Base Support of the University of Utah Cosmic Ray Group and Analysis of Telescope Array Data

The University of Utah cosmic ray group hosts an international collaboration that operates the Telescope Array Observatory. The observatory is for the study of the highest-energy particles known to humankind, with more than ten million times the energy of particles at the CERN Large Hadron Collider. The experiment uses the Earth’s atmosphere itself as part of the detector. These ultrahigh-energy cosmic rays are atomic nuclei arriving from outer space, with energies in a single subatomic particle equal to that of a well-pitched baseball. As these particles enter the atmosphere, they collide with nuclei in the air. This blasts the nucleus apart, producing many secondary particles. These particles travel a short distance before colliding with another air nucleus, generating additional new particles. This soon grows to a shower of billions of secondary particles. The observatory studies the primary cosmic rays by analyzing these air showers with two sets of detectors. The first is an array of more than 850 surface detectors sprinkled over more than 700 square miles. These measure the arrival time and density of shower particles as they reach Earth. There are also 60 telescopes on the perimeter of the surface detector array, overlooking the array. They have very fast, very sensitive cameras that capture movies of the air shower as it develops in the atmosphere. Together, these two sets of detectors enable the group to study the three biggest questions about ultrahigh-energy cosmic rays: their energy, chemical composition, and origins. These particles tell us about the most violent objects in the universe, as well as the space environment they pass through on their way to us. This research both teaches about our universe and provides a platform for training students to collect, analyze, and understand data. These are skills that students can use in their future careers. For example, past students have gone on to work on nuclear safety with the Atomic Energy Agency, lightning studies with NASA, and the Artemis lunar program. The Telescope Array observatory is based in rural Utah, in part, due to the need for dark skies; however, it also allows the collaboration to expose the local populace and students to this exciting science. The experiment is also located along a main route to the Great Basin National Park, providing an opportunity for park visitors from the public to also learn about the Telescope Array and scientific methods. The University of Utah Cosmic Ray Physics group hosts the Telescope Array (TA) Observatory. This is the largest instrument studying ultrahigh-energy cosmic rays (UHECR) in the Northern Hemisphere and is jointly operated by an international collaboration. UHECRs have energies of up to and exceeding 1020 eV in a single nucleus, seven orders of magnitude greater than protons accelerated by the Large Hadron Collider. Because of the low rate and steep E-3 flux vs. energy fall-off of UHECR, detectors of ~1000 square miles in size are needed. These detectors measure extensive air showers (EAS) produced by the collision of primary cosmic nuclei with target nuclei in air, producing secondary particles that in turn collide with more nuclei. This process leads to an avalanche of particles numbering in the billions. TA measures these air showers to extract information about the primary cosmic ray, using two complementary instruments. The first is an array of more than 850 plastic scintillation counters, each with two layers and 3m2 in area, spread over ~700 square miles. These units measure the arrival time and the density of the shower particles. The differences in timing at the hit detectors provide an estimate of the arrival direction of the original cosmic ray to within 1.5-2. The particle densities are analyzed to give the energy of the primary to within ~20%, with a dynamic range from 1018 to >1020eV. A complementary measurement of the longitudinal development of the shower is provided by the air fluorescence telescopes, which capture a 10 million frames/s movie of the ultraviolet (UV) light emitted by nitrogen molecules excited by the passage of the charged particles – analogous to the production of light in a fluorescence tube. The Energy of the shower can be inferred from the brightness of the UV signal, and the relative timing can be used to calculate the arrival direction of the primary cosmic ray. In addition, the longitudinal progression of the shower allows for the measurement of Xmax, the slant depth at which the shower achieves maximum intensity. Showers initiated by a larger incident nucleus would develop earlier in the atmosphere because of increased cross section and multiplicity of secondaries. Xmax thus provides a statistical measure of the mass number of the incident nucleus, hence the composition of UHECR. The University of Utah group was the first to positively identify cosmic ray air showers using the fluorescence technique in 1976. The current expansion of TA from 300 sq miles to 1200 (named TAx4) is the fourth major effort of the Utah group, motivated by indications of anisotropic excess of the highest energy cosmic rays from the direction of Ursa Major and Perseus-Pisces. The emphasis of the group continues to be the understanding of the source, composition, and acceleration mechanism of UHECR. TA is also in collaboration with the Auger Observatory, which is the only comparable detector and views the southern sky. Indications have been seen of differences in the composition and energy spectra observed by these two groups. An important focus for the ongoing observation and data analysis efforts is in discerning whether the differences are due to instrumental effects or possibly the result of actual astrophysical differences between the northern and southern skies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.