Cosmic Rays and the Discovery of the Unseen Particles


By Don LincolnFermilab

With the discovery of the positron, scientists encountered the first of what ended up being hundreds of particles that it would have been impossible for anyone to see without modern technology. It also was the first, inarguable, demonstration of Einstein’s E = mc2. But it wasn’t the last. The next step was taken by Carl Anderson and his interest in studying cosmic rays.

Illustration of subatomic particle in a particle
The quest for finding new particles started with the discovery of positrons. (Image: Rick_Jo/Shuitterstock)

It Was All Fueled by Carl Anderson’s Interest

Carl Anderson was a new postdoc when he discovered the positron, but time and tide wait for no man and all that. By 1936, he was now a newly minted professor at Caltech, with his first graduate student, Seth Neddermeyer. In fact, Anderson spent his entire professional life at Caltech, undergraduate, graduate, and faculty. That would be unusual these days. He must have really liked California.

In early 1936, he was still interested in studying cosmic rays, but he realized that there was a problem: cosmic rays were created at altitudes of about 12 miles, but he was doing his experiments at nearly sea level in Pasadena, California. If he wanted to learn more about where they were produced and before all that inconvenient air got in the way, he’d have to get closer to where they were made.

He did have an option and that option was to just drive his equipment to the top of tall mountains. So, Anderson and Neddermeyer decided to drag their equipment to the top of Pikes Peak, in Colorado.

Weird Data at Pikes Peak 

Pikes Peak has an elevation of over 14,000 feet, which is an improvement of Pasadena’s of 860 feet. In any event, all’s well that ends well, and the two scientists started taking data. And what data they found! They saw way more tracks in their cloud chamber than they did at sea level. They found lots of events where electrons and positrons were made. They found lots of protons getting kicked around. 

Photograph of the first positron ever observed
Weird tracks while collecting data using the cloud chamber at Pikes Peak led Carl Anderson to win a Nobel Prize in physics. (Image: Carl D. Anderson/Public domain)

But they also found some weird tracks that didn’t make any sense at all. They didn’t know what to make of them and some of their very smart colleagues thought that they were more likely to be electrons and their seeming oddities reflected something incomplete in Dirac’s theory. Perhaps the smartest critic was Robert Oppenheimer, soon to be of atomic bomb fame. So, Anderson and Neddermeyer published their results with little fanfare. 

Anderson won the Nobel Prize in physics in 1936, which topped off a good year, but he really hadn’t appreciated just how good it was, because they hadn’t figured out what those tracks meant yet. The two of them were just mulling it over, although, of course, they did take some time for Anderson’s trip to Sweden to collect his Nobel Prize.

This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We KnowWatch it now, on Wondrium.

The Discovery of Muon

It was in the Spring of 1937 that Anderson made a trip to Harvard where he learned that two physicists there, Jabez Street and E. C. Stevenson, had data similar to that of Anderson, but that they were considering announcing the discovery of a new particle. Not wanting to be scooped, Anderson wrote a quick article to the journal Physical Review, in which he claimed discovery of the particle the existence of which Oppenheimer’s earlier disbelief had caused him to soft pedal just a year earlier. 

Anderson’s paper was published in May, with Street’s paper presented at a meeting of the American Physical Society in late April, with final submission in October 1937. A new particle was added to the particle pantheon. This new particle seemed to have a mass of about 14% that of the proton and about 208 times that of the electron. There seemed to be positive and negative versions. 

Anderson named this particle the mesotron, because mesos means middle. As more particles were discovered in this intermediate mass range, it became evident that the mesotron wasn’t alone, so the name turned into the mu-mesotron, which eventually changed to mu meson, and finally muon.

The Finding of Pions

Portrait of Seth H. Neddermeyer
Seth Neddermeyer assisted Carl Anderson. (Image: Manhattan Engineer District/Public domain)

And the muon was a big deal. It’s a particle not found in any chemist’s book. It’s not a common particle found in matter. So, there you have it. The muon had been found. But that wasn’t the only particle that scientists found. Once researchers realized that particles could be found in cosmic rays, everyone started looking. And this was the period when particle accelerators began to become fairly common. 

Particle accelerators are more convenient than studying cosmic rays, as you don’t have to climb tall mountains and freeze your butt off or fight polar bears or whatever. You can just pop down into the basement of your laboratory, flip the switch, and, voila, you have a particle-making machine. The energy of the particles in your accelerator turned into particles that nobody had ever seen before.

One such particle was eventually called the pion, short for pi meson. After considerable effort, researchers proved that it was the particle that was initially created in cosmic ray collisions high in the atmosphere when a proton hit a proton. Pions were made, which fired down into the atmosphere, decaying into muons, which Anderson and Neddermeyer eventually found. Actually, pions were definitively found using a particle accelerator and they were not found until after World War II.

Common Questions about Cosmic Rays and the Discovery of the Unseen Particles

Q: What made Carl Anderson go to the top of Pikes Peak to collect data?

His interest in studying cosmic rays made him think of going to the place closer to where the rays are created, and so he chose Pikes Peak top to conduct his research with his graduate student Seth Neddermeyer. After some time, he managed to discover the previously unseen particles, positrons.

Q: How was muon named?

This particle was initially named mesotron, which means middle. Eventually, it was renamed mu-mesotron, and later, mu meson, and ultimately, muon.

Q: How are pion created?

This particle can be created when a proton hits another proton in cosmic rays high in the atmosphere, where pions are fired down and decay into smaller muons.

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