Scientists explain how a 300 TeV photon could reach Earth

On 9 October 2022 the gamma‑ray burst GRB 221009A produced an intense flood of high‑energy photons: the Large High Altitude Air Shower Observatory (LHAASO) recorded thousands of photons up to >10 TeV, and more than an hour later the Carpet detector registered a photon‑like event with estimated energy 300 (+43/−38) TeV. Standard physics predicts such a photon should be absorbed en route by the dense cosmic microwave background (via γγ → e+e−), giving an expected Carpet‑range flux of order 10^−96; the Carpet event also lacked the muon signature expected from a hadronic shower, with the collaboration estimating a hadron‑misidentification probability ≈3×10^−4.

Giorgio Galanti and Marco Roncadelli examine a two‑ingredient explanation: axion‑like particles (ALPs) plus Lorentz‑invariance violation (LIV). Photon↔ALP conversions in magnetic fields can account for the LHAASO photons at ~10-20 TeV but fall short by ~2 orders of magnitude at 300 TeV (predicted Carpet counts <10^−4 vs the 0.0513 needed at 95% CL). LIV alters the photon-photon absorption threshold so a 300 TeV photon interacts with much rarer, higher‑energy background photons, making the universe effectively more transparent; Galanti and Roncadelli derive 95%‑CL upper limits on the LIV scale of 1.22×10^21 GeV (linear) and 2.03×10^13 GeV (quadratic). An independent analysis finds the >1‑hour delay consistent with quadratic LIV at a similar scale. The authors stress a single event is not definitive and call for follow‑up observations from upcoming high‑energy gamma‑ray facilities.