On 9 October 2022, the gamma-ray burst GRB 221009A sent an extraordinary flood of high-energy radiation towards Earth. The Large High Altitude Air Shower Observatory (LHAASO) recorded thousands of photons from the event, some above 10 TeV. More than an hour later, the Carpet cosmic-ray detector registered something far harder to explain: a photon-like event with an estimated energy of 300 (+43/-38) TeV.
At that energy, standard physics says the photon should never have arrived. The space between galaxies is filled with background light, and the cosmic microwave background (CMB) is dense enough that a 300 TeV gamma ray should collide with it and convert into an electron-positron pair. In a paper in Physical Review Letters, Giorgio Galanti of INAF and Marco Roncadelli of INFN calculate that conventional propagation would yield only about 10^-96 expected photons in the Carpet energy range. The Carpet collaboration also found no muon signal of the kind expected from a hadronic cosmic-ray shower, estimating the probability of a hadron being misidentified as the event at roughly 3 x 10^-4.
The researchers examine two ingredients. Axion-like particles (ALPs) can mix with photons in magnetic fields, letting a photon travel as an ALP through absorbing regions and convert back later - a mechanism that fits the LHAASO photons detected between roughly 10 and 20 TeV. But at 300 TeV, Galanti says, "ALPs alone fall short by roughly two orders of magnitude". Among the ALP parameters considered, the expected number of Carpet photons stays below about 10^-4 - below the 0.0513 required at the 95% confidence level.
That led them to a possible violation of Lorentz invariance (LIV). In the scenario they studied, modified photon propagation changes the threshold for photon-photon absorption, so a 300 TeV photon can interact with higher-energy background photons that are far less abundant than CMB photons - making the universe more transparent to it. They derive upper limits of 1.22 x 10^21 GeV for the linear LIV scale and 2.03 x 10^13 GeV for the quadratic case, at 95% confidence. A separate analysis by Dmitry Ofengeim and Tsvi Piran found that the more-than-one-hour delay between LHAASO and Carpet can also be explained with quadratic LIV, at an estimated scale consistent with that limit.
Galanti keeps the result in proportion. "A single candidate photon from a single cosmic event cannot be claimed as an undisputed discovery," he says; confirmation would require repeated observations from multiple distant sources showing the same energy-dependent signatures. Facilities including the ASTRI Mini-Array, CTAO, SWGO and an expanded LHAASO could test whether similar signatures appear in gamma-ray bursts and distant active galaxies.
