The hunt for the muon neutrino
In order to understand how the weak force works and to explore the possible existence of two neutrinos, it was necessary to study neutrino interactions at high energies. Leon Lederman, Melvin Schwartz and Jack Steinberger were working to achieve a sufficiently intense neutrino beam for several years. This neutrino beam was produced in several steps:
- Protons were accelerated to an energy of 15 GeV (1 GeV = 109 eV) in the Brookhaven accelerator AGS (Alternating Gradient Synchrotron) on Long Island, USA.
- The intense proton beam was directed onto a target of beryllium. In each collision a handful of particles were produced, mainly pi-mesons.
- The many pi-meson decaysresulted in a collimated beam of muons and neutrinos.
- The neutrinos, the muons and the surviving pi-mesons crashed into a 13 m thick steel shield, which stopped all particles except the neutrinos.
The steel shield, which
was made of armour plates from scrapped warships, was
essential for the experiment. All particles, except
the neutrinos, had to be prevented from reaching the
detector, as they could give rise to a large number
of particle reactions and completely swamp the
incredibly few neutrino reactions. The pi-mesons,
which interact through the strong force, were stopped
after less than half a metre of armour plates, while
the muons, which interact mainly through the
electromagnetic force, penetrated much further. The
steel shield, which had a weight of about 2000 tons,
was dimensioned to stop the penetrating muons.
The muon neutrino was discovered!
The experiment of the Nobel
Prize winners not only showed that the muon and the
muon neutrino form a pair, it also showed that the
other two leptons, the electron and the electron
neutrino, form a pair. The grouping in pairs of
elementary particles is fundamental in particle
physics and became even more significant after the
establishment of the so called standard model.