Ultra High Energy Cosmic Rays - The Extreme Universe

The most energetic particles are actually not produced by LHC, but by our mother nature somewhere in the universe. They are called cosmic rays whose energy go up as high as 1E20 eV that is hundred million times higher than the energy of the proton beam at LHC. The surface of the Earth is constantly bombarded by these cosmic rays. Who accelerates these particles? Or perhaps are they messengers from the Big Bang? After a century of extensive studies of cosmic rays, we still have not identified the true origin yet.


The above figure illustrates two possibilities: Top-Down (from the Big Bang) and Bottom-Up (by a natural accelerators such as Active Galactic Nucleus (AGN) and Gamma Ray Burst (GRB). If indeed the origin turns out to be the Top-Down, the consequence is astonishing; we are getting the messenger from the Big Bang itself! The key to identity the Top-Down signature is by detecting UHE neutrinos and/or gamma rays above 1E19 eV.

Pierre Auger Observatory in Argentina

There is one caveat; unfortunately since these cosmic rays above 1E19 eV are extremely rare, a gigantic array of detectors must be built. This was the Pierre Auger international project located in Argentina, where Arisaka's group played a key role in construction, commissioning, operation and data analysis, starting in 1999.


The Pierre Auger observatory is located in Argentina on the east side of the Andes near Malargue, covering 50 km x 50 km of the Pampas. As shown above, it consists of two kinds of detectors: (1) The Shower Detectors (SD) made by 1,600 water tanks (shown as red dots, spaced by 1.5 km each), and (2) the Fluorescence Detector (FD) at four locations.


Arisaka's group, consisting of five PhD students and a senior researcher, Arun Tripathi, was in charge of testing ~5,000 of 8 inch PMTs for the Shower Detector. We developed a complete test facility (including a potting station for water proof) at UCLA in 2002, and shipped the entire facility to Malargue in Argentina where the Pierre Auger observatory was located. The above picture shows the local lab (top-left), 16 PMT testing station with Arun Tripathi (top-right), David Barnhill at the control station (bottom-left), and Federico Suarez (a local grad student) + Tohru Ohnuki (bottom-right).


The above picture shows one of the Shower Detector (SD) in front, located near the Fluorescence Detector (FD) on the hill. Near the FD, a tall antenna for communication with the SD is standing. The experiment is still going on for many years to come, although Arisaka's group has reduced activities since 2008.

Publications by Pierre Auger Collaboration

A complete list of the publications by the Pierre Auger can be found by InSPIRE => Pierre Auger. UCLA Auger group played a critical role to set the best limit on the ultra high-energy gamma ray flux. This stringent limit excluded a possibility of Wimpzilla (a messenger from the Inflation) as a dark matter. We also observed the suppression of cosmic ray flux above 40 EeV due to the GZK Cut-off. Below describes these recent results.

Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter.

By Pierre Auger Collaboration (P. Abreu et al.).
[arXiv:1009.1855 [astro-ph.HE]].
10.1016/j.astropartphys.2010.08.010. Astropart.Phys. 34 (2010) 314-326.

Upper limit on the cosmic-ray photon flux above 1E19 eV using the surface detector of the Pierre Auger Observatory and its Theoretical Implications.

By Pierre Auger Collaboration (J. Abraham et al.).
[arXiv:0712.1147 [astro-ph]].
10.1016/j.astropartphys.2008.01.003. Astropart.Phys. 29 (2008) 243-256.

Observation of the suppression of the flux of cosmic rays above 4E19 eV.

By Pierre Auger Collaboration (J. Abraham et al.).
[arXiv:0806.4302 [astro-ph]].
10.1103/PhysRevLett.101.061101. Phys.Rev.Lett. 101 (2008) 061101.

PhD Thesis by Arisaka's Group

The following five grad students have completed PhD Thesis, based on the Pierre Auger data.

Search for Ultra High Energy Cosmic Ray Anisotropy with Auger

  • Pedram Boghrat
  • 2008

The Cosmic Ray Spectrum as Measured by the Surface Detector of the Pierre Auger Observatory

  • Joong Lee
  • 2007

Composition of the Highest Energy Cosmic Rays

  • Matthew Healy
  • 2007

Composition Analysis of Ultrahigh Energy Cosmic Rays Using the Pierre Auger Observatory Surface Detector

  • David Barnhill
  • 2005

Small Scale Angular Clustering of the Highest Energy Cosmic Rays in the Surface Detector Data of the Pierre Auger Cosmic Ray Observatory

  • Tohru Ohnuki
  • 2005

Talks by Arisaka on the Origin of UHE Cosmic Rays

Below is Arisaka's talks in 2007 when Pierre Auger announced the correlation of UHE Cosmic rays with the nearby AGN.

Origin of the Most Energetic Particles in the Universe: New Results from Pierre-Auger [PDF, PPT]

  • Place : UCLA Physics Colloquium
  • Date : November 15, 2007
  • Topic : The first report on the angular correlation of UHE cosmic rays and AGN within 100 Mpc.

Origin of the Most Energetic Particles in the Universe: New Results from Pierre-Auger [PDF, PPT]

  • Place : Seminar at UC Irvine
  • Date : December 4, 2007
  • Topic : The report on the angular correlation of UHE cosmic rays and AGN plus mass composition studies.

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