Pierre Auger Observatory

The toe of the spectrum of ultra-high energy cosmic rays (UHECRs), above ≈50 EeV, is an extremely interesting region for studying the origins of CRs. The potentially small magnetic deflections at these energies are coupled with the presence of the flux suppression, which could be a signature of the maximum acceleration potential of the sources, or could find its explanation in the interactions of cosmic rays with background photons, effectively limiting the region of interest in the search for UHECR sources to a relatively small bubble around us. In this talk we present the latest anisotropy searches carried out by the Pierre Auger Collaboration in the energy range above 32 EeV. The dataset used is that collected in the phase one of the Observatory between 2004 and 2020, before the AugerPrime upgrade, for a cumulative exposure of 120000 km2 sr yr. We have conducted both blind, model-independent searches for overdensities, correlation analyses with astrophysical structures, and cross-correlation studies with catalogs of candidate sources. We have found evidence for a deviation from isotropy at an angular scale of 25 degrees at the 4𝜎 level for UHECRs with energy above 38 EeV.

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 2004 January 1 and 2020 December 31 are given for cosmic rays that have energies in the range 78–166 EeV. Details are also given on 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.

After over 60 years, the powerful engines that accelerate ultra-high-energy cosmic rays (UHECRs) to the formidable energies at which we observe them from Earth remain mysterious. Assuming standard physics, we expect UHECR sources to lie within the local Universe (up to a few hundred Mpc). The distribution of matter in the local Universe is anisotropic, and we expect this anisotropy to be imprinted on the distribution of UHECR arrival directions. Even though intervening intergalactic and Galactic magnetic fields deflect charged UHECRs and can distort these anisotropies, some amount of information on the distribution of the sources is preserved. In this proceedings contribution, we present the results of the joint Pierre Auger Observatory and Telescope Array searches for (a) the largest-scale anisotropies (the harmonic dipole and quadrupole) and (b) correlations with a sample of nearby starburst galaxies and the 2MRS catalogue tracing stellar mass within 250 Mpc. This analysis updates our previous results with the most recent available data, notably with the addition of 3 years of new Telescope Array data. The main finding is a correlation between the arrival directions of 12.1%−3.1%+4.5% of UHECRs detected with E ≥ 38 EeV by Auger or with E ≳ 49 EeV by TA and the positions of nearby starburst galaxies on a 15.1°−3.0°+4.6° angular scale, with a 4.7σ post-trial significance, up from 4.2σ obtained in our previous study.

The Pierre Auger Observatory in Argentina, built to study the physics of highest-energy cosmic rays, has a tremendous emotional appeal given the Pampa Amarilla environment at 1400 m asl in Mendoza, coupled with the aura of a pioneering experiment that explores the Universe. Here, we present some of the Outreach, Education, and Communication programmes carried out within the international collaboration. Since early times the Observatory has been communicating its existence and purpose through the Visitor Center, where guided tours with supporting presentations are frequently offered to make the experiment, astroparticle physics, and scientific research, in general, accessible to the public. The limitation of the on-site access by the ongoing pandemic fuelled the development of virtual tours and enlarged the number of international communication to the public. The use of high-school students' specific programs has been one of the elements of the Observatory upgrade. A part of the detectors used in the upgrade has been built in cooperation with students, setting the basis for a proficient citizen science program. A fraction of the data processed from the collaboration is made available to the public in a usable format. The collaboration developed special masterclasses, providing the resources to analyse the public data set together with special training sessions at teachers' or students' level. Finally, a glance at the worldwide inspirational activities, based on the mentioned emotional appeal of the Observatory and conducted by the International Collaboration members, will be given.

Presently large systematic uncertainties remain in the description of hadronic interactions at ultra-high energies and a fully consistent description of air-shower experimentaldata is yet to be reached.The amount of data collected by the Pierre Auger Observatory using simultaneously the fluorescence and surface detectors in the energy range 10 18.5 -10 19.0 eV has provided opportunity to perform a multi-parameter test of model predictions.We apply a global method to simultaneously fit the mass composition of cosmic rays and adjustments to the simulated depth of shower maximum (X max ), and hadronic signals at ground level (R Had ).The best description of hybrid data is obtained for a deeper scale of simulated X max than predicted by hadronic interaction models tuned to the LHC data.Consequently, the deficit of the simulated hadronic signal at ground level, dominated by muons, is alleviated with respect to the unmodified hadronic interaction models.Because of the size of the adjustments ∆X max and R Had and the large number of events in the sample, the statistical significance of these assumed adjustments is large, greater than 5σ stat , even for the combination of the systematic experimental shifts within 1σ sys that are the most favorable for the models.

