Introduction

An elementary particle, the neutrino, is a key particle in revealing the universe and its evolution. In the Standard Model, the neutrino is a type of neutral lepton and can interact only with other particles via the weak force and has an extremely small cross section. This unique property enables neutrinos the possibility of scatter-free propagation in matter and a probe inside the matter. Another unique property of neutrinos is oscillation in flavor. A reactor is one of the intense neutrino sources and is utilized to measure the neutrino oscillation properties. This chapter reviews the fundamental properties of neutrinos and those measurements. In addition, the example of the target of the neutrino probe is stars, especially in supernovae. The core-collapse supernova is a catastrophic phenomenon at the death of stars, and it emits most of its energy as neutrinos. SN1987A is the only supernova with the observation of neutrinos from a supernova. The neutrinos bring direct information inside the supernova. While there is a limited chance to observe supernova bursts, the diffused neutrinos from distant supernovae exist in the universe. This is called the Diffuse Supernova Neutrino Background (DSNB) or Supernova Relic Neutrino (SRN) and has not yet been discovered. The observation of DSNB is one of the milestones for revealing the supernova mechanism and the star formation history. The review of DSNB is also described in this chapter.