# Ultra-High-Energy Photons: New Horizons Ahead?

Piotr Homola, Mariangela Settimo

► To cite this version:

Piotr Homola, Mariangela Settimo. Ultra-High-Energy Photons: New Horizons Ahead?. Universe, 2024, 10 (12), pp.466. 10.3390/universe10120466. [hal-04870221]

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Editorial

# Ultra-High-Energy Photons: New Horizons Ahead?

Piotr Homola <fnref n="1" />,*, and Mariangela Settimo <fnref n="2" />,*,

This Special Issue is dedicated to photons of energies above $10^{18}$ eV, i.e., at the very end of the cosmic ray energy spectrum, which is often called the ultra-high-energy (UHE) regime, ending at the record observed energy of $3.12 \times 10^{20}$ eV [1]. The bare existence of these high-energy cosmic ray particles raises some of the most long-standing questions in astrophysics: what are they? Where do they come from? How do they reach these extraordinary energies? The investigation of the origin of ultra-high-energy cosmic rays (UHECR) has engaged the scientific community for decades, yielding significant progress. Trying to explain the cosmic ray origin at UHE, one might follow the conventional approach, known also as the “bottom-up” approach, which relies on established physics extrapolated over several orders of magnitude. The processes involved include mainly an acceleration of charged nuclei and their subsequent interactions with ambient background radiation. An alternative path engages exotic models, known as “top-down” scenarios, and assumes that UHE cosmic rays could be generated as a result of a decay in, or annihilation of, the hypothetical supermassive particles (hence “exotic”) of energies in the order of $10^{23}$ eV. Given the general difficulty in modeling the acceleration of charged nuclei to UHEs [2] and a limit of their mean free path, refs. [3,4] the “top-down” approach offers a conceptually appealing solution, albeit with the price of admitting some unconfirmed physics beyond the Standard Model of particles. The predictions of these two classes of scenarios—the bottom-up and top-down—differ significantly in the fraction of photons in the observed UHECR fluxes: the former predicts only very small fractions of the order of 1% (depending on the chemical composition of UHECRs), while the other promises even a 50% contribution of photons to the overall cosmic ray flux at the highest energies (depending on the model). Thus, the observation of UHE photons could serve as a unique probe for exploring physics beyond the Standard Model. Notably, the two largest cosmic ray infrastructures, the Pierre Auger Observatory [5] and the Telescope Array [6], using state-of-the-art knowledge, converged at a common conclusion: the existence of UHE photons cannot be confirmed with the presently available cosmic ray data. Furthermore, the very stringent upper limits on UHE photon fluxes impose significant constraints on “exotic” models, as mentioned in the review by The Pierre Auger Collaboration [7] in this Special Issue and in [8]. Additionally, the ever-expanding datasets, with UHE photons persistently missing, are increasingly constraining the hypotheses adopted in conventional scenarios. In this article, we illustrate, with a few examples, how, despite the absence of UHE photons in the current data, opportunities might still be possible when humbly considering the many unknowns in the field. When trying to schematize the logical path behind research in the field (see Figure 1), one might highlight four classes of bricks:

1. a priori: adopting assumptions needed to proceed with UHE photon studies (e.g., trusting the extrapolations of hadronic models and quantum electrodynamics processes by many orders of magnitude or specific exotic models);

2. Methodological: applying theoretical modeling and simulations to determine the expectations concerning the UHE photon fluxes;

3. Technical: collecting as much high-quality data as possible

4. a posteriori: adopting assumptions needed to interpret the relevant observations.

