Joint Multi-Period Fermi-LAT and LHAASO Constraints on Axion-Like Particles from Mrk 421 Using Profile Likelihood with Gaussian Copula Correlation

Abstract

We propose a joint multi-epoch profile-likelihood analysis of axion-like particles (ALPs) using five sets of simultaneous Fermi-LAT and LHAASO observations of the TeV blazar Mrk 421. Photon-ALP oscillations are calculated self-consistently together with EBL absorption for two representative jet emission models: a two-zone hybrid model and a single-zone hadronic model. To account for weak correlations among different observational epochs, we introduce a Gaussian copula with a conservative correlation coefficient $\rho =0.03$ and perform a global optimization of nuisance parameters under the no-ALP hypothesis before profiling the ALP parameters. In the low-mass regime relevant to CAST ($m_a\lesssim 1$ neV), we obtain a 95\% CL upper limit of $g_{a\gamma }=7.46\times 10^{-13}{\mathrm{GeV}}^{-1}$. Over the full mass range $0.1$--$500$ neV, the most conservative 95\% CL upper limits are $g_{a\gamma }<6.50\times 10^{-12}{\mathrm{GeV}}^{-1}$ (two-zone) and $g_{a\gamma }<7.34\times 10^{-12}{\mathrm{GeV}}^{-1}$ (single-zone). These constraints benefit from the broadband VHE coverage and long-term monitoring provided by LHAASO. The analysis framework developed here offers a statistically consistent approach for future ALP searches with multi-messenger gamma-ray data.

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