Over the past decades the photon emission from astronomical objects has been measured across 20 orders of magnitude in energy, from radio up to TeV gamma rays. This has not only led to many astronomical discoveries, but also, thanks to the extreme distances and energies involved, allowed researchers to test some of the fundamental tenets of physics. For example, the 2017 joint measurement of gravitational waves and gamma-rays from a binary neutron-star merger made it possible to determine the speed of gravity with a precision of less than 10-16 compared to the speed of light. Now, the High-Altitude Water Cherenkov (HAWC) collaboration has pushed the energy of gamma-ray observations into new territory, placing constraints on Lorentz-invariance violation (LIV) that are up to two orders of magnitude tighter than before.
Models incorporating LIV allow for modifications to the standard energy—momentum relationship dictated by special relativity, predicting phenomenological effects such as photon decay and photon splitting. Even if the probability for a photon to decay through such effects is small, the large distances involved in astrophysical measurements in principle allow experiments to detect it. The most striking implication would be the existence of a cutoff in the energy spectrum above which photons would decay while traveling towards Earth. Simply by detecting gamma-ray photons above the expected cutoff would put strong constraints on LIV.
Increasing the energy limit for photons with which we observe the universe is, however, challenging. Since the flux of a typical source, such as a neutron star, decreases rapidly (by approximately two orders of magnitude for each order of magnitude increase in energy), ever larger detectors are needed to probe higher energies. Photons with energies of hundreds of GeV can still be directly detected using satellite-based detectors equipped with tracking and colorimetry. However, these instruments, such as the US-European Fermi-LAT detector and the Chinese-European DAMPE detector, require a mass of several tonnes, making launching them expensive and complex. To get to even higher energies ground-based detectors, which detect gamma-rays through the showers they induce in Earth’s atmosphere, are more popular. While they can be more easily scaled up in size than can space-based detectors, the indirect detection and the large background coming from cosmic rays make such measurements difficult.
It is likely that LIV will be further constrained in the near future, as a range of new high-energy gamma-ray detectors are developed
Recently, significant improvements have been made in ground-based detector technology and data analysis. The Japanese-Chinese Tibet air shower gamma-ray experiment ASγ, a Cherenkov-based detector array built at an altitude of 4 km in Yangbajing, added underground muon detectors to allow hadronic air showers to be differentiated from photon-induced ones via the difference in muon content. By additionally improving the data-analysis techniques to more accurately remove the isotropic all-sky background from the data, in 2019 the ASγ team managed to observe a source, in this case the Crab pulsar, at energies above 100 TeV for the first time. This ground-breaking measurement was soon followed by measurements of nine different sources above 56 TeV by the HAWC observatory located at 4 km altitude in the mountains near Puebla, Mexico.
These new measurements of astrophysical sources, which are likely all pulsars, could not only lead to an answer on the question where the highest-energy (PeV and above) cosmic rays are produced, but also allows new constraints to be placed on LIV. The spectra of the four sources studied by the collaboration did not show any signs of a cutoff, allowing the HAWC team to exclude the LIV energy scale to 2.2×1031 eV — an improvement of one-to-two orders of magnitude over previous limits.
It is likely that LIV will be further constrained in the near future, as a range of new high-energy gamma-ray detectors are developed. Perhaps the most powerful of these is the Large High Altitude Air Shower Observatory (LHAASO) located in the mountains of the Sichuan province of China. The construction of the detector array is ongoing while the first stage of the array commenced data taking in 2018. Once finished, LHAASO will be close to two orders of magnitude more sensitive than HAWC at 100 TeV and capable of pushing the photon energy into to the PeV range. Additionally, the limit of direct-detection measurements will be pushed beyond that from Fermi-LAT and DAMPE by the Chinese European High Energy cosmic Radiation Detector (HERD), a 1.8-tonne calorimeter surrounded by a tracker scheduled for launch in 2025 which is foreseen to be able to directly detect photons up to 100 TeV.