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check for
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**Citation:** Homola, P.; Settimo, M. Ultra-High-Energy Photons: New Horizons Ahead? *Universe* **2024**, *10*, 466. https://doi.org/10.3390/universe10120466

Received: 9 December 2024

Accepted: 16 December 2024

Published: 20 December 2024

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**Copyright:** © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland</footnote>

<footnote n="2">² SUBATECH, CNRS, IMT-Atlantique, Nantes Université, 44300 Nantes, France</footnote>

<footnote>* Correspondence: piotr.homola@ifj.edu.pl (P.H.); mariangela.settimo@in2p3.fr (M.S.)</footnote>
Given the above perspective, it is a matter of experience, scientific intuition, and state-of-the-art knowledge that influences how the experimental results are interpreted in the context of the initial assumptions, with a potential and unavoidable influence on future research directions. In particular, one might conclude that UHE photons are unlikely to exist and undertake research in another area, or that they might be a key missing piece in the whole UHECR picture and explore more exotic scenarios. Both approaches are legitimate, but each obviously leads to different scientific outcomes and experimental efforts.

<figure><img src="image_5.png" /><figcaption>**Figure 1.** The logical landscape of UHE photon studies: The final outcome is determined by a combination of individual choices concerning both the *a priori* and *a posteriori* assumptions, experimental and methodological research actions, and the interpretations of the obtained results. This can lead to distinctly different scientific results, including some uncharted territories beyond the UHE photon, and even the UHE cosmic ray, paradigm.</figcaption></figure>

Let us now briefly highlight some representative example constituents of the UHE photon landscape mentioned above, with a particular focus on the opportunities for novel research. Firstly, regarding class 1 bricks, one could, for instance, point out that to begin any research on the observation of UHE photons requires assuming that these particles have a significant probability of reaching Earth.

Interestingly, this does not need to be the case. In the quest for a more complete explanation of the processes that occur in nature, one of the major channels of investigation is the effort towards developing a theory which would describe gravitational processes in the micro scale, so-called quantum gravity. A number of possible experimental opportunities are being considered to observe manifestations of quantum gravity (see e.g., [9]), and one of the main expectations is related to Lorenz Invariance Violation (LIV). Various types of LIVs are being considered and probed experimentally. Here, we want to mention only one effect called "photon decay", which is allowed with some modifications of the photon dispersion relation connected with LIV. The key parameters of these modifications have been extensively tested and severe constraints have been set (see e.g., [10]), but with all the available experimental evidence, one cannot exclude that the photon decay effect occurs in reality. If it does, the lifetime of a sufficiently high-energy photon would be extremely short (in the order of 1 s), and, as a consequence, UHE photons would have only negligible chances to reach Earth. In such a scenario, all of the current upper limits to UHE photons would immediately lose their conclusive power in terms of constraining the exotic UHECR models. On the other hand, the bare admission of the possibility of the photon decay effect, or other scenarios leading to the early cascading of photons in the *a priori* assumptions, open new
opportunities such as searching for correlated groups of cosmic rays and/or air showers of lower primary energies, which might be observable over even extremely large areas and thus give unique signal patterns, as illustrated in Figure 2. Such particle/shower groups are dubbed cosmic ray ensembles (CREs), and they have already been studied [11,12]. The possibility of observing large-scale cosmic ray correlations naturally brings us to class 2 bricks. To give an example of the currently ongoing simulations, let us mention potentially spectacular CRE with unique observable properties that might originate from interactions between UHE photons and the magnetic field of the Sun. The related physics processes might lead to a generation of very thin (up to 1 m) and very long (thousands or even millions of kilometers) fronts of secondary particles (mostly photons) of energies covering the whole cosmic ray energy spectrum, with a maximum around 100 GeV. These secondary photons, upon entering the Earth's atmosphere, would generate phenomena called air shower walls (ASWs) (see [12] and Poncyljusz et al., this Special Issue [13]). The peculiar spatial characteristic of ASW constitutes a unique signal signature. However, presently, neither the largest cosmic ray observatories, the Pierre Auger Observatory or Telescope Array, nor individual detectors or small local arrays can identify ASWs. The former are tuned to observe only air showers induced by individual high-energy particles, and they disregard low-energy events which might be tightly correlating and consistent with an ASW tail. On the other hand, small local arrays of detectors or individual devices can detect ASW particles, but due to the very limited extent of the detection sites, the correlations between signals coming from separate showers that constitute an incoming ASW are likely to be missed. Simulations of ASWs could also provide estimates of gamma ray fluxes in the TeV energy range, which would be directly comparable with data from satellite observatories. In the latter case, high-energy photons arriving from the direction of the solar disk, although not generated in the Sun, would originate from the interactions between UHE photons and the solar magnetic field. A possible case of interest could be, for example, a few high-energy (above 100 GeV) photons arriving from the direction of the solar disk which were observed by Fermi-LAT during the 1.4 year of the solar minimum in the Cycle 24, unobserved in the next 7.8 years [14]. Of course, all the other experimental evidence would have to fit with this newly proposed interpretation.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Cosmic ray ensembles: A qualitatively novel channel in UHECR research, bringing up a new potential to search for UHE photons.</figcaption></figure>