Lorentz invariance violation (LIV) is often described by dispersion relations of the form E i 2 = m i 2+p i 2+δi,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 δi,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 1019 eV, we obtain δγ,0 > -10-21, δγ,1 > -10-40 eV-1 and δγ,2 > -10-58 eV-2 in the case of a subdominant proton component up to 1020 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 δhad,0 < 10-19, δhad,1 < 10-38 eV-1 and δhad,2 < 10-57 eV-2 at 5σ CL.

The Surface Detector of the Pierre Auger Observatory for the detection of ultra-high energy cosmic rays, an array of 1600 water-Cherenkov stations in a triangular grid with 1500 m separation between stations, has also the capability to detect neutrinos with energy above 1017eV. The identification, through the special characteristics of highly inclined showers, is efficiently performed for neutrinos of all flavours interacting deep in the atmosphere at large zenith angles as well as for τ-neutrinos skimming the Earth crust. In this talk I review the status of the neutrino search at the Observatory using about 15 years of data. Restrictive upper bounds on the neutrino flux from diffuse sources and from point-like steady sources were stablished, placing strong constraints on several models of neutrino production at EeV energies and on the properties of the sources of ultra-high-energy cosmic rays. Unrivaled sensitivity in searches for transient sources has been also achieved.

To obtain direct measurements of the muon content of extensive air showers with energy above 10$^{16.5}$ eV, the Pierre Auger Observatory is currently being equipped with an underground muon detector (UMD), consisting of 219 10 m$^{2}$-modules, each segmented into 64 scintillators coupled to silicon photomultipliers (SiPMs). Direct access to the shower muon content allows for the study of both of the composition of primary cosmic rays and of high-energy hadronic interactions in the forward direction. As the muon density can vary between tens of muons per m2 close to the intersection of the shower axis with the ground to much less than one per m2 when far away, the necessary broad dynamic range is achieved by the simultaneous implementation of two acquisition modes in the read-out electronics: the binary mode, tuned to count single muons, and the ADC mode, suited to measure a high number of them. In this work, we present the end-to-end calibration of the muon detector modules: first, the SiPMs are calibrated by means of the binary channel, and then, the ADC channel is calibrated using atmospheric muons, detected in parallel to the shower data acquisition. The laboratory and field measurements performed to develop the implementation of the full calibration chain of both binary and ADC channels are presented and discussed. The calibration procedure is reliable to work with the high amount of channels in the UMD, which will be operated continuously, in changing environmental conditions, for several years.

AMIGA (Auger Muons and Infill for the Ground Array) is an upgrade of the Pierre Auger Observatory to complement the study of ultra-high-energy cosmic rays (UHECR) by measuring the muon content of extensive air showers (EAS). It consists of an array of 61 water Cherenkov detectors on a denser spacing in combination with underground scintillation detectors used for muon density measurement. Each detector is composed of three scintillation modules, with 10 m2 detection area per module, buried at 2.3 m depth, resulting in a total detection area of 30 m2. Silicon photomultiplier sensors (SiPM) measure the amount of scintillation light generated by charged particles traversing the modules. In this paper, the design of the front-end electronics to process the signals of those SiPMs and test results from the laboratory and from the Pierre Auger Observatory are described. Compared to our previous prototype, the new electronics shows a higher performance, higher efficiency and lower power consumption, and it has a new acquisition system with increased dynamic range that allows measurements closer to the shower core. The new acquisition system is based on the measurement of the total charge signal that the muonic component of the cosmic ray shower generates in the detector.

Cosmic rays arriving at Earth collide with the upper parts of the atmosphere, thereby inducing extensive air showers. When secondary particles from the cascade arrive at the ground, they are measured by surface detector arrays. We describe the methods applied to the measurements of the surface detector of the Pierre Auger Observatory to reconstruct events with zenith angles less than 60̂ using the timing and signal information recorded using the water-Cherenkov detector stations. In addition, we assess the accuracy of these methods in reconstructing the arrival directions of the primary cosmic ray particles and the sizes of the induced showers.