Axion-like particles (ALPs) are well-motivated extensions of the standard model (SM) that appear in numerous new physics scenarios. In this paper, we concentrate on searches for long-lived ALPs predicted by the photophobic scenario at the HL-LHC with the center-of-mass energy $\sqrt{ s}=14$ TeV and the integrated luminosity $\mathcal{ L}=$ $3$ ab$^{ -1}$. We consider the process $pp \to \gamma a$ with the ALP $a$ decaying into a pair of displaced charged leptons and perform a detailed analysis of two types of signals: $\pi^+ \pi^- \gamma E\mkern-10.5 mu/_T $ and $\ell^+ \ell^- \gamma E\mkern-10.5 mu/_T $. For the $\pi^+ \pi^- \gamma E\mkern-10.5 mu/_T $ signal, we find that the prospective sensitivities of the HL-LHC can reach $g_{ aWW}\in [8.72 \times 10^{ -3}, 6.42 \times 10^{ -2}]$ TeV$^{ -1}$ for the ALP mass $m_a \in [4, 10]$ GeV. While for the $\ell^+ \ell^- \gamma E\mkern-10.5 mu/_T $ signal, the HL-LHC can probe a broader parameter space, with the sensitivities covering $m_a \in [4, 10]$ GeV and $g_{ aWW} \in [4.17 \times 10^{ -3}, 2.00 \times 10^{ -1}]$ TeV$^{ -1}$. These long-lived searches complement some previous prompt decay studies from LEP and LHC experiments, extending the parameter space explored by the LHCb collaboration. Our results show that the HL-LHC has significant potential to probe long-lived ALPs via their displaced vertex signatures.
Categories: hep-ph