Ultrahigh Energy Cosmic Rays (2004 – 2012)

Numerous theoretical, phenomenological and experimental efforts have been undertaken to study the most energetic and least well-understood component of cosmic ray radiation: ultrahigh energy cosmic rays (UHECRs). The observation of UHECR particles that reach the terrestrial atmosphere with energies exceeding 10 ²⁰ eV is puzzling as a mechanism that could be responsible for their acceleration remains either unknown or elusive. As a member of the Pierre Auger Collaboration, my colleagues and I analyzed UHECRs in order to answer fundamental questions pertaining to their origins, make-up, and propagation from their sources to the Earth.

My graduate studentship and my first two postdoctoral appointments (Laboratoire AstroParticule et Cosmologie; The University of Chicago) were dedicated to projects associated with the Pierre Auger Collaboration, which studies UHECR radiation from EeV energies (1 EeV = 10 ¹⁸ eV) up to the highest energies. The properties of the UHECR, such as arrival direction, energy and mass, are inferred from the extensive air shower (EAS) that occurs as the UHECRs enter the atmosphere. To analyze the EAS, the Pierre Auger Observatory, located near Malargüe (Argentina), employs both water-Cherenkov detectors (SD; which sample the particle densities as the EAS arrives) and fluorescence detectors (FD; which collect the UV light from the de-excitation of atmospheric nitrogen as the EAS develops). The Pierre Auger Observatory is not a pointing telescope, rather it records all incoming flux of UHECRs from the South equatorial Pole up to ∼25° in declination with an angular resolution of about 1.5°.

During my time in the Pierre Auger Collaboration, I improved the data quality by studying the effect of changes in the atmosphere, in terms of pressure and density, on the EAS development. I developed a model that corrects the reconstructed energy of the events and the counting rate of events for atmospheric variations. Additionally, I undertook the calculation of the geometrical aperture of the SD as a function of time and the measurement of the UHECRs energy spectrum. I have also proposed a novel method to estimate the relative exposure of the SD. It relies on data estimates only and is faster and more accurate than the traditional method. I have worked extensively on the analysis of the arrival directions of the UHECRs and on the associated astrophysical interpretations. These include a search for Galactic point sources of EeV neutrons, anisotropy studies around the Galactic centre and searches for correlation between the UHECRs arrival directions and the positions of Galactic and nearby extragalactic astrophysical objects.

My most recent UHECR work entails estimations of the expected anisotropy signal that will be detected by the next generation of UHECR detectors. My colleagues and I simulated realistic UHECR sky maps for a wide range of possible astrophysical scenarios, taking into account the energy losses and photo-dissociation of the UHE protons and nuclei as well as their deflections by intervening Galactic and extragalactic magnetic fields. These sky maps were analyzed from the point of view of their intrinsic anisotropies, using the two-point correlation function. Finally, we performed a statistical study of the resulting anisotropies for each astrophysical scenario, varying the UHECR source composition and energy spectrum as well as the source density; and considering a large set of independent realizations for each choice of a parameter set in order to explore the so-called cosmic variance.

While a postdoctoral fellow at the University of Chicago, I was also able to take part in other cosmic ray experiments. These projects include: (i) the measurement of the absolute yield of fluorescence photons at the Fermilab test beam with the AIRFLY (AIR FLuorescence Yield) experiment; (ii) the MIDAS (MIcrowave Detection of Air Showers) experiment that examined the possibility to detect EAS through molecular bremsstrahlung; (iii) the MAYBE (Microwave Air Yield Beam Experiment) performed at the Van de Graaff facility of the Argonne National Laboratory to characterize the microwave emission from an electron beam induced air plasma; and (iv) the design of a Radio Imaging Atmospheric Cherenkov Telescope (RIACT) to detect, in the microwave regime, the Cherenkov emission induced by the charged particles of the EAS.