Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session X3: The Nature of the Highest Energy Cosmic Rays |
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Sponsoring Units: DAP Chair: Corbin Cavault, Case Western Reserve University Room: Thurgood Marshall South |
Tuesday, February 16, 2010 10:45AM - 11:21AM |
X3.00001: Observations of Ultra-High Energy Cosmic Rays by the Pierre Auger Observatory Invited Speaker: |
Tuesday, February 16, 2010 11:21AM - 11:57AM |
X3.00002: Composition Studies of the Highest Energy Cosmic Rays with the High Resolution Fly's Eye Observatory Invited Speaker: The existence of a strong break at $6 \times 10^{19}$~eV in the cosmic ray energy spectrum may be most simply explained by the Greisen-Zatsepin-Kuzmin (GZK) mechanism of interactions with the cosmic microwave background (CMB), provided the highest energy particles observed are both protons and extragalactic in origin. Mass studies of cosmic rays can test both of these conditions, by observing a transition from a heavy galactic to light extragalactic composition and the persistence of the light composition at the highest energies. Air fluorescence observatories such as the High Resolution Fly's Eye (HiRes) probe primary cosmic ray composition by studying of the shape of cosmic-ray induced extensive airshowers. The average depth of airshower maximum $X_{max}$ depends logarithmically on the primary energy and atomic mass, and the {\em elongation rate}, $d\hspace{-0.1cm}<\hspace{-0.2cm}X_{max}\hspace{-0.2cm}>\hspace{-0.1cm}/d\log{E}$, will be constant for unchanging primary compositions. Further, shower-to-shower fluctuations in $X_{max}$ will be smaller in airshowers induced by heavy nuclei due to averaging effects. We report the results of composition studies of ultra-high energy cosmic rays observed by the stereoscopic HiRes observatory, for particles with energies above 1.6~EeV. The mean shower maximum, elongation rate, and fluctuations observed by HiRes are all consistent with a predominantly protonic composition when interpreted via the QGSJET01 and QGSJET-II high energy hadronic interaction models. The HiRes data thus supports the CMB-interaction explanation of the end of the energy spectrum and severely constrains models in which the galactic-to-extragalactic transition occurs above $10^{18}$~eV. [Preview Abstract] |
Tuesday, February 16, 2010 11:57AM - 12:33PM |
X3.00003: Ultra-High Energy Cosmic Ray Origin and Model Building in Light of new data Invited Speaker: Ultra-high energy cosmic ray detection recently made a considerable leap forward due to results of the world's current largest cosmic ray detector, the Pierre Auger Observatory in Argentina, and also due to new data from the Northern hemisphere, such as from the High Resolution Fly's Eye. As often the case when science enters new sensitivity regimes, the emerging picture also holds some surprises. For example, there is a significant correlation of highest energy events with the large scale galaxy structure on the Southern hemisphere but at the same time considerable deflection by cosmic magnetic fields would be expected if many of these events are heavy nuclei, as hinted to by the data. We will discuss how this and other open questions may be addressed by theoretical models for the origin of the most energetic particles in Nature. [Preview Abstract] |
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