Puzzle of neutrino oscillations

Abstract

The 20th century was a thriving era for many fields in sciences, including particle physics with the establishment of the Standard Model. When the Standard Model was first proposed, neutrinos were supposed to appear as three flavours corresponding to the lepton produced when the neutrino interacts and having no mass. However late 1960s, scientists were confronted to the solar neutrino problem and their conception of neutrinos had to change. The long journey up to the discovery of neutrino oscillations in solar and atmospheric neutrinos -which enables one flavour to change into another during its course over time- imposes those particles have a mass. Consequently neutrinos have been extensively studied on the one hand to determine their properties (i.e. to put constraints on their mass, to know how they interact with matter, know if they are their own antiparticle,...) and on the other hand to study their oscillations and constraint the allowed range of parameter values. There are six parameters: two squared mass differences, three angles and at least one phase. The formalism was well established and the two mass differences allowed correspond to the mass difference observed in solar neutrino oscillations and to the mass difference observed in atmospheric neutrino oscillations. By 2000, the situation was much clearer. However, some experiments reported unexpected parameter values, challenging our comprehension of neutrino oscillations. In total three types of anomalies are reported; the LSND, the Gallium and the reactor antineutrino anomaly. We will focus here on the first one, which was an anomaly initially reported by the Liquid Scintillator Neutrino Detector experiment in 1995 and was confirmed in 2009 by the MiniBooNE experiment. These new parameter values do not fit in the actual scheme of three neutrino mixing and may imply new physics.

In this work, we will start with an introduction to neutrino oscillations in chapter 1, where we will then present the formalism of three neutrino mixing as understood nowadays. In chapter 2 we will describe the MiniBooNE experiment and review its results. Next, in chapter 3, we will derive the deviation of the observed number of events compared to the expected one to understand better the importance of the anomaly. We will also derive the oscillation parameter values with this new set of data to see how these insert into the actual scheme. We will continue with a discussion about the currently most investigated solution; the existence of additional sterile neutrinos whose further investigation experiments are presented in chapter 4.