The mysterious origin of the highest energy particles in the Universe has been designated as one of the primary outstanding problems in the physical sciences of this new Millenium. These elusive particles arrive at Earth from unknown sources in deep space. Their energies are 100,000,000 times greater than the most energetic particles produced in particle accelerators on Earth. Moreover, their energies exceed what has been inferred in any astrophysical setting. Arriving almost equally from all directions, we deduce that they do not originate in our Milky Way Galaxy. On the other hand, their interactions with cosmic microwave radiation should render the universe almost opaque to the most energetic of these particles, so that understanding how they can reach us from cosmic distances is a profound puzzle. Although they have been the subject of intense pursuit by science teams for several decades, our understanding of these highest energy cosmic rays remains hampered by their exceptional rarity. We are trying to study particles that arrive only once a century per square kilometer. Our method is to collect them with a detector array, called the Pierre Auger Observatory that, when completed, will span 3000 square kilometers in Argentina.
The Auger Observatory's cumulative data set at the highest energies, obtained during the construction phase of the project, already exceeds that of any previous cosmic ray experiment. This data set will expand by more than an order of magnitude over the next three years. One major advantage of Auger is that it utilizes simultaneously for the first time both techniques traditionally used by previous experiments to determine the energy of the primary cosmic ray. First, an array of detectors on the ground samples and detects the cascade of particles, or "air shower," produced in the atmosphere by the interactions of the primary cosmic ray. On dark nights, special telescopes also image the pattern of nitrogen fluorescence, or sky glow, generated by the passage of the particle cascade through the atmosphere. These detection methods have complementary advantages and limitations, so that by combining them we will be in a position to minimize the systematic uncertainties inherent in either technique alone. In this manner, we have the best possible measurements of these rarest of particles. We will determine if there are protons, heavy atomic nuclei, photons, neutrinos, or possibly more exotic objects. The Auger group at Penn State University has experimental and analytical expertise in both detection techniques. We will conduct a variety of studies, both experimental and analytical, which will address some of the primary questions that must be resolved in minimizing the uncertainties in the combination of the two techniques. For instance, it is crucial to reduce the systematic uncertainty in fluorescence energy assignments, which result partly from uncertainty in the air fluorescence yield and partly from uncertainty in detector calibration. It is also essential to minimize uncertainty in atmospheric modeling. We will conduct a five-prong program of experimental work aimed at reducing fluorescence detection energy uncertainties. We will also continue to be heavily involved in the Auger spectrum analysis and in resolving the apparent discrepancy between the energy reconstruction of the two techniques. We will concentrate particular effort on areas of analysis in which we have special strengths and interests: anisotropy studies and the search for neutrinos near 10^18 eV or above.
High energy cosmic ray observations are an important domain of astrophysics that will produce new insights about extreme processes in the universe. Exciting news in cosmology and high energy astrophysics will be the theme of a week-long summer workshop that we are developing for high school teachers in Pennsylvania, to be offered in each of 3 consecutive years. The concepts of basic college physics are interwoven into the curriculum, so teachers strengthen their working knowledge of their core curriculum while getting an inside look at what is happening at the forefront of astrophysics research. We will take advantage of existing infrastructure and experience at Penn State from similar workshops in astronomy, chemistry and material sciences, and we will work with our colleagues in particle physics and particle astrophysics who have expressed a common interest in this endeavor.
