Particles that rarely leave signals
Neutrinos carry no electric charge and rarely interact with ordinary material. Many pass through substantial amounts of matter without producing a detectable event. Experiments therefore focus on rare interactions. This weak detectability does not make neutrinos identical to photons or mean they have no physical effects.
Three flavours and oscillation
The three known flavours are associated with the electron, muon and tau. Detection probabilities can change during propagation, a phenomenon called neutrino oscillation. Flavour and mass states differ. Oscillation establishes that neutrinos are not all massless without directly fixing every individual mass value.
Sources for experiments
Nuclear processes in the Sun, radioactive decay, reactors and accelerators provide neutrinos or antineutrinos. Their energy distributions and composition differ, enabling complementary tests. These sources should not be collapsed into one universal production mechanism, because different reactions and decays contribute in different settings.
References: [1]
Questions beyond discovery
The 2015 Nobel Prize recognized the discovery of neutrino oscillations. Absolute masses, the relationship between neutrinos and their antiparticles, and possible roles in matter–antimatter asymmetry remain research questions. Oscillation is well established, while these broader questions remain under experimental investigation.