The broad definition of CREs, which includes groups of particles with a dispersion in both arrival locations and times, also points to a range of possible applications in research concerning the foundation of physics. One of the examples featured in this Special Issue (Ng
and Perlman [15]) has the potential to probe the spacetime structure, which could be possible if we observe temporally extended (with considerably different arrival times) CREs composed of photons. Analyses in this direction will resemble the studies on spacetime graininess performed by gamma astronomers [16], where X-ray and gamma ray cosmic data (energies up to TeV) were studied to conclude that spacetime is smooth down to distances smaller than $10^{-21}$ m. The CRE approach may explain the case of cosmic ray event clusters with dispersions of arrival times from seconds to minutes in small air shower arrays [17–19], and, when applied to the high-energy regime, can potentially lead to correspondingly improved sensitivities to spacetime graininess.

An experimental approach oriented on observing CREs (and ASWs) could be a global network of large observatories and diverse sensors providing data for a global analysis in a wide energy range to identify large-scale cosmic ray correlations. Such an approach has been envisaged and is currently being implemented within the CREDO Collaboration [20]. It would shift the organizational paradigm in cosmic ray research from the currently practiced data exchange via scientific reports and conference proceedings towards a permanently open alliance between the cosmic ray-oriented infrastructures and teams, supported by the data from sensors where cosmic ray signals are considered to be part of the background (gamma ray, neutrino, dark matter, and astronomical observatories, accelerator experiments in the off-beam mode). Of course, the success of such a global approach is conditional: if we do not observe CREs repeatedly, it would signify scientifically valuable constraints to exotic physics, but the ruling paradigm of UHECR will remain unaltered.

It is unquestioned that the conventional path in UHECR methodology and organization (individual observatories) should continuously be maintained and developed in a thoughtful manner. A great example here is the Pierre Auger Observatory upgrade [21], which brings us closer to event-by-event identification of primary masses of UHECR. As in both exotic and conventional models, UHE photons are expected to contribute to the observed UHECR fluxes, their unambiguous observation is not unlikely, and, in case of success, it will undoubtedly provide unprecedented insight into physics at the highest energies known. On the other hand, the non-observation of UHE photons is a meaningful result to constraint models in top-down scenarios. Moreover, in the more conventional cosmogenic interpretation, tensions between the observation of UHECR mass and energy spectra and the UHE photons can open new alternative interpretations with modified photon dispersion relations, including the photon decay effect or other early cascading processes.

The third class of the bricks which form the UHE photon landscape concerns the collection of the experimental evidence. It is important to remain aware that a set of choices are typically made for the trigger definition and for data reduction with an online event classification approach. These choices are typically determined by the expected rate of noise and limitations in data acquisition and storage capabilities, but can also be influenced by knowledge of the physics processes and the expected scientific outcomes. As a consequence, unexpected physics effects might be missed. It is therefore crucial to consider strategies for mitigating the risk of losing valuable data. Furthermore, adopting the FAIR principles (Findable, Accessible, Interoperable, Reusable) in open data policies [22] could be highly beneficial. By facilitating data sharing to a broader community—a practice already used in astrophysics observatories—it becomes possible to reuse and reanalyze data even years later, leveraging improved knowledge of specific phenomena or advancements in simulations and models.

