We use the surface detector of the Pierre Auger Observatory to search for air showers initiated by photons with an energy above \(10^{19}\) eV. Photons in the zenith angle range from 30\(^\circ\) to ...60\(^\circ\) can be identified in the overwhelming background of showers initiated by charged cosmic rays through the broader time structure of the signals induced in the water-Cherenkov detectors of the array and the steeper lateral distribution of shower particles reaching ground. Applying the search method to data collected between January 2004 and June 2020, upper limits at 95\% CL are set to an \(E^{-2}\) diffuse flux of ultra-high energy photons above \(10^{19}\) eV, \(2{\times}10^{19}\) eV and \(4{\times}10^{19}\) eV amounting to \(2.11{\times}10^{-3}\), \(3.12{\times}10^{-4}\) and \(1.72{\times}10^{-4}\) km\(^{-2}\) sr\(^{-1}\) yr\(^{-1}\), respectively. While the sensitivity of the present search around \(2 \times 10^{19}\) eV approaches expectations of cosmogenic photon fluxes in the case of a pure-proton composition, it is one order of magnitude above those from more realistic mixed-composition models. The inferred limits have also implications for the search of super-heavy dark matter that are discussed and illustrated.
The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy ...physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.
Using the data of the Pierre Auger Observatory, we report on a search for signatures that would be suggestive of super-heavy particles decaying in the Galactic halo. From the lack of signal, we ...present upper limits for different energy thresholds above \({\gtrsim}10^8\)\,GeV on the secondary by-product fluxes expected from the decay of the particles. Assuming that the energy density of these super-heavy particles matches that of dark matter observed today, we translate the upper bounds on the particle fluxes into tight constraints on the couplings governing the decay process as a function of the particle mass. Instantons, which are non-perturbative solutions to Yang-Mills equations, can give rise to decay channels otherwise forbidden and transform stable particles into meta-stable ones. Assuming such instanton-induced decay processes, we derive a bound on the reduced coupling constant of gauge interactions in the dark sector: \(\alpha_X \lesssim 0.09\), for \(10^{9} \lesssim M_X/\text{GeV} < 10^{19}\). Conversely, we obtain that, for instance, a reduced coupling constant \(\alpha_X = 0.09\) excludes masses \(M_X \gtrsim 3\times 10^{13}~\)GeV. In the context of dark matter production from gravitational interactions alone during the reheating epoch, we derive constraints on the parameter space that involves, in addition to \(M_X\) and \(\alpha_X\), the Hubble rate at the end of inflation, the reheating efficiency, and the non-minimal coupling of the Higgs with curvature.
Instantons, which are non-perturbative solutions to Yang-Mills equations, provide a signal for the occurrence of quantum tunneling between distinct classes of vacua. They can give rise to decays of ...particles otherwise forbidden. Using data collected at the Pierre Auger Observatory, we search for signatures of such instanton-induced processes that would be suggestive of super-heavy particles decaying in the Galactic halo. These particles could have been produced during the post-inflationary epoch and match the relic abundance of dark matter inferred today. The non-observation of the signatures searched for allows us to derive a bound on the reduced coupling constant of gauge interactions in the dark sector: \(\alpha_X \lesssim 0.09\), for \(10^{9} \lesssim M_X/{\rm GeV} < 10^{19}\). Conversely, we obtain that, for instance, a reduced coupling constant \(\alpha_X = 0.09\) excludes masses \(M_X \gtrsim 3\times 10^{13}~\)GeV. In the context of dark matter production from gravitational interactions alone, we illustrate how these bounds are complementary to those obtained on the Hubble rate at the end of inflation from the non-observation of tensor modes in the cosmological microwave background.
A catalog containing details of the highest-energy cosmic rays recorded through the detection of extensive air-showers at the Pierre Auger Observatory is presented with the aim of opening the data to ...detailed examination. Descriptions of the 100 showers created by the highest-energy particles recorded between 1 January 2004 and 31 December 2020 are given for cosmic rays that have energies in the range 78 EeV to 166 EeV. Details are also given of a further nine very-energetic events that have been used in the calibration procedure adopted to determine the energy of each primary. A sky plot of the arrival directions of the most energetic particles is shown. No interpretations of the data are offered.
The Pierre Auger Observatory, being the largest air-shower experiment in the world, offers an unprecedented exposure to neutral particles at the highest energies. Since the start of data taking more ...than 18 years ago, various searches for ultra-high-energy (UHE, \(E\gtrsim10^{17}\,\text{eV}\)) photons have been performed: either for a diffuse flux of UHE photons, for point sources of UHE photons or for UHE photons associated with transient events like gravitational wave events. In the present paper, we summarize these searches and review the current results obtained using the wealth of data collected by the Pierre Auger Observatory.
