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Search for gamma-rays in the EeV sky at the Pierre Auger Observatory using universality

Abstract : The search for photons of ultra-high energy (UHE), above 10¹⁸ eV, lies in the highest energy range of gamma-ray astronomy, a prominent discipline in multimessenger astronomy, as at high energies it is pursued primarily in the context of the astrophysics of cosmic rays, the progenitors of the gamma rays. The highest energy cosmic rays are expected to produce UHE gamma-rays, either in interactions with the source ambient, or with the soft universal background photons in their propagation to Earth. These cosmogenic photons probe ultra-high energy cosmic rays as their flux depends on the characteristics of the sources, as well as on the nature of the parent nuclei. UHE-photons could also probe new physics, as their detection could be a smoking gun for dark matter composed of super-heavy particles decaying in photons. At UHE, cosmic rays and photons are measured through the extensive air showers produced when entering the Earth’s atmosphere. The key signatures to distinguish photons showers from the overwhelming background due to hadrons are a larger atmospheric depth at the shower maximum (Xmax) and a lower number of muons. These two observables can be measured at the Pierre Auger Observatory, which combines, into a hybrid instrument, a fluorescence detector (FD) with a round array of particle detectors (SD). In hybrid events, FD measures Xmax, while from the SD the muon content can be estimated. In this thesis, developed in the Auger Collaboration, we have conceived a new mass-sensitive variable, Fμ, related to the muonic content of the shower, by exploiting the air-shower universality property, in combination with the reconstruction of hybrid events. This new variable is then combined with Xmax, the mass-sensitive observable by excellence, into an analysis method that increases the photon/hadron separation power for the search of photons with energies above 10¹⁸ eV. In chapter 1, we discuss the phenomenology of UHE gamma rays, in terms of their production and propagation, as well as the principle of their detection through the discrimination of photons showers from hadron ones. Chapter 2 will focus on the Pierre Auger Observatory, describing its technical features and some of the key results obtained in more than 15 years of operation. In chapter 3 we explain the universality concept and how it is applied to build a universality-based model of the signals in the detectors of the ground array. Chapter 4 introduces the first part of this thesis work: the building of the simulations and of the data sets that are used in the following chapters. Particular attention is devoted to the simulation procedure, and to the event selection. In chapter 5, we use the universality concept, in combination with the hybrid reconstruction, to set up a technique to derive Fμ from the signal recorded in individual SD stations. The potential of Fμ as photon-hadron discriminator is also assessed. Chapter 6 describes the combination of Xmax and Fμ, by using multivariate analysis techniques. A data-driven approach for the estimation of the background expected is exploited to derive the photon selection cut. Finally, in chapter 7 the photon selection cut is applied to the hybrid data. As no UHE photons are observed, upper limit to their flux are set, and the physics implications are discussed.
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Submitted on : Friday, July 30, 2021 - 1:01:11 AM
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  • HAL Id : tel-03309027, version 1

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Pierpaolo Savina. Search for gamma-rays in the EeV sky at the Pierre Auger Observatory using universality. High Energy Astrophysical Phenomena [astro-ph.HE]. Université Paris-Saclay; Università del Salento, 2021. English. ⟨NNT : 2021UPASP041⟩. ⟨tel-03309027⟩

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