# Tag Info

Neutrinos are light, uncharged leptons. The neutrino tag should be applied to question relating to neutrino properties or interactions involving neutrinos.

Neutrinos are produced in nuclear reaction involving the weak force. Sources that are useful for experimental efforts include the sun (matter type, electron flavored neutrinos), nuclear fission reactors (anti-matter type, electron flavored neutrinos), the interactions of cosmic rays with the atmosphere and the interactions of man-made particle beams with matter (both matter and anti-matter, and all flavors)

Having neither charge nor color, neutrinos interact only by way of gravity and the weak nuclear force. Both of these forces are, well, weak and the neutrinos have relatively low cross-section for interactions with ordinary matter.

## Flavor?

Both the charged and the un-charged leptons come in three type which seem to be identical except for mass. The charged leptons are the electron, the muon, and the tau-lepton (often just called "a tau"). For each of these there is a corresponding neutrino, but see the section on mixing below.

## Brief History

A light uncharged particle was first proposed in 1930 by Wolfgang Pauli to solve the problem of the beta decay spectrum. Pauli called his particle a "neutron", but that name was later adopted for the uncharged nucleon. The name "neutrino" (meaning "little neutral one") was coined by Enrico Fermi in 1934. Neutrinos were originally modeled as massless for simplicity and in the absence of any measurable mass the assumption was adopted as a given. Neutrinos (actually anti-neutrinos) from a fission reactor were first detected experimentally in 1956 by Cowan and Reines using a delay coincidence technique that remains the standard for reactor neutrinos to this day.

Starting in 1970 Raymond Davis Jr., Kenneth C. Hoﬀman and Don S. Harmer tried to measure the solar neutrino flux using a large tank full of cleaning fluid placed deep in the Homestake mine in South Dakota. They got a figure too low to match theories of stellar structure. This mis-match persisted for two decades, and required a change of theory to resolve: the neutrinos must be considered as massive (albeit light) and allowed to mix.

Experiments at Sudbury Canada, the Kamioka mine facility in Japan, various nuclear reactor complexes, and at several accelerator sites around the world would eventually show clear evidence of neutrino mixing.

Current efforts are focused on determining the parameters of the mixing matrix (two mass differences and all three mixing angles are known), searching for evidence of CP violation in the neutrino sector, and determining if the neutrinos are Dirac or Majorana particles.

## Mixing

Mixing occurs because the flavor states of the neutrinos, written $\nu_e, \nu_\mu, \nu_\tau$ are not eigenstates of the free Hamiltonian. Those are called the "mass states" and are written $\nu_1, \nu_2, \nu_3$. In a mixing experiment, (anit-)neutrinos are produced in one location (production occurs in flavor state) and allowed to propagate to another location where they are detected (again, detection is of flavor states). During the time the neutrinos travel, they are acted upon by the free Hamiltonian which does not keep pure flavor state pure---that is, it mixes them. The result is that the distribution of flavor states detected may not match the distribution of flavor states created.

Mixing was actually proposed by Gribov and Pontecorvo in 1968 (even before the Homestake experiment). [Phys. Lett, 28B, vol. 7, p. 493]

## Open Questions

• Measure the remaining parameter of the mixing ($\delta_{CP}$) and refine the values of the known parameters.
• Does neutrino mixing violate CP (i.e. is $\delta_{CP} \ne 0$?)
• Mass hierarchy problem.
• Dirac of Majorana nature?
• Are there additional neutrinos states (either heavy weakly interacting neutrinos or sterile neutrinos)?
• What is up with the new result from OPERA? Do they really go faster than light? This appears to be solved, and Einstein is still right.