A promising energy range to look for angular correlation between cosmic rays of extragalactic origin and their sources is at the highest energies, above few tens of EeV (\(1\:{\rm EeV}\equiv ...10^{18}\:\)eV). Despite the flux of these particles being extremely low, the area of \({\sim}\:3{,}000 \: \text{km}^2\) covered at the Pierre Auger Observatory, and the 17-year data-taking period of the Phase 1 of its operations, have enabled us to measure the arrival directions of more than 2,600 ultra-high energy cosmic rays above \(32\:\text{EeV}\). We publish this data set, the largest available at such energies from an integrated exposure of \(122{,}000 \: \text{km}^2\:\text{sr}\:\text{yr}\), and search it for anisotropies over the \(3.4\pi\) steradians covered with the Observatory. Evidence for a deviation in excess of isotropy at intermediate angular scale, with \({\sim}\:15^\circ\) Gaussian spread or \({\sim}\:25^\circ\) top-hat radius, is obtained at the \(4\:\sigma\) significance level for cosmic-ray energies above \({\sim}\:40\:\text{EeV}\).
The development of an extensive air shower depends not only on the nature of the primary ultra-high-energy cosmic ray but also on the properties of the hadronic interactions. For energies above those ...achievable in human-made accelerators, hadronic interactions are only accessible through the studies of extensive air showers, which can be measured at the Pierre Auger Observatory. With its hybrid detector design, the Pierre Auger Observatory measures both the longitudinal development of showers in the atmosphere and the lateral distribution of particles that arrive at the ground. This way, observables that are sensitive to hadronic interactions at ultra-high energies can be obtained. While the hadronic interaction cross-section can be assessed from the longitudinal profiles, the number of muons and their fluctuations measured with the ground detectors are linked to other physical properties. In addition to these direct studies, we discuss here how measurements of the atmospheric depth of the maximum of air-shower profiles and the characteristics of the muon signal at the ground can be used to test the self-consistency of the post-LHC hadronic models.
For several decades, the origin of ultra-high-energy cosmic rays (UHECRs) has been an unsolved question of high-energy astrophysics. One approach for solving this puzzle is to correlate UHECRs with ...high-energy neutrinos, since neutrinos are a direct probe of hadronic interactions of cosmic rays and are not deflected by magnetic fields. In this paper, we present three different approaches for correlating the arrival directions of neutrinos with the arrival directions of UHECRs. The neutrino data is provided by the IceCube Neutrino Observatory and ANTARES, while the UHECR data with energies above \(\sim\)50 EeV is provided by the Pierre Auger Observatory and the Telescope Array. All experiments provide increased statistics and improved reconstructions with respect to our previous results reported in 2015. The first analysis uses a high-statistics neutrino sample optimized for point-source searches to search for excesses of neutrinos clustering in the vicinity of UHECR directions. The second analysis searches for an excess of UHECRs in the direction of the highest-energy neutrinos. The third analysis searches for an excess of pairs of UHECRs and highest-energy neutrinos on different angular scales. None of the analyses has found a significant excess, and previously reported over-fluctuations are reduced in significance. Based on these results, we further constrain the neutrino flux spatially correlated with UHECRs.
Lorentz invariance violation (LIV) is often described by dispersion relations of the form \(E_i^2=m_i^2+p_i^2+\delta_{i,n} E^{2+n}\) with delta different based on particle type \(i\), with energy ...\(E\), momentum \(p\) and rest mass \(m\). Kinematics and energy thresholds of interactions are modified once the LIV terms become comparable to the squared masses of the particles involved. Thus, the strongest constraints on the LIV coefficients \(\delta_{i,n}\) tend to come from the highest energies. At sufficiently high energies, photons produced by cosmic ray interactions as they propagate through the Universe could be subluminal and unattenuated over cosmological distances. Cosmic ray interactions can also be modified and lead to detectable fingerprints in the energy spectrum and mass composition observed on Earth. The data collected at the Pierre Auger Observatory are therefore possibly sensitive to both the electromagnetic and hadronic sectors of LIV. In this article, we explore these two sectors by comparing the energy spectrum and the composition of cosmic rays and the upper limits on the photon flux from the Pierre Auger Observatory with simulations including LIV. Constraints on LIV parameters depend strongly on the mass composition of cosmic rays at the highest energies. For the electromagnetic sector, while no constraints can be obtained in the absence of protons beyond \(10^{19}\) eV, we obtain \(\delta_{\gamma,0} > -10^{-21}\), \(\delta_{\gamma,1} > -10^{-40}\) eV\(^{-1}\) and \(\delta_{\gamma,2} > -10^{-58}\) eV\(^{-2}\) in the case of a subdominant proton component up to \(10^{20}\) eV. For the hadronic sector, we study the best description of the data as a function of LIV coefficients and we derive constraints in the hadronic sector such as \(\delta_{\mathrm{had},0} < 10^{-19}\), \(\delta_{\mathrm{had},1} < 10^{-38}\) eV\(^{-1}\) and \(\delta_{\mathrm{had},2}< 10^{-57}\) eV\(^{-2}\) at 5\(\sigma\) CL.
