Pitt Physics REU | Cosmic Rays

University of Pittsburgh
Research Experience for
Undergraduates in Physics
Focus On Minorities

Ultra High Energy Cosmic Rays

Cosmic Rays are the name given to high-speed subatomic particles (for instance, neutrons, bare protons, or nuclei of light elements) or high-energy electromagnetic radiation (i.e., photons, and thus also particles) that impinge upon Earth from space.

Cosmic rays were discovered around the beginning of the 1900s: Attempting to perform experiments in an environment free of the known radiation that comes from within the earth (as had then just recently been discovered), physicists took electroscopes high off the ground (for instance, Theodor Wulf took an electroscope to the top of the Eiffel Tower), and to their surprise, continued to observe electroscope discharge. Physicists such as Albert Gockel and then Victor Hess took electroscopes up in hot-air balloons, and found that the rate of electroscope discharge actually increased with altitude, thus conclusively discovering what Robert Milikan (who researched the matter extensively) termed Cosmic Rays (Hess shared the 1936 Nobel Prize for this discovery.

Here is a photo of Hess before one of his ballon flights.
Here is another copy of the same photo.
And here is a similar brief history of cosmic rays, including a breakdown of the relative particle constituents of cosmic rays, courtesy of the High Resolution Fly's Eye project at Utah.

Many such particles are given off by the Sun, but many also originate from outside the solar system, and even outside our Galaxy. Most have relatively low energies, but a few have energies on the order of 10²⁰ electron volts (by way of comparison, this is nearly a billion times higher than the energies achieved in man-made particle accelerators). The first of these rare, ultra-high-energy cosmic rays was observed in 1961, at the MIT "Volcano Ranch" research station near Albuquerque, New Mexico (nicknamed at the time "Desert Queen"--you can see a vintage picture of a portion of the site here). But in 1991, at the so-called "Fly's Eye" detector in Utah, a cosmic ray was detected having an energy of 3x10²⁰ eV, by far the most energetic cosmic ray ever detected, and a startling event in physics. In 1994, the AGASA group in Japan detected a cosmic ray nearly as energetic, with an energy of 2x10²⁰ eV.

These ultra high energy cosmic rays can not currently be explained. When physicists look into space in the direction from which the ultra-high energy rays appear to have come, nothing unusual (like perhaps a supernova or pulsar or other exotic astronomical phenomenon) is observed that is known to possibly accelerate a particle to such energies. Furthermore, whatever the mechanism, it is generally assumed that it must be something exotic, and outside our own Milky Way galaxy. But this raises another difficulty, because the universe is known to be permeated with low-energy background radiation (in the microwave region of the EM spectrum), left over from the Big Bang. It was pointed out by Kenneth Greisen, Georgi Zatsepin, and Vadem Kuzmin that, by quantum mechanical mechanism, high-energy subatomic particles would interact with these background photons. The interaction would reduce the cosmic ray's energy, so that particles traveling long intergalactic distances should not have energies greater than 5x10¹⁹ eV when they reach the earth (this value is known as the GZK cutoff). So observations like the one made in 1991 are a real mystery for physics. It is interesting to note that the significance of the 1961 event was not realized until after the discovery of the cosmic background radiation in 1965.

Here is a description of the 1991 event, from Adelaide University.
Here is an enthusiastic description of the 1991 event, from physicist John Walker.
"The mystery of Ultra High Energy Cosmic Rays"
20-minute long multimedia presentation, from the "Astrophysics Science Project, Integrating Research and Education" (ASPIRE). Available in several formats. (click on bottom right side, where it says "watch movie").

Several theories propose ways that highly energetic cosmic rays might originate from within (or at least "near") our own galaxy, which would then not violate the GZK cutoff. These theories range from shock waves of supernovas, to acceleration effects from magnetic fields in space, all the way to speculation of the existence of new exotic particles or even geometric "defects" in spacetime.

