Searching for axion-like particles with ultra-peripheral heavy-ion collisions

TL;DR

The paper proposes using ultra-peripheral heavy-ion collisions at the LHC to search for axion-like particles with masses between 5 and 100 GeV through photon fusion, leveraging the Z^4-enhanced γγ luminosity and the clean, exclusive final state. It develops the theoretical framework for ALP production in UPCs, models the photon-photon luminosity with exclusivity effects via STARlight, and outlines a diphoton search strategy with robust background rejection. By recasting LEP and LHC multi-photon searches and computing UPC sensitivities, the study demonstrates that Pb-Pb UPCs can provide leading constraints on the ALP-photon couplings in this mass range, surpassing existing limits for certain operators. The work highlights heavy-ion UPCs as a unique, high-sensitivity channel for ALP searches with potentially significant implications for beyond-Standard-Model photon-coupled sectors.

Abstract

We show that ultra-peripheral heavy-ion collisions at the LHC can be used to search for axion-like particles with mass below 100 GeV. The $Z^4$ enhanced photon-photon luminosity from the ions provides a large exclusive production rate, with a signature of a resonant pair of back-to-back photons and no other activity in the detector. In addition, we present both new and updated limits from recasting multi-photon searches at LEP II and the LHC, which are more stringent than those currently in the literature for the mass range 100 MeV to 100 GeV.

Figures (4)

• Figure 1: Exclusive ALP production in ultra-peripheral Pb-Pb collisions.
• Figure 2: The dominant backgrounds to the ALP signal are from light-by-light scattering in ultra-peripheral Pb-Pb collisions, and $e^+ e^-$ production where both electrons fake a photon.
• Figure 3: Differential cross section for signal and background. Shown from bottom to top are the stacked background distributions for bremsstrahlung photons from electrons (purple), fake photons from electrons (blue) and light-by-light scattering (LBL) (green). The red (orange) line shows an injected signal with a 5 nb production cross section for $m_a = 15$ GeV and $\Lambda=17$ TeV ($m_a =40$ GeV and $\Lambda=8$ TeV), taking an energy resolution of 0.5 GeV.
• Figure 4: Left: Expected sensitivity to the operator $\frac{1}{4}\frac{1}{\Lambda}a F\tilde{F}$ in heavy-ion UPCs at the LHC (green solid and dashed curves, for a ${\rm Pb}$-${\rm Pb}$ luminosity of 1 nb$^{-1}$ and 10 nb$^{-1}$, respectively). Shown for comparison is the limit from 36 pb$^{-1}$ of exclusive p-p collisions Chatrchyan:2012tv (red dot-dash). New and updated exclusion limits from LEPII (OPAL $2 \gamma$, $3 \gamma$) Abbiendi:2002je and from the LHC (ATLAS 2$\gamma$, 3$\gamma$) Aad:2014ioaAad:2015bua are indicated by the various shaded regions (see text). Right: The analogous results for the operator $\frac{1}{4\cos^2 \theta_W}\frac{1}{\Lambda}a B\tilde{B}$. The LEPI, $2\gamma$ (teal shaded) region is taken from Jaeckel:2015jla.