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: $$ \gtrsim $$ 10 TeV with Large High Altitude Air Shower Observatory (LHAASO) and $$ \gtrsim $$ 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.