1. Building the UHE photon paradigm My major early achievement is the authorship, maintenance and development of the PRESHOWER code, now a standard program enabling complete simulations of ultra-high energy (UHE) photons as cosmic rays. I wrote the code in 2003 during my PhD studies, then maintained it and developed, also with my master students in the next years. PRESHOWER handles the propagation of UHE photons above the atmosphere, where an interaction with the geomagnetic field is likely, leading to electromagnetic cascading (the preshower effect) above the Earth atmosphere and the consequent change of the properties of extensive air showers induced by the cascade (preshower). PRESHOWER was integrated as a module with the most popular cosmic-ray simulation programs: CORSIKA and CONEX. Using the PRESHOWER module is now the “must” setting in the state of the art simulation/analysis related to UHE cosmic-ray photons. PRESHOWER is the first and only UHE code of its kind with both open source and description published in peer reviewed journals (Homola et al. 2003 and 2013). Examples of applications of PRESHOWER can be found, among others, in T. Abu-Zayyad et al. (Telescope Array Collaboration), Phys. Rev. D88, 112005 (2013); A.V. Glushkov et al. JETPLett. 85 (2007) 131; G. I. Rubtsov et al., Phys. Rev. D73, 063009 (2006); O. E. Kalashev et al, Phys. Rev. D80 (2009) 103006. I’ve also performed and published as a corresponding author a number of own analyses related to UHE photons. One of the results of this UHE photon activity was an invitation to become a co-author of a review article on UHE photon research: M. Risse and P. Homola, Mod. Phys. Lett. A 22, 749 (2007), remaining up to now the key reference for the UHE photon studies.
The use of a complete UHE photon simulation chain enabled the search for UHE photons among the cosmic rays observed on Earth. The result of this search can be nowadays summarized very briefly: we do not observe extensive air showers initiated by UHE photons, no matter undergoing or not the preshower effect. The fact of not observing UHE photons was commonly used, also by myself, to conclude about the constraints on exotic scenarios postulating the existence of UHE photons as products of decay or annihilation of hypothetical, super-massive particles, so called top-down scenarios. “Non-observation of UHE photons” result together with “practically ruling out top-down scenarios” became part of the cosmic-ray paradigm, part of state of the art knowledge, and my achievement was to largely contribute to this paradigmatic picture.
2. Questioning the UHE photon paradigm – questioning myself The years of studies on UHE photons with a “standard” QED approach did not allow me realizing how much room there is for a deeper investigation of the topic whis is now “paradigmatically” closed: electromagnetic cascading above the Earth atmosphere. Only during my postdoctoral contracts, when I decided to change the scientific environment to which I was used and in which I felt comfortable, a special phase transition in scientific thinking occurred to me – triggering the opening of a new and exciting investigation direction: the super-preshower channel. As explained in more detail in the forthcoming description of my proposal, given the fundamental uncertainties about the physics in the considered energy range (above 1020eV), one can not be sure about the properties of cascades initiated by extremely energetic photons or by other particles. Such cascades, named by me super-preshowers, can be considered in a general way: without any assumptions about their properties, like e.g. spatial and temporal spread, energy spectrum of particles or the front shape. The observational question in such a consideration is strikingly simple: what types of super-preshowers can be observed on Earth with the presently available infrastructure? There is an obvious, although yet not probed, „detection horizon” for super-preshowers. It can be located somewhere between an obviously detectable air shower induced by a super-preshower composed of tightly collimated particles (the preshower effect – yet not observed), and obviously undetectable super-preshower composed of particles spread so widely that only few of them can reach Earth. Probing this horizon with a global approach to the cosmic ray data and defines an extensive scientific program oriented on the search for physics manifestations at largest energies known, with potential impact on ultra-high energy astrophysics, the physics of fundamental particle interactions and cosmology. So far, the cosmic-ray research has been oriented on detecting single air showers only, while the search for ensembles of cosmic-ray events induced by super-preshowers is a scientific terra incognita. I consider the opening of the perspective to explore this uncharted realm my major scientific achievement. I’ve presented this view and got a positive reception in a number of conferences and collaboration meetings: the Pierre Auger Collaboration meeting, Malargue, November 2015; Next-Generation Techniques for Ultra-High Energy Astroparticle Physics, Chicago, February 2016; Warsaw Workshop on Non-Standard Dark Matter, June 2016; Baikal-GVD Collaboration meeting, Dubna, June 2016; ASTERICS Citizen Science Workshop, Oxford, July 2016. My next step was to organize a collaborative effort based on the good reception of my idea.