Searching for ALP Lepton Flavor Violation via ALP Decays at the LHC

Abstract

In the ALP model, lepton flavor violation (LFV) can arise from off-diagonal ALP-lepton couplings ($g_{a\ell_i\ell_j}$), which are absent in the Standard Model. We focus on ALP production via gluon fusion ($pp \to a$) at the Large Hadron Collider (LHC), which dominates the ALP mass range of 5-1000 GeV due to its high cross section and manageable backgrounds. We are interested in the decay of an axion into an electron and a muon, two oppositely charged leptons of different flavors. After background suppression, we obtain the sensitivity to ALP in this mass range, finding significantly improved limits for $5 < m_a < 1000$ GeV, where SM backgrounds are suppressed. Our research is complementary with relevant studies.

Figures (3)

• Figure 1: In high-energy collisions at the Large Hadron Collider (LHC), axion-like particles (ALPs)—denoted as "ax" in the diagram—are produced via gluon–gluon fusion, where gluons are the force carriers of the strong interaction. The left panel shows the ALP decaying into $e^-\mu^+$, while the right panel depicts its decay into $e^+ \mu^-$. As charged leptons with relatively low mass, electrons and muons provide clean experimental signatures in particle detectors, enabling precision studies of non-perturbative QCD effects and the confinement mechanism.
• Figure 2: The distributions in Fig.2 clearly characterize the event topology for different observables, where signal and background components are distinguished by the legend in the upper-right panel. Specifically: (a) shows the missing transverse energy ( $E_{\mathrm{T}}^{\mathrm{miss}}$ ) distribution. Signal events (red) exhibit a pronounced tail at high $E_{\mathrm{T}}^{\mathrm{miss}}$ , arising from the undetected axion-like particle (ALP) in the process $pp \to a \to e^\pm \mu^\mp + E_{\mathrm{T}}^{\mathrm{miss}}$. In contrast, background events (blue) cluster predominantly at low $E_{\mathrm{T}}^{\mathrm{miss}}$ , primarily due to instrumental effects and measurement uncertainties; (b) presents the $m_{e\mu}$ (electron-muon invariant mass) spectrum, with signal events peaking sharply near the axion mass $m_a$ (dashed line) from resonant $a\to e\mu$ decays, contrasted with the smooth continuum distribution of backgrounds. These distinctive features enable effective kinematic separation through optimized selection cuts.
• Figure 3: In this paper, the derived exclusion limits are compared with the most stringent experimental constraints from dilepton final states, specifically the $Mu-\bar{Mu}$ experiment. Related theoretical studies predicting axion production cross sections in this mass range are referenced to provide context for the experimental constraints. The kinematic distributions used in this analysis differ from those employed in previous collider searches, offering complementary sensitivity to axion-like particle production.