# What are neutrinos? Where are they found?

May 30, 2016

Neutrinos are small neutral particles weakly interacting particle with very little mass. They are classified as leptons a subgroup of fermions. There are three "flavours" of neutrinos, electron, muon and tau neutrinos, each of these flavours has a particle and an antiparticle, so there are six kinds of neutrinos in total.

Neutrinos are everywhere, following through you me, the earth, but they don't interact much. The best place in the solar system to look for neutrinos is flowing out of the sun as they are created by nuclear fusion processes in its core.

#### Explanation:

Neutrinos are small neutral particles weakly interacting particle with very little mass. They occur everywhere, flowing through you and me and the earth, but they don't interact much.

They are classified as leptons a subgroup of fermions. There are three "flavours" of neutrinos, electron, muon and tau neutrinos, each of these flavours has a particle and an antiparticle, so there are six kinds of neutrinos in total.

The six neutrinos have the following symbols (base on the small Greek letter "nu")

${\nu}_{e}$ electron neutrino lepton number 1
${\overline{\nu}}_{e}$ anti-electron neutrino lepton number -1

${\nu}_{\mu}$ muon neutrino lepton number 1
${\overline{\nu}}_{\mu}$ anti-muon neutrino lepton number -1

${\nu}_{\tau}$ tau neutrino lepton number 1
${\overline{\nu}}_{\tau}$ anti-tau neutrino lepton number -1

Because Neutrinos are neutral leptons they only interact via the weak force and gravity.

Interactions

Gravity: Neutrinos are effected by gravity, but they have such little mass the effect is quite small.

Weak: This is the primary way material effects neutrinos in the standard model. This means that neutrinos are involved in certain nuclear reactions. To a student they appear or disappear when a electron (or muon or tau particle) appear or disappears. This is because lepton number is conserved. Neutrinos occur in beta decay and related processes, some fission and fusion reactions and scatter with electrons (theoretically muons and tau particles too)

They are most associated with beta decay, where a neutron decays into a proton, an electron and ... a neutrino. The neutrino was discovers when it was realized that momentum was not conversed in beta decay (as measured at the time) when seeing a neutron break up into an electron and a proton, it was realized there was another small neutral particle not accounted for, the name "neutrino" essentially means "little neutral one."

Non-Interactions

Electromagnetism: They aren't effected by the electromagnetic force, this means that they aren't attracted or repulsed by charged particles, nor do they interact directly with any magnetic field and photons (light) does not effect them.

Strong: They aren't effected by the strong nuclear force, this means that they are not bound to the nucleus.

Taken together means that should a neutrino be generated in an atom due to a weak interaction it will leave it quite readily.

Mass/Matter oscillations/solar neutrino problem.

Neutrino's are produced in three nuclear reactions which occur in the sun's core, most predominately in the proton proton reaction to starts fusion.

${p}^{+} + {p}^{+} \to {d}^{+} + {e}^{-} + {\nu}_{e}$

By counting neutrinos we could measure nuclear fusion in the sun by measuring the electron neutrinos!

But we found too few neutrinos, but only 1/3 of the expected number was found! It was realized that neutrinos had mass and that this allowed them to switch flavor, some electron neutrinos becoming muon or tau neutrinos, when the passed though matter (leaving the sun or passing though the earth), so we found the missing neutrinos