What is a neutrino?
A neutrino is the electron's partner under the weak force, a particle with no electric charge and no color, the strong force's charge. Matter here is built by picking slots out of five, three for color and two for the weak force. One pick makes a pair that the weak force can rotate into each other, the electron at charge minus one and the neutrino at zero. Each family of matter has its own. Every decaying muon leaves an electron and two neutrinos behind. In the 1990s, four detectors at CERN counted how often the Z particle decays into things nobody sees, which is neutrinos, and found 2.984 ± 0.008 kinds of light neutrino, three to within a percent. Neutrino masses need a mechanism beyond that minimal list, which is work in progress.
Also in Chapter 28: What Is a Particle, What Is a Wave, and Why Does It Look Like Both?, Chapter 30: Why Do the Masses Land Where They Do?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 27: Why This Exact List of Matter, and Why Three Times Over?watch a universe make its own particles