Scientists explain how a 300 TeV photon could reach Earth

On 9 October 2022, the Large High Altitude Air Shower Observatory (LHAASO) observed a gamma-ray burst named GRB 221009A, which emitted a multitude of high-energy photons, some surpassing 10 TeV. An hour after, the Carpet cosmic ray detector recorded an anomalous photon-like event with an estimated energy of 300 (+43/-38) TeV. According to prevailing physics, a photon of such high energy should have disintegrated en route to Earth.

Giorgio Galanti of INAF and Marco Roncadelli of INFN have since investigated what could enable this phenomenon.

Their findings, published in Physical Review Letters, suggest two speculative mechanisms: axion-like particles (ALPs) and a potential breach of Lorentz invariance (LIV). The cosmos is permeated with background light, including cosmic microwave background (CMB) radiation from the early universe. A 300 TeV gamma ray could collide with these low-energy photons, producing an electron and a positron.

At this energy, the CMB represents the primary absorber. Galanti notes that the CMB is exceedingly dense, rendering the universe nearly opaque to a 300 TeV photon. The researchers estimate only around 10⁻⁹⁶ photons would survive conventional propagation to the Carpet energy range. The Carpet event itself was peculiar, lacking the muon signal typically associated with a hadronic cosmic-ray shower.

The likelihood of a hadron being misidentified as the event was approximately 3 × 10⁻⁴. While Galanti and Roncadelli treat the published Carpet result as presented, they leave open the question of whether it could signify novel physics.

Regarding ALPs, hypothetical particles capable of mixing with photons in magnetic fields, a photon can transform into an ALP, traverse a region where photons would be absorbed, and revert back to a photon. This mechanism may account for the LHAASO photons detected in the 10-20 TeV range. However, ALPs alone fall short by roughly two orders of magnitude at 300 TeV.

Moreover, CMB absorption becomes excessively severe at the Carpet energy. The researchers observed that, given the ALP parameters they considered, the anticipated number of Carpet photons remains below about 10⁻⁴, lower than the 0.0513 required at the 95% confidence level. This finding prompted them to explore another possibility: a potential violation of Lorentz invariance, one of relativity's foundational principles.

In their scenario, LIV-induced modifications of photon propagation alter the threshold for photon–photon absorption. A 300 TeV photon could then interact with higher-energy background photons, which are significantly less abundant than the CMB photons responsible for standard absorption. The universe thus appears more transparent to the photon.

The researchers calculate upper limits for linear LIV (1.22 × 10²¹ GeV) and quadratic LIV (2.03 × 10¹³ GeV) at 95% confidence. Galanti emphasizes that ALPs function at LHAASO energies but not at Carpet energies, while LIV operates at Carpet energies but not at LHAASO energies. Their model integrates both effects into a unified theoretical framework.

An additional indication is provided by a separate analysis by Dmitry Ofengeim and Tsvi Piran, who noted an hour-long delay between the LHAASO and Carpet events, which quadratic LIV could also explain. The estimated scale aligns with the limit determined by Galanti and Roncadelli. Nevertheless, Galanti cautions against interpreting a single candidate photon from a single cosmic event as a definitive discovery.

Confirmation would necessitate repeated observations from multiple distant sources exhibiting the same energy-dependent signatures. Future observations from facilities such as ASTRI Mini-Array, CTAO, SWGO, and the expanded LHAASO could assess whether similar high-energy signatures materialize in gamma-ray bursts and distant active galaxies.

For now, the 300 TeV event remains an exceptional observation with a possible explanation that extends beyond standard photon propagation.