Testing New-physics Scenarios with the Combined LHAASO and Carpet-3 Fluence Spectrum of GRB 221009A: Axion-like Particles and Lorentz-invariance Violation

From gamma-ray burst 221009A, very high-energy photons were detected: 10 TeV with Large High Altitude Air Shower Observatory (LHAASO) and 100 TeV with Carpet-3. Such energetic photons are expected to be absorbed via electron-positron pair production on their way to the Earth. Their observation might be explained by new physics, including Lorentz invariance violation or photon mixing with axion-like particles. Here, we construct a joint fluence spectrum by combining flux measurements from both experiments, and fit it under these hypotheses.While Lorentz invariance violation can account for the Carpet-3 observation, it provides only a modest improvement over standard physics in the overall fit and requires parameters excluded by other constraints. Axion-like particle mixing improves the description of both LHAASO and Carpet-3 data, yielding a substantial enhancement in fit quality for a specific region of the axion-like particle parameter space.