Doug Bonderud

Sep 9th 2020

Particle Physics Pion-eers: Mastering the Meson


Particle physics gets really strange, really fast. Everything started off simply enough with the discovery of electrons in 1897; add in neutrons and protons, and you’ve got the atom everyone knows and loves from chemistry class. Photons came next, then the revelation that antimatter electrons — positrons — were also possible.

But the particle party was just getting started. Work by Ernest Rutherford in 1907 highlighted a concentration of mass in the atom’s nucleus, but this necessitated a new particle — called the meson by Hideki Yukawa — responsible for carrying the nuclear strong force capable of holding nuclei together. While Yukawa proposed his meson in 1937, discovery didn’t come until 1947 when experiments by Cecil Powell uncovered yet another new particle: the pion.

But what exactly is a pion? What role does it play in particle physics? And how does it impact our understanding of small-scale interactions?

Case of the Missing Meson

While rapid scientific advancement in particle interactions, combinations and limitations produced substantive insight, it also gave rise to unique issues. As more sophisticated measurement tools and mathematical frameworks were developed, new challenges emerged. For example, to account for energy conservation in beta decay — in which an electron is emitted from a particle nucleus — Wolfgang Pauli proposed the neutrino in 1930. In 1956, these particles were detected at the University of California at Irvine.

Hideki Yukawa also saw a potential particle problem: No strong nuclear force carriers existed under current models. So in 1935, he postulated the meson — a middleweight particle with a mass somewhere between protons and electrons — that could account for the strong force binding between neutrons and protons. Using the radius of atomic nuclei as a guide, Yukawa inferred the range of the strong nuclear force and derived a potential mass for his meson: 100 MeV/c2. And in 1936, the muon (mu meson) was discovered with a mass that was very close to Yukawa’s calculation at 106 MeV/c2.

But an issue quickly emerged: Further research found that muons weren’t team players when it came to strong nuclear interaction. So where was the missing meson?

The Pionic Man

In 1947, Cecil Powell was working with high-energy cosmic rays to isolate subatomic particles at the University of Bristol along with Cesar Lattes and Giuseppe Occhialini. Without the benefit of modern-day particle accelerators, Powell leveraged a more hands-on approach by carrying specially-designed photographic plates to high altitudes on Pic-du-Midi mountain in the Pyrenees. Cosmic rays striking the plates created microscopic particle “trails” that became apparent once the plates were developed.

On March 7, 1947, one of Powell’s assistants found evidence of a slow-moving meson that stopped briefly and then emitted a second meson. Another emission was found the next day, indicating that the original meson had decayed, creating one new, lighter meson and a neutral particle. Upon closer inspector, this lighter meson matched the characteristics of the muon discovered 10 years earlier. The original, heavier particle was Yukawa’s elusive meson.

Named the pion (pi meson), this particle contains one quark and one antiquark and accounts for most of the mass in atomic nuclei. More importantly, pions are responsible for the residual strong nuclear force between protons and neutrons, in turn making it possible for nuclei to exist at all. There are three types of pions: positive, negative and neutral. Both charged pions decay in 26.033 nanoseconds (2.6003 x 10-8 seconds), while their neutral counterpart decays in just 84 attoseconds (8.4 x 10-17 seconds).

Particle Potential

Prior to the discovery of mesons, muons and pions, Yukawa published a 1935 paper that helped address critical issues within particle physics. Called the Yukawa potential, it takes the form:

{\displaystyle V_{\text{Yukawa}}(r)=-g^{2}{\frac {e^{-\alpha mr}}{r}},}

The goal of Yukawa’s work was to reconcile the results of James Chadwick’s tightly-packed atomic nucleus model with the impact of electromagnetic force — at such small distances, particles inside the nucleus should be pushed apart, not pulled together. Werner Heisenberg postulated that neutrons were in fact combinations of protons and electrons; when these “composite” neutrons emitted electrons, an attractive force was created that helped secure the atom’s structure. Further investigation showed several potential problems, including the impossibility of an electron of spin 1/2 and a proton of spin 1/2 to equal a neutron spin of 1/2.

Heisenberg’s work inspired Enrico Fermi’s ideas of beta decay with the emission and absorption of neutrinos and electrons. While this solved the spin problem, the force produced by beta decay wasn’t sufficient for atomic binding. Yukawa combined both theories to fix the issue by recognizing and articulating the key role of an “exchange particle” — the meson. Using his equation, Yukawa deduced the mass of this missing particle. And 12 years later, he was proven right: Pions were the linchpin in atomic links.

Elementary Evolution

As new particles and potential interactions emerge, real-life applications — from pinpoint navigation accuracy to the creation of custom states of quantum light — are evolving. But these cutting-edge efforts owe their existence to the tireless work of scientists like Cecil Powell and Hideki Yukawa who helped solve some of the most pressing problems in particle physics by accurately predicting — and empirically verifying — the presence and potential of pions.