Are there photons at the highest energies?

In a recent review article current searches for ultra-high-energy (UHE) photons at the Pierre Auger Observatory were summarized.

The Pierre Auger Observatory is not only the world's largest cosmic-ray observatory, but it can also be used to efficiently search for neutral particles, such as photons and neutrinos, at ultra-high energies. In a recent review article for a special issue of the journal "universe" focussing on ultra-high-energy (UHE) photons, current searches for these particles at the Pierre Auger Observatory were summarized. Even though no UHE photon has been unambiguosly identified so far, the upper limits on their incoming flux determined from Auger data are the most stringent to date over almost three decades in energy (see Fig. 1). These limits can be used to constrain astrophysical scenarios, for example involving interactions of cosmic rays with the cosmic microwave background (so-called "GZK interactions"). Such constraints can provide another handle, in a multi-messenger approach, on the origin and nature of ultra-high-energy cosmic rays. Besides that, upper limits on the incoming photon flux were also be used to constrain scenarios beyond the standard model of particle physics, e.g., involving super-heavy dark matter.


Figure 1: Current upper limits on the integral photon flux determined from data collected by the Pierre Auger Observatory (red, blue and gray circles). Shown are also upper limits published by other experiments as well as expected photon fluxes for different theoretical scenarios. For details, see the journal article.

The biggest challenge in the search for UHE photons is distinguishing air showers initiated by these particles from the vast background of showers induced by charged cosmic rays, i.e., protons and heavier nuclei. Depending on the energy range, different analysis methods and detector systems are used to fully exploit the capabilities of the Observatory. In the energy range below 1018 eV, data from the 750 m surface detector array combined with data from the high-elevation fluorescence telescopes HEAT are used, thus benefitting from a direct determination of the observable Xmax, the atmospheric depth of the shower maximum - a key quantity directly related to the type of the primary particle - and additional information on the lateral shower profile on ground. The corresponding analysis has also been published in 2022 in Astrophysical Journal. At higher energies, a similar analysis based on Xmax and the lateral shower profile is used, albeit with data from the 1500 m surface detector array and the regular fluorescence telescopes. A key feature of the analysis in this energy range is that it is based on air-shower universality, which is used for the first time in the context of a photon search. Preliminary results have been shown at the International Cosmic Ray Conference (ICRC) 2021, and they will be reported on in detail in a forthcoming journal publication. Lastly, at the highest energies, an analysis based solely on data from the 1500 m surface detector array is used to overcome the lack of statistics in the data set of the fluorescence detector. Hence, this search for photons relies on the lateral shower profile combined with features of the recorded data in the individual stations of the surface detector, such as the risetime of the signals. Details on this analysis will be published soon in the Journal of Cosmology and Astroparticle Physics (JCAP).

Besides these searches for photons, which led to upper limits on the diffuse, i.e., direction-independent, unresolved, flux of photons, also searches for point sources of UHE photons as well as searches for photons in coincidence with gravitational wave events have been performed. Especially the later underline the capabilities of the Pierre Auger Observatory as a multi-messenger observatory, taking on a leading role in the context of multi-messenger astronomy at the highest energies.


Related papers:

Searches for Ultra-High-Energy Photons at the Pierre Auger Observatory
The Pierre Auger Collaboration, Universe 8 (2022) 579
[arxiv.org/abs/2210.12959] [doi: 10.3390/universe8110579]

A search for photons with energies above 2x1017 eV using hybrid data from the low-energy extensions of the Pierre Auger Observatory
The Pierre Auger Collaboration, The Astrophysical Journal, 933 (2022) 125
[arxiv.org/abs/2205.14864] [doi: 10.3847/1538-4357/ac7393]

A search for ultra-high-energy photons at the Pierre Auger Observatory exploiting air-shower universality
P. Savina for the Pierre Auger Collaboration, a detailed journal publication is currently in preparation

Search for photons above 1019 eV with the surface detector of the Pierre Auger Observatory
The Pierre Auger Collaboration, JCAP 05 (2023) 021
[arxiv.org/abs/2209.05926 ] [doi: 10.1088/1475-7516/2023/05/021]

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