Axion-Photon Conversion of LHAASO Multi-TeV and PeV Photons Guangshuai Zhang I1and Bo-Qiang Ma l123y 1School of Physics Peking University Beijing 100871 China

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Axion-Photon Conversion of LHAASO Multi-TeV and PeV Photons∗

Guangshuai Zhang (张光帅)1and Bo-Qiang Ma (马伯强)1,2,3†

1School of Physics, Peking University, Beijing 100871, China

2Center for High Energy Physics, Peking University, Beijing 100871, China

3Collaborative Innovation Center of Quantum Matter, Beijing, China

The Large High Altitude Air Shower Observatory (LHAASO) has reported the detection of a large

number of multi-TeV-scale photon events including also several PeV-scale gamma-ray-photon events

with energy as high as 1.4 PeV. The possibility that some of these events may have extragalactic ori-

gins is not yet excluded. Here we propose a mechanism for the traveling of very-high-energy (VHE)

and ultra-high-energy (UHE) photons based upon the axion-photon conversion scenario, which al-

lows extragalactic above-threshold photons to be detected by observers on the Earth. We show

that the axion-photon conversation can serve as an alternative mechanism, besides the threshold

anomaly due to Lorentz invariance violation, for the very-high-energy features of the newly observed

gamma ray burst GRB 221009A.

PACS numbers: 14.80.Va; 11.30.Cp; 12.60.-i; 98.70.Rz

It has long been known that the Universe is opaque to very-high-energy (VHE) photons with energy above 100 GeV

and ultra-high-energy (UHE) photons with energy above 100 TeV, since these photons may collide with low-energy

background photons, such as cosmic microwave background (CMB) photons or extragalactic background light (EBL)

photons, and produce electron-positron pairs [1–3] during their propagation in the Universe. In other words, gamma

ray photons whose energy is above certain threshold cannot travel very long in the Universe due to the absorption of

background low-energy photons.

Suppose a UHE photon and a low-energy background photon collide with each other and produce the electron-

positron pair. For the process to be kinematically possible, it requires that

s > 4m2

ec4,(1)

where s= (pγ+pb)2= (pe++pe−)2is the invariant mass of the process. It follows that, for given background photon

energy, the energy of the gamma ray photon should exceed certain threshold,

Eγ> Eth ≡2m2

ec4

(1 −cos θ),(2)

where Eγis the energy of the gamma ray photon, is the energy of the soft photon and θis the angle between the

three-momenta of the two photons in the observer frame. Conversely,

>th ≡2m2

ec4

E(1 −cos θ).(3)

The cross section for the process γγ →e+e−in tree level is well-known,

σ(v) = π

α2

(1 −v2)h(3 −v4) ln 1 + v

1−v−2v(2 −v2)i,(4)

which is the so-called Breit-Wheeler formula [4]. Here vis the speed of the outgoing electron-positron in the center

of mass frame of the process

v(Eγ, , θ) = s1−2m2

ec4

Eγ(1 −cos θ)(5)

and αis the ﬁne-structure constant. The cross section reaches its maximum of around 1.70 ×10−25 cm2when vis

around 0.7. When v0 as well as v→1, the cross section reaches 0. In other words, suppose the collision is face

∗Preprint arXiv:2210.13120, published in Chinese Phys. Lett. 40 (2023) 011401, https://doi.org/10.1088/0256-307X/40/1/011401

†Corresponding author. Email: mabq@pku.edu.cn

arXiv:2210.13120v5 [hep-ph] 6 Jan 2023

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Axion-PhotonConversionofLHAASOMulti-TeVandPeVPhotonsGuangshuaiZhang(I)1andBo-QiangMa(l/:)1;2;3y1SchoolofPhysics,PekingUniversity,Beijing100871,China2CenterforHighEnergyPhysics,PekingUniversity,Beijing100871,China3CollaborativeInnovationCenterofQuantumMatter,Beijing,ChinaTheLargeHighAltitudeAirShow...

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Axion-Photon Conversion of LHAASO Multi-TeV and PeV Photons Guangshuai Zhang I1and Bo-Qiang Ma l123y 1School of Physics Peking University Beijing 100871 China

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