Setup of a Drift Tube Muon Tracker and Calibration of Muon ...

Chapter 2NeutrinosThe standard model of particle physics comprises only a few elementary particles:six quarks and six leptons as well as their anti particles plus the gauge bosonsmediating the forces between them. Out of these particles, the neutrinos are of amost interesting kind. They account for three of the leptons (ν e , ν µ , ν τ ), they areuncharged, each being a partner of one of the remaining—charged—leptons (e − , µ − ,τ − ). Originally assumed to be massless, it has recently been shown that neutrinoshave a tiny mass indeed. Although the standard model assumes a strict leptonnumber conservation within each family, neutrinos have the ability to change intoone another when traveling distances. This already shows that neutrinos offer alarge potential for physics beyond the standard model. Neutrinos hardly interactwith matter, making it hard to detect them on the one hand, but on the other handalso offering a great opportunity to use them as messenger particles. Their abilityto reach us from any source without any significant disturbance on their way makesit possible to use them as a probe of whatever they originate from.The first section of this chapter will give a brief introduction to the historyof neutrino physics. This will be followed by a look at neutrino oscillations andthe influence of matter on this effect. A particular look on solar neutrinos is thenpresented, followed by an overview of other neutrinos sources.2.1 A Brief History of Neutrino PhysicsThe neutrino was first postulated by Wolfgang Pauli in 1930 [1]. To explain thecontinuous energy spectrum of electrons in β decays as well as to maintain angularmomentum conservation, he assumed a neutral, invisible particle nowadays knownas the electron neutrino ν e to take part in the process.It was not until 1956 that the neutrino could actually be observed by FrederickReines and Clyde L. Cowan in an experiment called “Project Poltergeist” [2]. Atthe Savannah River reactor in the US state of South Carolina, they succeeded toobserve the inverse beta decay:¯ν e + p → e + + n.Reines and Cowan arranged to detect the γ-emission following the capture of theneutron in a nucleus, which together with two photons from the annihilation of thepositron with an electron gives a distinct signature for the process.5