Finally, the last class of bricks brings us to the interpretation of the experimental results and its agreement with expectations, which, as already discussed, depends on models and initial hypotheses. The challenge, where personal experience often comes into play, arises when dealing with outliers. While the significance of such an observation provides the mathematical and scientific basis to reject or accept the outliers, its relevance to further investigations may be shaped by personal experience. For an example, let us consider the
measurements of the UHECR energy spectrum above $10^{19}$ eV made by the two largest
observatories built so far, the Pierre Auger Observatory and the Telescope Array. The
results are in tension, and a joint working group was established to explain the discrepancy.
The latest reports on this group refer to an energy-dependent difference that is beyond the
systematic uncertainties associated with the energy scale in the declination band visible at
the two observatories [23]. Can this unexplained difference be relevant for the UHE photon
search? The answer to this question would be “yes, potentially relevant” if we have in mind
a possible scenario which connects, at least qualitatively, the outlying observations with
our field of research, or else we “feel” that such a scenario is likely to exist; otherwise, the
reasonable choice would be to answer that it is “irrelevant”. To better illustrate our point,
let us now propose a candidate, albeit highly speculative, scenario. One of the differences
between the two largest observatories is the strength of the local geomagnetic field: it is
twice larger at the Telescope Array location (USA) than at the Pierre Auger Observatory
(Argentina). Interestingly, the simulations of UHE photons in the vicinity of Earth show
significant energy-dependent effects above $10^{19}$ eV, the so-called “preshowering”, which can
affect the primary energy and mass reconstruction [24]. These simulations are included in
the studies undertaken by the two collaborations. However, this non-observation result can
be undermined by inherent weaknesses in the tools, such as the far-going extrapolations of
the models and unsolved and/or unknown technical issues in the simulation programs (see,
e.g., [25]). Hence, while one might not be eager to immediately connect the experimental
tension at the high-end of the UHECR energy spectrum with UHE photons, it seems to be a
viable scenario capable of qualitatively explaining the discrepancy. Whether one chooses
to attach importance to the above physically plausible but highly speculative connection
is a personal decision which will determine future research actions. Naturally, one could
consider other yet unexplained observations in the UHECR field and around it in a similarly
deliberative manner. To the aforementioned air shower event clusters [17–19] and gamma
rays from the solar disk [14], let us also add a few more results which might potentially be
relevant for UHE photon studies: modified dispersion relations as a possible explanation
of the discrepancies in the cosmological data with the leading model [26], uncertainties in
the particle interaction models especially highlighted by the recent unexplained results
concerning muon properties (muon excess in air showers [27–29], muon deficit in B meson
decays [30], the muon g-2 anomaly [31]), or the intriguing precursor-like and quasi-periodic
correlations between the dynamics of the cosmic ray detection rates and global seismic
activity [32]. The above collection might (or might not) be a list of inspirations, but it
definitely points to a number of possible new investigations which might be potentially
relevant to the search for UHE photons.

To summarize, with this introductory article, we want to highlight the potential for
UHE photon research to still have a significant impact even beyond the field of UHECR.
Despite the non-observation of UHE photons to date, it is important to maintain a consis-
tently humble scientific perspective regarding the many unknowns in the realm of UHECR.
Such an approach is crucial to minimize the risk of overlooking major discoveries which
potentially might even be round the corner or findable in the existing data.

The reader is offered a scientific perspective on the opportunities—albeit not an
exhaustive list—that exist in the field, with the hope of sparking new inspiration and
motivation. And even if one tends to think that the assumption of the non-existence of
UHE photons remains the most popular, a collection of excellent articles written by a mix of
distinguished and young researchers who consider UHE photons a scientifically attractive
field might provide a refreshing intellectual journey.

**Conflicts of Interest:** The authors declare no conflict of interest.
## Abbreviations

The following abbreviations are used in this manuscript:

ASW Air shower walls

CRE Cosmic ray ensembles

CREDO Cosmic Ray Extremely Distributed Observatory

UHF Ultra-high-energy

UHECR Ultra-high-energy cosmic rays

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