Introduction

natural units:

• MeV
• fm (1 fm = $10^{-15}$ m)
• barn (1 b = 10$^{-28}$ m$^2$)
• $\hbar = c = 1$
• mass, energy and momentum have the same units
• $\hbar c = 197.3$ MeV fm
• $(\hbar c)^2 = 0.3894$ GeV$^2$ mb

fine structure constant $\alpha=\frac{e^2}{4\pi \hbar c}$. Defining a 'strong nuclear charge' $\alpha_s \sim 1$ at a distance of a few fermi.

The scale of weak interactions is given by the Fermi constant $G_F/(\hbar c)^3 = 1.166 \times 10^{-5} GeV^{-2}$.

1. the muon by Anderson and Neddermeyer in 1936 using a cloud chamber within a magnetic field;
2. the pion in 1947 by Powell’s group in Bristol using specially developed photographic emulsion;
3. 'strange particles' by Rochester and Butler (1946–47) using a coincidence-counter-controlled cloud chamber. a much longer lifetime than would be expected for a ‘normal’ strongly interacting particle with a comparable mass.
4. A new quantum number, strangeness, Gell-Mann and Nishijima in 1953
5. early 1950, synchrotrons with GeV energy