Neutrinos are among the most poorly understood particles of the Standard Model.
Their existence was first postulated by Wolfgang Pauli to explain the properties of β decays.
In a theory with just nucleons (neutrons and protons) and electrons, β decay proceeds as
n → p+ + e-
If the neutron is at rest, conservation of momentum and energy require that nearly all the energy go to the electron.
Both the proton and electron will show a monoenergetic energy spectrum.
In reality, a range of energies is measured, with the electron spectrum showing a maximum (endpoint) energy where the monoenergetic peak should be.
To explain this, Pauli proposed the existence of a third particle that participates in the interaction:
n → p+ + e- + ν̅e
This third particle does not interact much, so it can carry momentum and energy away from the decay without being detected.
It must be electrically neutral to satisfy conservation of charge and nearly massless to maintain the endpoint energy.
Hence, the neutrino (little neutral thing) was created.
It was several decades before neutrinos were finally definitively discovered by searching for their emission from a nuclear reactor at Savannah River.
In the Standard Model, neutrinos only interact through the weak interactions.
They are realized as parts of weak isospin doublets along with the left-handed charged leptons.
The weak SU(2) gauge symmetry of the Standard Model creates interactions between these doublets and the W bosons.
Interactions where a neutrino is transformed into a charged lepton (or vice versa) are known as charge-current interactions because charge is transferred between the different interacting particles.
For example, we can look at a charge-current quasi-elastic (CCQE) reaction of a neutrino scattering off of a neutron:
n + νe → p+ + e-
The interaction with the W boson allows the electron neutrino to be converted into an electron.
The neutron is converted into a proton to maintain charge conservation, with the charge carried by the W (quarks have their own weak isospin doublets).
Interactions with the Z boson (the third boson associated with the broken weak SU(2) symmetry) allow for things such as elastic scattering of neutrinos, neutrino pair production, and neutrino-antineutrino annihilation.