Light scalar explanation for the 18 TeV GRB 221009A

The recent astrophysical transient Swift J1913.1 thorn 1946 may be associated with the ?-ray burst GRB 221009A. The redshift of this event is z ? 0.151. Very high-energy ? rays (up to 18 TeV) followed the transient and were observed by the Large High Altitude Air Shower Observatory (LHAASO); additionally, Carpet-2 detected a photonlike air shower of 251 TeV. Photons of such high energy are expected to readily annihilate with the diffuse extragalactic background light (EBL) before reaching Earth. If the ?-ray identification and redshift measurements are correct, new physics could be necessary to explain these measurements. This paper provides the first CP-even scalar explanation of the most energetic 18 TeV event reported by LHAASO. In this minimal scenario, the light scalar singlet S mixes with the Standard Model Higgs boson h. The highly boosted S particles are produced in the GRB and then undergo the radiative decay diphoton S ? ?? while propagating to Earth. The resulting photons may thus be produced at a remote region without being nullified by the EBL. Hence, the usual exponential reduction of ? rays is lifted due to an attenuation that is inverse in the optical depth, which becomes much larger due to the scalar carriers.