The University of Utah Cosmic Ray Physics Group has a long history of studying the spectrum, composition, and sources of the highest energy particles in the universe. The west desert of Utah, where their experiments have been located, is an excellent site for cosmic ray experiments. Their current experiment is the Telescope Array (TA), which is the largest cosmic ray experiment in the northern hemisphere. This group is the host institution of the TA experiment. The physics issues (what is the nature of the sources? and how do they accelerate particles to such high energies?) are among the most important questions in astrophysics today. Measurements of the spectrum and composition of cosmic rays are basic to answering these questions, and searching for anisotropy in their arrival directions is a direct way of identifying sources.
This award provides the baseline support of the group, covering salaries, travel and data analysis. The goals of the coming three-year grant period include: measuring the cosmic ray spectrum with both surface and fluorescence detectors and making cosmic ray composition studies in stereo and hybrid modes, as well as searching for anisotropy.
For Broader Impacts, the group's main outreach arm is the ASPIRE Project, which creates and maintains some of the Web's most engaging and interactive science lessons and labs. ASPIRE provides direct outreach to area teachers, runs summer workshops for students, and makes visits to rural schools. The Lon and Mary Watson Cosmic Ray Center (owned by the University of Utah) in Delta, Utah, is the center of the TA outreach work and for the field operations.
The Telescope Array was conceived and built by members of the High Resolution Fly’s Eye (HiRes) and AGASA (Akeno Giant Air Shower Array) collaborations to study the origin and nature of the highest energy cosmic rays. As a part of this, one of the collaboration’s goals was to try to understand the differences in the measurements of ultra high energy cosmic ray (UHECR) properties as measured by HiRes, AGASA, and the Pierre Auger Observatory (PAO). Planning for the Telescope started in the late 1990’s. Construction was approved by the Japanese funding agency in 2003 and by the US NSF in 2006. Construction of the high energy portion of the Telescope Array was completed in 2008 and data collection began. The Telescope Array is a hybrid detector consisting of both surface detectors which sample the charge density in the footprint of a cosmic ray induced air shower and fluorescence telescopes which measure the longitudinal development of the shower via the ultra-violet light generated as the shower develops in the atmosphere. The detector has an area of about 350 square miles and is located in Utah’s west desert. It measures the origin direction, energy, and chemical composition of UHECRs. One of the discrepancies under study is the end of the cosmic ray spectrum. In 1964, Penzias and Wilson of AT&T labs discovered the microwave background. This led to calculations by Greissen, Zatsepin, and Kuzmin to theorize an end to the cosmic ray spectrum. They calculated that cosmic rays with energy greater than ~6x10^19eV would interact with the background and lose their energy. This GZK cutoff was observed by HiRes, but not AGASA or other experiments. The HiRes observation of the cutoff is confirmed by the Telescope Array TA data indicates the probability of the spectrum continuing without cutoff is about 2x10^-11 which is about 6.5sigma. While the PAO also sees the suppression, their cutoff is at a slightly different energy; this is a subject for further study. The HiRes/MIA and HiRes experiments observed a chemical composition that was heavy like iron around 10^17eV and became light/protonic above ~10^18eV. The interpretation of this was that at 10^17eV, we were observing the last remnants of the galactic cosmic rays and >10^18eV the cosmic rays were extra-galactic. PAO observes a light composition at 10^18eV, but finds the composition getting significantly heavier starting ~4x10^18eV. HiRes measured the composition using the stereo technique which used telescopes at two different locations to get a precise geometry and redundant measurement of the shower development. PAO used a hybrid measurement which uses surface detectors to help the telescopes get a geometry lock and then finds the shower profileusing their telescopes. TA is able to utilize both techniques. TA finds a light/protonic composition from 10^18eV to the highest energies. This is an area of joint work between TA and PAO to understand this difference. As TA has been collecting data, we have noticed a grouping of the highest energy events, those with E>5.7x10^19eV. With the 5year data-set, we attempted to quantify the statistical significance of the spot. We found that about 26% of the events were in about 6% of the observable area. We estimated the probability of this happening by chance to be just over 3sigma. This provides some evidence, but is not convincing enough to call an observation. We published this information and declared that we would continue to watch the region. When we added a 6th year of data, the signal became stronger, 4.6sigma, which is encouraging. If the signal continues to grow, in a year or two, we should be able to declare an observation. A larger detector and more data may enable us to see structure in this spot (which is currently 20degrees) and provide some insight into the sources of UHECRs. Using the start-up funds of two professors in the group, we have added a low energy extension to the TA. It adds 10 telescopes looking higher in the sky to one of the sites. This is important since the showers of lower energy cosmic rays develop higher in the atmosphere. Using these new telescopes, TA is now able to lower its energy threshold to ~3x10^16eV and observe the transition from galactic to extra-galactic cosmic rays. It also enables TA to reach closer to the direct measurements of cosmic rays by balloon and space based detectors. TA is now able to make measurements of cosmic rays over 4.5 orders of magnitude in energy. This is the first time this has been done with one detector and with energy is set by the GZK cutoff. The group visits many schools, underrepresented and refugee groups, libraries, farmer’s markets etc each the year. A web site was made to teach common core physical science topics and an education center was built and maintained near the experimental site.