Here is some discussion of "traditional" (back to Fermi) theories of possible acceleration mechanisms, problems with those theories, and some more recent speculations. Courtesy of the High Resolution Fly's Eye project in Utah.
Here is story from year 2000 about possible acceleration effects from magnetic fields in space.
Here is a story from 2002 about ultra-high energy cosmic rays possibly originating from the black holes at the center of nearby galaxies; here is another writeup on the same theory.
Here is a story from 1999 about possible acceleration effects due to supernova shock waves.
Here is a 1997 Scientific American article on the topic, listing some of the more speculative explanations. (Note that many of the links within the story no longer work.)
Here is an article from 2002 describing an explanation for cosmic rays based on a violation of Relativity theory. (You can read some discussion of the article here, archived from the sci.physics.research discussion group archive.)
The various hypotheses regarding ultra high energy cosmic rays should not be confused with this relatively new finding that other, lower-energy cosmic rays, called anomalous for a different reason, may arise from ionization of dust within our own solar system.

Cosmic ray particles impinging upon the Earth strike particles of the upper atmosphere, causing the primary cosmic ray to decay into other, less-energetic subatomic particles, which then in turn decay into other particles (so that what actually hits the ground is not the original single particle). Such behavior is called an "air shower."

Here is a description of this effect, and a cartoon representation of an air shower.
At this link is a schematic representation of a typical cosmic ray particle cascade.
Here is yet another artist's rendering of an air shower.
Here is a nice representation of an air shower, illustrating the many different types of detectors that can be used. Includes brief explanation at bottom.
This page, courtesy of the High Res Fly's Eye project in Utah, discusses air showers and their detection in slightly more detail.

To accurately reconstruct the energy and trajectory of the original particle, several (ideally, as many as possible) of the final particles should be detected. But due to quantum mechanical effects, the higher the energy of the original incoming cosmic ray, the wider the area over which the resulting particle cascade gets spread. For the highest energy cosmic rays, the area of the spread can be on the order of hundreds of square kilometers at ground level. This makes it more difficult to detect enough final shower products with a single detector. One relatively recent novel solution is to link together many small detectors placed on rooftops, thus effectively creating one very large detector. Several groups, including Pitt, have formed various collaborations whose goals are to place such detector arrays on the roofs of high schools, thus solving the original problem while at the same time getting high school students and teachers directly involved in leading research.

North American Large-scale Time-coincidence Array (NALTA)
The collaboration of various groups, of which Pitt Physics is affiliated, whose goal is to construct wide-area arrays of detectors on school rooftops. The page includes:
- Description of air showers,
  
  and
- how the rooftop detectors are invisioned to work.

More cosmic ray info:

Timeline of the History of Cosmic Rays
From the Pierre Auger Observatory in Argentina.
Cosmic Rays by NASA
A relatively recent (December 2002) summary of the same basic information as on this page; and listing some of the more speculative theories of the origin of the high energy rays, and mentioning the (as of this writing) soon to be completed Pierre Auger Cosmic Ray Observatory.

A short page on cosmic rays
from the National Oceanic and Atmospheric Adminstration.
A page on Galactic Cosmic Rays
by James Schombert of U. Oregon. The page describes the deflection of cosmic rays due to the earth's magnetic field; and discusses in some detail the analysis of cosmic ray composition. NOTE: Be aware of the missing exponent notation in several places; so for instance, where it reads "1020 eV", he actually means "ten TO THE twentieth electron Volts."
Probing the Proton
A history of proton investigation, beginning with the story of cosmic rays, and containing some nice description of the devices used to view subatomic particles. From Stanford Linear Accelerator.
"Cosmic Ray" entry from the MacMillan Encyclopedia of Physics
A brief introduction to the topic, from 1996; similar to the information on this page.
Cosmic Ray links
an extensive list of links, from the Max Plank Institute

last updated on 21 January 2003 by Gordon Weinberg.
All information subject to change without appearance here.

this page: http://www.phyast.pitt.edu/~reupfom/cosmic-rays.html
main REU page: http://www.phyast.pitt.edu/~reupfom

Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors
and do not necessarily reflect the views of the NSF.
This program is supported by NSF grant #9987904.