Dr. Brian Fick Professor PhD, Virginia Polytechnic Institute and State University Experimental Astro-Particle Physics Office: Dow 302 Phone: 906.487.3212 Fax: 906.487.2933

The Pierre Auger Cosmic Ray Observatory Auger Observatory at Michigan Tech Fick Lab: Fisher 005B Lab Phone: 906.487.1795 Email: fick@mtu.edu

Research Interests

Where do the highest energy cosmic rays come from? How does Nature endow nano-scopic particles with such macro-scopic energies? What kind of astrophysical or cosmological process can accelerate or create such particles? The phenomenon of Ultra High Energy Cosmic Rays is the single largest departure from thermal equilibrium of anything anyone has ever seen in the Universe!

Workers at MTU are trying to find the answers to these questions using the Pierre Auger Cosmic Ray Observatory. We (Profs Fick and Nitz) have contributed to the design, construction, and operation of this powerful new scientific instrument. The group at MTU is responsible for the Auger surface detector trigger electronics, atmospheric monitoring equipment, and hybrid reconstruction software. We are also heavily involved in analyzing the incoming data.

Primary cosmic rays (particles from space) must interact with the earth's atmosphere to be seen. An extensive shower of secondary particles is produced for every incoming primary cosmic ray. Ground based cosmic ray observatories observe the passage of this extensive shower of particles using two complementary types of detector. The first records the passage of relativistic charged particles using an array of radiation detectors at ground level. We refer to this method as a surface detector or SD (Figure 1). The second type of detector images the uv light emitted as shower particles interact with the nitrogen of the atmosphere. This technique, which only works on clear moonless nights is referred to as a fluorescence detector or FD (Figure 2). PAO is unique as it employs both techniques to make simultaneous and complementary measurements of cosmic rays.

Ground based cosmic ray observatories can only hope to measure three aspects of primary cosmic rays. They can determine the energy spectrum, the arrival direction distribution, and the general composition of cosmic rays. All scientific questions about the highest energy cosmic rays must be answered using this information alone. The highest energy cosmic rays are also very rare. In one year, one expects to see one particle of interest in a square kilometer! An enormous detector is required to make any scientific progress. The PAO, when completed will have an active area of 3000 square kilometers. This is about 10 times the size of previous observatories working in this field.

In the past year we have begun to analyze data from the observatory as it is being constructed. The instrument is approximately half-completed, but already equals the sensitivity of previous projects in this field. We have produced a first cosmic ray energy spectrum above 3 EeV (Figure 3) . (1 EeV = 10¹⁸ eV ~ 0.1 Joules) . The data set used is about equivalent to that used by previous, smaller experiments. This spectrum is the first to be constructed from SD data which has been calibrated by the more reliable FD. The general shape of the spectrum agrees with previous work. But it also shares the weakness of low statistics at the highest energies. At present it is impossible to determine whether the spectrum falls off steeply or continues at the highest energies. A steep fall-off would be expected when high energy protons (E > 60 EeV) produced uniformly throughout the Universe interact with the all-pervasive microwave background radiation. A continuing spectrum would indicate that the bulk of the highest energy cosmic rays must have come from distances closer than about 300 million light years. A definitive distinction between models will have to wait for the completion of the PAO in about a year.