Explaining a 300 TeV photon candidate from GRB 221009A takes Lorentz invariance violation, not axion-like particles alone

The paper, by Giorgio Galanti of INAF and Marco Roncadelli of INFN in Physical Review Letters, starts from an expected count of about 10^-96 photons in the Carpet energy range under conventional propagation ^[4][5]. Set that beside the Carpet collaboration's estimate of about 3 x 10^-4 for the chance that a hadron was misidentified as the event ^[6]. The two are different kinds of quantity. Read together, though, they make a misread cosmic ray about 92 orders of magnitude likelier than a photon that travelled by known physics ^[2]. The thing this does not tell you is whether the event was a photon. Galanti and Roncadelli take the published Carpet result at face value and leave open whether it is a signal of new physics ^[7].

By standard physics, a photon at that energy should vanish on the way ^[3]. At 300 TeV it can hit a photon of the cosmic microwave background and produce an electron and a positron, and the CMB is the main absorber at that energy ^[8]. "The CMB is extremely dense," Galanti said ^[9]. The photon's mean free path is far shorter than the distance to GRB 221009A ^[8].

The first remedy the pair tested was axion-like particles. In a magnetic field a photon can convert into an ALP, cross the region where it would have been absorbed, then convert back ^[10]. That can account for LHAASO's photons between about 10 and 20 TeV ^[10]. It fails at Carpet's energy. "ALPs alone fall short by roughly two orders of magnitude at 300 TeV," Galanti said ^[11]. Across the ALP parameters they considered, the expected Carpet count stays below about 10^-4, against 0.0513 needed at 95% confidence ^[12]. On those figures the shortfall is more than 500-fold ^[1].

The pair then added a violation of Lorentz invariance. In their scenario, modified photon propagation moves the threshold for photon-photon absorption. The 300 TeV photon then interacts with higher-energy background photons, which are much scarcer than CMB photons, and the universe becomes more transparent to it ^[13].

In my view, the combined model is the part to weigh most carefully. Each hypothetical effect operates only in the energy band where one of the two detectors needs it: ALPs for LHAASO, Lorentz violation for Carpet ^[15].

The timing result from Dmitry Ofengeim and Tsvi Piran is a separate calculation. But the delay it explains is the gap between the LHAASO and Carpet detections, so it depends on the same Carpet event ^[16].

Galanti is plain about how far the result goes. "A single candidate photon from a single cosmic event cannot be claimed as an undisputed discovery," he said ^[17]. Confirmation would take repeated observations from multiple distant sources showing the same energy-dependent signatures ^[18].