UVa Physics Review: Better Know a Particle: The Pion

Yukawa proposed a theory of strong interactions involving a "meson mediator particle" in 1935. The following year the physics community thought they had discovered the particle but in fact they have found the mu meson. Eventually in 1947 experiments conducted in the Pyrenees and the Andes mountain ranges discovered this elusive particle which have come to know as the pion.

The reason for the mountains was simply that high energy particle accelerators were not a common tool for physics yet. The Cosmotron was not powered up until 1953 and the Bevatron wouldn't see beam till the following year. So physicists had to hike up a mountain in the hopes of seeing a pion created by cosmic collisions in the atmosphere.

Jumping forward a few decades we now know plenty about the pion. There are three flavours: $\pi^+, \pi^0, \mathrm{and\,\,} \pi^-$ . With identical masses for the charged flavours of approximately 140 MeV/c/c and slightly lower mass for the neutral flavour. The charged pions are each other's antiparticles but the discussion of the neutral pion's antiparticle is best left to another post. Pions are bound states of the lightest quarks, and as mesons are a quark-antiquark pair ( $\pi^+ \sim u\bar{d}$ ).

Finally pions are unstable particles. The neutral pion decays electromagnetically into two photons with a mean lifetime around $10^{-16}\mathrm{\,s}$ . The decay of the charged pion is very interesting and follows the form $\pi^{\pm}\rightarrow\mu^{\pm}\nu_{\mu}$ where the neutrino has opposite lepton number of the muon. Not only is this decay a weak decay and thus on a much slower timescale (26ns) but naively one would expect $\pi^{\pm}\rightarrow \mathrm{e}^{\pm}\nu_{\mathrm{e}}$ to be a more common decay mode due to larger phase space. The topic of why the muon channel dominates is again a topic for another post.