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How exactly was the Oh-My-God particle (ultra-high energy cosmic ray) observed and its energy measured?

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had you followed the link in your link you would get a detailed "how"… – anna v Nov 28 '12 at 4:54
Wow this was one hard paper to find. I was extremely skeptical of this initially and thought it was a hoax because the Wikipedia article cites "J. Walker (January 4, 1994). "The Oh-My-God Particle"", of Fourmilab (er...) who in turn cited "Star Trek: The Next Generation Technical Manual"... it all seemed pretty fishy to me. – nervxxx Mar 26 '13 at 5:11
Also… – Pacerier Jul 31 '15 at 10:39
up vote 9 down vote accepted

The OMG particle was observed by the Fly's Eye experiment located on Dugway Proving Ground in Utah. The Fly's Eye experiment was the first experiment to successfully employ the air fluorescence technique developed by Dr. Alan Bunner (Cosmic Ray Detection by Atmospheric Fluorescence, Ph. D. Thesis, Cornell University, 1967).

The air fluorescence technique observes light emitted isotropically by nitrogen molecules in the atmosphere. The nitrogen molecules are excited by the passage of charged particles in the extensive air shower initiated when the cosmic ray particle collided with a nucleus in the upper atmosphere. This light is not Cherenkov radiation.

Using simple arguments it can be shown that the total amount of light emitted at the peak of the extensive air shower is proportional to the energy of the primary cosmic ray particle. Measurement of this light is used to provide calorimetric determination of the energy of the primary particle. This is one of the strengths of the air fluorescence technique. Unlike ground array measurements, the air fluorescence measurement does not depend on detailed physics models or simulations.

This is one reason that the Pierre Auger experiment and it's counterpart in the northern hemisphere, the Telescope Array Project, both depend upon air fluorescence telescopes to calibrate and cross check the measurements of their ground array detectors.

In addition, air fluorescence observes the extensive extensive air shower development in the atmosphere. The ground based detectors can only sample the particle shower at a few discrete points on the surface of the earth.

The primary drawback of the air fluorescence technique is that it can only be used on moonless nights with good atmospheric conditions while ground arrays operate 24 hours and 7 days a week.

I should mention that I am an author on the OMG particle paper.

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Ultra-high energy cosmic rays create enormous cascades of charged particles as they and interact and re-interact and re-re-interact in the atmosphere. This generates a lot of Cerenkov radiation and nitrogen fluorescence in the atmosphere and many of the charged particles reach the ground in a cone that may be miles wide.

So you build an array of ground station (typically water tanks a meter or two on a side instrumented with PMTs) for detecting ionizing radiation and a number of fluorescence telescope pointed at the sky over the array. The premiere example of such a facility is the Pierre Auger Observatory

Then you wait for a whopping big coincidence.

It also takes a lot of careful work and Monte Carlo simulation to tune the energy estimation from the results.

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But how traceable are these jets, especially if a major portion of the particles produced do not reach any detector? (Also, what does PMT stand for?) – Benji Remez Nov 28 '12 at 5:17
PMT = Photo-multiplier tube (a very sensitive light detector). The ground array only samples the event, but for these big events the sampling is statistical (there are many hundreds or thousands of surface hits) so you know about what fraction of the total you measured (at least if the whole event lands inside the instrumented ground). Similarly the telescopes only see a small fraction of the total light, but you know something about the geometry so you can take a pretty good guess at how much there was. – dmckee Nov 28 '12 at 5:24
And you do a lot of simulation, and compare with the result from many other experiments. Especially for the much more common events in the $10^{12}$--$10^{16}\text{ eV}$ range (which is pretty well understood). That gives you some confidence in your tools when it comes time to estimate the true energy of the big ones. – dmckee Nov 28 '12 at 5:26

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