This is a proposal to continue the participation of the University of Chicago (UC) in the Pierre Auger Observatory (PAO). The observatory is designed to study the origin and nature of the highest energy particles ever observed, namely cosmic rays with energies greater than 10^19 eV, by measuring their energy, their composition, and the distribution of arrival directions with large statistics and careful control of systematic effects. The Southern PAO, nearing completion in Malargue, Argentina, is the first hybrid detector to combine the two best-developed techniques of measuring showers caused by ultra-high energy cosmic rays (UHECRs). The observatory detects shower particles that reach the surface with a surface array and the longitudinal development of the shower through the fluorescence produced in the atmosphere. This combination is essential for accurate energy measurements and reliable composition studies.
The integration of theory, observations, and experiments at UC into a single effort has greatly benefited both the research goals and the training of students by involving them in all aspects of science: from the construction and operation of the PAO to the analysis of its data. This project will solidify the international leadership in the analysis and interpretation of the PAO data at UC.
The discovery of the origin of the highest energy particles will impact both astrophysics and particle physics. The Southern PAO has had a major impact on public education and outreach throughout the participating countries across the globe: from Argentina to Vietnam. In collaboration with the Auger Visitors Center, the Malargue Planetarium, and the Adler Planetarium, the UC effort strengthens the depth of physics and astrophysics content of the education and public outreach efforts by promoting the 3D and 2D visualizations developed at UC of the highest energy showers, their detection methods, and possible cosmic origins. In collaboration with the American Natural History Museum, the group will develop a Science Bulletin that will further provide the public access to this fascinating mystery.
The Hidden Sources of the Highest Energy Cosmic Particle NSF supported work by researchers at the University of Chicago has highlighted the mysterious origin of the highest energy cosmic particles ever observed. The group is part of an international collaboration that built the Pierre Auger Cosmic Ray Observatory the largest observatory of cosmic rays ever built, covering 3,000 square kilometers of the Argentinean pampas. The Auger Observatory observed the largest number of particles at the highest energy frontier of physics, above 1018 eV (an Exa-electron-volt). Cosmic rays are studied over a vast energy range. At moderate energies, their properties are well measured by satellites and high-altitude balloons. Protons are the dominant particle type, with only minor components of other nuclei, and those cosmic rays are known to originate in our Galaxy, probably through processes associated with supernova explosions. At the extremely high energies studied by the Auger Observatory, the flux is less than 1 cosmic ray per km2 per year, so Auger detects them indirectly over an area of 3,000 square kilometers. Identifying the type of particle relies on measuring how deep into the atmosphere the measured cascade of particles penetrates. The distribution of the highest energy events in the sky (above 55 Exa-electron-volt as shown in figure 1) shows that cosmic rays of ultrahigh energies are not arriving equally from all directions in the sky. The departure from isotropy is subtle, however current data show hints of clustering similar to the distribution of galaxies in the nearby extragalactic universe. These anisotropies should become more significant with data expected from the next few years of operations of the Observatory. In particular, the area around Centaurus A, the nearest Active Galactic Nucleus to our Galaxy, the Milky Way, has more events than expected on average. Higher statistics will tell if Centaurus A is the first source of ultrahigh energy particles to be identified and to begin solving the long time mystery of the origin of cosmic rays. The Pierre Auger Cosmic Ray Observatory has also found evidence suggesting that the highest energy cosmic rays are large nuclei rather than simply protons. The data indicate that the chemical composition of cosmic rays changes with energy from mainly protons around 10^18 eV (an Exa-electron-volt) to a population that is almost devoid of protons and small nuclei at higher energies. This puzzling change occurs over an energy range which is commonly associated with a transition from cosmic rays that are produced in the Milky Way Galaxy to a different population which come from outside our Galaxy. This result was not expected, partly because protons are far more prevalent in the Universe than large nuclei, and also because protons dominate the cosmic rays at lower energy. It is also surprising because the directions of highly charged large nuclei would be strongly affected by the magnetic field of the Galaxy, so the reported correlation of their arrival directions with local matter in the nearby universe (at only slightly higher energy) should be washed out. An alternative view is that the models in the figure (red and blue regions) cannot be trusted at these extremely high energies, and unexpected features of proton interactions might explain the trends seen in the data. The University of Chicago group has been working closely with data from Auger as well as theoretical models of possible cosmic accelerators with the goal of unveiling this century old mystery of the origin of cosmic rays. The group is also working on new techniques to make observations of high energy particles more affordable in order to plan for next generation observatories that may cover even larger areas.
