Fig. 8.

Diphoton invariant mass distribution at $\sqrt{s}=5.02$ TeV in Pb–Pb collisions, for integrated luminosity $L=10{\mathrm{nb}}^{-1}$. The result due to the production of an ALP of mass 10 and 30 GeV is shown, with coupling $g_a=5\times {10}^{-5}$ ${\mathrm{GeV}}^{-1}$, in both cases with a width of 0.5 GeV included to roughly mimic the effect of experimental resolution. The continuum light-by-light background is also shown. The photons are required to have transverse momentum $p_{\perp }^{\gamma }>3$ GeV and pseudorapidity $|{\eta }^{\gamma }|<2.4$. The ALP is assumed here only to couple to photons