The Pierre Auger Observatory: latest results and future perspectives

The Pierre Auger Observatory is the largest ultrahigh-energy cosmic ray observatory in the world. The huge amount of high quality data collected since 2004 up to now led to great improvements in our knowledge of the ultra-energetic cosmic rays. The suppression of the cosmic-ray flux at highest energies was clearly established, and the extra-galactic origin of these particles was confirmed. On the other hand, measurements of the depth of shower maximum indicate a puzzling trend in the mass composition of cosmic rays at energy around the ankle up to the highest energy. The just started upgrade of the Observatory, dubbed AugerPrime, will improve the identification of the mass of primaries allowing us to disentangle models of origin and propagation of cosmic rays.


Hadronic Shower
Ground level How to study UHECRs?
Detection of the fluorescence light emitted by de-excitacion of atmospheric N2 after interactions with the secondary particles of the shower Measurement of the particle density at ground level (e, γ, μ) Amount of fluorescence light is proportional to the energy that the shower dissipates in the atmosphere Calorimetric measurement of the primary particle energy Xmax depends on primary particle mass The distribution of particles at ground level depends on the energy and mass of the primary particle

Lateral Distribution
Longitudinal development

Mass Composition
Average of X max Std. Deviation of X max

Mass composition is not the same at all energies
Large proton fraction at the energy of the ankle Mean mass increases at highest energy

Method:
sky model as the sum of an isotropic fraction plus the anisotropic component from selected sources. The model predictions are compared to the data using the maximum likelihood ratio method.

vertical events and 1118 inclined ones @ E>20EeV
Correlation of UHECRs with the brightest AGNs of the Swift-BAT catalog under the assumption that all the selected sources contribute equally to the UHECR flux.

Auger Collaboration @ICRC 2017
15 Arrival directions of the UHECRs Departures from isotropy: ∼3σ C.L. region of Centaurus A (19 observed events vs 6.0 expected on ∼ average from an isotropic flux) ∼2.7σ C.L. excess has been found in the directions of the active galaxies from Fermi-LAT ∼4σ C. L. starburst galaxies in direction of Cen. A and in the South Galactic pole (NGC 4945, NGC 1068 NGC 253 and M83)

Auger Collaboration @ICRC 2017
UHE photons are tracers of the Greisen-Zatsepin-Kuzmin (GZK) process. If these predicted GZK photons were observed, it would be an indicator for the GZK process being the reason for the observed suppression in the energy spectrum of UHE cosmic rays

Search for photons E > 1EeV
Photons vs Hadrons showers • Higher value of the Xmax • Lower average number of muons • Steeper LDF and consequently a smaller footprint on ground

photons candidate above 10 EeV (SD) 3 photons candidate between 1-2 EeV (Hybrid)
The current upper limits impose tight constraints on current top-down scenarios proposed to explain the origin of UHE cosmic rays Down-going (all flavors) neutrinos that develop deep in the atmosphere generating inclined showers and triggering the Auger surface detector can be identified provided their zenith angles exceed 60 degrees.
Tau neutrinos entering the Earth with a zenith angle close to 90 degrees can interact and produce a tau lepton that decays in the atmosphere inducing an "upward-going" shower that triggers the surface detector.

Neutrinos of 10
∼ 18 eV are expected from interactions of UHECR in the sources or during propagation through the Universe.

No neutrino events observed
Different models for cosmogenic neutrinos that attempt to explain the origin of cosmic rays are excluded at the 90% C.L particularly those that assume proton primaries dN/dE = k E -2 → k ~ 6.4 x 10 -9 GeV cm -2 s -1 sr -1

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Hadronic physics excess of muons in UHECR air showers compared to predictions of hadronic interaction models PRL 117, 192001 (2016) R E energy rescaling parameter to allow for a possible shift in the FD energy calibration, R had multiplicative rescaling of the hadronic component of the shower The measured longitudinal profile with its matching simulated showers, using QGSJet-II-04 for proton and iron primaries The observed and simulated ground signals for the same event. Enhancement of the capability of the Surface Detector to identify the mass of the primary particle on a shower-by-shower basis Auger Collaboration @ICRC 2017 A thin scintillation detector, which is mounted above the larger WCD, provides a robust and well-understood scheme for particle detection that is sufficiently complementary to the water-Cherenkov technique and permits a good measurement of the density of muons.

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AugerPrime -The project of the upgrade Extension of the dynamic range of the WCD The dynamic range of the WCD will be enhanced by a factor 32 with an additional small (1") PMT that will be inserted in the WCD

Upgrade of WCDs
New electronics of the SD It will increase the data quality thanks to better timing accuracy and a faster ADC sampling. The number of collected events will be doubled in comparison to the statistics collected up to now by the existing Pierre Auger Observatory, with the advantage that every future event will have mass information and will allow us to better address some of the most pressing questions in UHECR physics 22

Summary
Spectrum →strong flux suppression Mass composition → light @ ankle mixed @ UHE

Photons and neutrinos search → constraints on p-dominated sources
Source → compatible with maximum rigidity scenario