LHC as an Axion Factory: Probing an Axion Explanation for $(g-2{)}_{\mu }$ with Exotic Higgs Decays

We argue that a large region of so-far unconstrained parameter space for axionlike particles (ALPs), where their couplings to the standard model are of order $(0.01–1){\mathrm{TeV}}^{-1}$, can be explored by searches for the exotic Higgs decays $h\to Za$ and $h\to aa$ in run 2 of the LHC. Almost the complete region in which ALPs can explain the anomalous magnetic moment of the muon can be probed by searches for these decays with subsequent decay $a\to \gamma \gamma$, even if the relevant couplings are loop suppressed and the $a\to \gamma \gamma$ branching ratio is less than 1.

Figure 1

ALP-induced one-loop contributions to the anomalous magnetic moment of the muon.

Figure 2

Regions in ALP coupling space where the experimental value of $(g-2{)}_{\mu }$ is reproduced at 68% (red), 95% (orange), and 99% (yellow) confidence level (C.L.), for $m_a=1\mathrm{GeV}$.

Figure 3

Constraints on the ALP mass and coupling to photons derived from various experiments (colored areas without boundaries, adapted from [5]) along with the parameter regions (shaded in light green) that can be probed in LHC run 2 ($300{\mathrm{fb}}^{-1}$ integrated luminosity) using the Higgs decays $h\to Za\to {\ell }^{+}{\ell }^{-}\gamma \gamma$ (top) and $h\to aa\to 4\gamma$ (bottom). We require at least 100 signal events in each channel. The contours in the upper panel correspond to $|C_{Zh}|/\mathrm{\Lambda }=0.72{\mathrm{TeV}}^{-1}$ (solid), $0.1{\mathrm{TeV}}^{-1}$ (dashed), and $0.015{\mathrm{TeV}}^{-1}$ (dotted). Those in the lower panel refer to $|C_{ah}|/{\mathrm{\Lambda }}^2=1{\mathrm{TeV}}^{-2}$ (solid), $0.1{\mathrm{TeV}}^{-2}$ (dashed), and $0.01{\mathrm{TeV}}^{-2}$ (dotted). The red band shows the preferred parameter space where the $(g-2{)}_{\mu }$ anomaly can be explained at 95% C.L.

Figure 4

Constraints on the ALP mass and coupling to electrons derived from various experiments (colored areas without boundaries, adapted from [37, 38]) along with the parameter regions (shaded in light green) that can be probed in LHC run 2 ($300{\mathrm{fb}}^{-1}$ integrated luminosity) using the Higgs decays $h\to Za\to {\ell }^{+}{\ell }^{-}{e}^{+}{e}^{-}$ (top) and $h\to aa\to e^{+}{e}^{-}{e}^{+}{e}^{-}$ (bottom). We require at least 100 signal events in each channel. The contours in the upper panel correspond to $|C_{Zh}|/\mathrm{\Lambda }=0.72{\mathrm{TeV}}^{-1}$ (solid), $0.1{\mathrm{TeV}}^{-1}$ (dashed), and $0.01{\mathrm{TeV}}^{-1}$ (dotted). Those in the lower panel refer to $|C_{ah}|/{\mathrm{\Lambda }}^2=1{\mathrm{TeV}}^{-2}$ (solid), $0.1{\mathrm{TeV}}^{-2}$ (dashed), and $0.01{\mathrm{TeV}}^{-2}$ (dotted).