This award will fund the group to study ultrahigh energy cosmic rays with the High Resolution Fly's Eye (HiRes) and Telescope Array collaborations. The study of cosmic rays has intellectual impact in several areas of physics and astrophysics. Ultrahigh energy cosmic rays are the highest energy particles in the universe, and are accelerated in the most violent astronomical objects.
The HiRes collaboration has made several important measurements, with this group playing an important part. The Telescope Array is the largest cosmic ray experiment in the northern hemisphere. It is a hybrid experiment and has been collecting data since early 2008. The group is involved in the analysis of both surface detector (SD) and fluorescence detector (FD) data. They lead the collaboration in the SD work, developing a new method of SD analysis, and have calculated the spectrum of cosmic rays from the SD. Their FD analysis centers on the two fluorescence detectors located in the southern portion of the experiment. They are making the first steps in FD analysis: monocular reconstruction, followed by hybrid reconstruction.
The broader impacts of the research include the education of students and postdocs and the group will join the Aspire Outreach activity.
, D.R. Bergman, G.B. Thomson, Department of Physics and Astronomy, University of Utah Intellectual Merit This grant funded experimental research studying the physics of ultrahigh energy cosmic rays with two experiments located in the west desert of Utah, the High Resolution Fly’s Eye (HiRes) and Telescope Array (TA) experiments. The aim of the research is to understand what ultrahigh energy cosmic rays are, where they come from, and how they are accelerated. The ultrahigh energy regime covers approximately 1x1018 eV < E < 3x1020 eV. Ultrahigh energy cosmic rays are the most energetic particles in the universe, and the highest energy cosmic ray ever seen had the enormous energy of 51 Joules, the energy of a well-pitched fast ball, all concentrated in a single elementary particle. The highest energy cosmic rays are rare, occurring at a rate of about one per square kilometer per century, so very large experiments are needed to collect these events. HiRes (which concluded in this grant period) and TA (its successor) are the largest such experiments in the northern hemisphere. There is a theoretical upper limit to the energy of a cosmic ray, called the Greisen-Zatsepin-Kuzmin cutoff, which is caused by interactions of cosmic rays with photons of the cosmic microwave background radiation. Our group and others in the HiRes experiment made the first experimental observation of the cutoff. This observation has been confirmed by work done by our group in the TA experiment. Modern cosmic ray experiments use two detector techniques: observing air shower particles using arrays of particle detectors deployed on the surface, and observing the nitrogen fluorescence light emitted as cosmic ray air showers propagate through the atmosphere. TA has both kinds of detectors, and is engaged in developing the experimental techniques necessary for their use. We made a major breakthrough in the analysis techniques of surface array data. All such analyses involve making a simulation of the data by the Monte Carlo technique, and until now simulating air showers has been very inaccurate due to the enormous amount of computer time necessary. The main approximation technique used, called "thinning", has been inadequate for the needs of experiments. Our group has solved this problem by inventing a technique we call "dethinning" which restores the information lost in this approximation. This has allowed us to perform an analysis of TA surface detector data that is of unprecedented accuracy. Our data is now being used to measure the spectrum of cosmic rays and to search for anisotropy in their pointing directions. Broader Impacts In addition to improving the state of knowledge of the field of cosmic ray physics and educating postdocs, graduate students, and undergraduates, the proponents of this grant worked in several outreach areas: judging local science fairs, giving talks to undergraduates on our research, giving tours of TA to Utah state legislators and resource managers, working on the design of the TA Visitors’ Center in Delta, Utah, and participating in the ASPIRE project of the cosmic ray group of the University of Utah.
