Searching for long-lived axion-like particles via displaced vertices at the HL-LHC

TL;DR

This work investigates long-lived axion-like particles in the photophobic scenario at the HL-LHC by targeting displaced-vertex signatures from the process $pp \to \gamma a$, with $a$ decaying to $\tau^+\tau^-$. It performs a MC-based cut-based analysis in two DV channels, hadronic $\pi^+\pi^-\gamma\slashed{E}_T$ and leptonic $\ell^+\ell^-\gamma\slashed{E}_T$, using inner-tracking detector observables to suppress backgrounds and derive 95% CL sensitivities for $m_a\in[4,10]$ GeV and $g_{aWW}$ in the ranges $[8.72\times10^{-3}, 6.42\times10^{-2}]$ TeV$^{-1}$ (ππ) and $[4.17\times10^{-3}, 2.00\times10^{-1}]$ TeV$^{-1}$ (ℓℓ). The leptonic channel offers broader reach, with cross sections and efficiencies enabling stronger constraints, while the hadronic channel provides complementary coverage; both channels yield sensitivities that extend beyond existing prompt-decay searches and LHCb limits. The results demonstrate HL-LHC potential to probe displaced-vertex ALP signatures in the 4–10 GeV mass window and motivate future studies at lepton colliders and LHCb.

Abstract

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 $pp \to γa$ with the ALP $a$ decaying into a pair of displaced charged leptons and perform a detailed analysis of two types of signals: $π^+ π^- γE\mkern-10.5 mu/_T$ and $\ell^+ \ell^- γE\mkern-10.5 mu/_T$. For the $π^+ π^- γE\mkern-10.5 mu/_T$ 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 $\ell^+ \ell^- γE\mkern-10.5 mu/_T$ 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.

Figures (7)

• Figure 1: The branching ratios for different photophobic ALP decay modes as functions of its mass $m_a$.
• Figure 2: The proper decay length of the ALP as a function of its mass $m_a$ for different values of the coupling $g_{aWW}$.
• Figure 3: The Feynman diagrams for the process $pp \to \gamma a~(a \to \tau^+ \tau^-)$ with subsequent hadronic tau decay $\tau^+ \tau^- \to \pi^+ \pi^- \slashed{E}_T$ (a) and leptonic tau decay $\tau^+ \tau^- \to \ell^+ \ell^- \slashed{E}_T$ (b).
• Figure 4: The normalized distributions of the observables $|d_0^{\tau}|$, $v_0^{\tau}$, $v_z^{\tau}$, $\Delta R_{\tau^+\tau^-}$ and $m_{\tau^+\tau^-}$ for the signal of selected ALP-mass benchmark points and the SM background for $g_{aWW}=5 \times 10^{-3}$ TeV$^{-1}$ at the 14 TeV HL-LHC with $\mathcal{L}=$$3$ ab$^{-1}$.
• Figure 5: The normalized distributions of the observables $|d_0^{\ell}|$, $v_0^{\ell}$, $v_z^{\ell}$, $\Delta R_{\ell^+\ell^-}$ and $m_{\ell^+\ell^-}$ for the signal of selected ALP-mass benchmark points and the SM background for $g_{aWW}=5 \times 10^{-3}$ TeV$^{-1}$ at the 14 TeV HL-LHC with $\mathcal{L}=$$3$ ab$^{-1}$.