central exclusive diphoton (${ }\xspace{ }\xspace \to \gamma\gamma$, right) production in ultraperipheral PbPb collisions. The $\,^{(*)}$ superscript indicates a potential electromagnetic excitation of the outgoing ions.
• Figure 2: Acoplanarity distribution of exclusive ${ }\xspace{ }\xspace$ events measured in data (circles), compared to the expected QED ${ }\xspace{ }\xspace$ spectrum in the starlight MC simulation (histogram), scaled as described in the text. The curve shows a $\chi^2$ fit to the sum of two exponential distributions corresponding to exclusive ${ }\xspace{ }\xspace$ plus any residual (nonacoplanar) background pairs. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histogram include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size.
• Figure 3: Comparison of data (circles) and starlight MC expectation (histogram, scaled as described in the text) for the exclusive ${ }\xspace{ }\xspace$ events passing all selection criteria, as a function of dielectron acoplanarity (top left), mass (top right), $p_{\mathrm{T}}$ (bottom left), and rapidity $y$ (bottom right). Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panels.
• Figure 4: Diphoton acoplanarity distribution for exclusive events measured in the data after selection criteria (squares), compared to the expected LbL scattering signal (orange histogram), QED ${ }\xspace{ }\xspace$ (yellow histogram), and the CEP+other (light blue histogram, scaled to match the data in the $\text{A}_{\phi} > 0.02$ region as described in the text) backgrounds. Signal and QED ${ }\xspace{ }\xspace$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size.
• Figure 5: Distributions of the single photon $E_{\mathrm{T}}$, $\eta$, and $\phi$, as well as diphoton $p_{\mathrm{T}}$, rapidity, and invariant mass measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange histogram), QED ${ }\xspace{ }\xspace$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text{A}_{\phi} > 0.02$ region). Signal and QED ${ }\xspace{ }\xspace$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size.