### definite unit | lepton:

Neutrino
* All neutrinos are stable.
* Most neutrinos pass right through the earth without ever interacting with a single atom of it
  - through the Earth unimpeded and escape detection
  - passing through the Earth come from the Sun
* also play an important role in astrophysics
  - result from collisions between cosmic rays and Earth's atmosphere
  - travel at the speed of light
* are also the only identified candidate for dark matter , specifically hot dark matter.
* are among the main particles of the universe
  - oddest discoveries of modern physics
  - amongst the most mysterious of 'fundamental particles', the building blocks of matter
  - an abundant source that can be found in numerous decay processes
  - difficult particles to detect
* are elementary particles which are a product of nuclear fusion reactions in stars
  - elusive particles
  - essentially massless particles emitted in enormous quantities by supernovas
  - everywhere
  - formed in the proton- proton chain
  - fundamental particle building blocks of matter
  - ghostly subatomic particles produced by thermonuclear reactions in the sun's core
  - hard to spot because they are small and have no charge and very little mass
  - infinitesimally small bits of sub-atomic matter
  - inoffensive
  - leptons
  - like cosmic ghosts
  - mass-less subatomic particles that couple weakly to ordinary matter
  - massless, so they just carry energy
* are nearly massless and have no electric charge
  - atomic particles that carry a lot of energy in small packages
  - neutral electron-like particles that originate from the sun or cosmic rays in space
  - normally very passive particles, electrically neutral and with a mass at or near zero
* are one of nature's basic building blocks but they are very tricky to track
  - the basic building blocks of atoms and, in turn, everything else
  - tool to investigate the origin of mass
* are particles associated with radioactive phenomena
  - that are known to exist
  - with almost no mass and no electric charge
  - without electric charge and, as far as scientists know, they have no mass
  - probably the strangest particles in the universe
  - produced in weak interactions as chirality eigenstates
  - reluctant to interact with everyday matter
  - so elusive that they can pass through the entire earth without interacting
  - sub-atomic particles that are created when two hydrogen nuclei fuse together
  - subatomic particles produced during nuclear fission and fusion processes
  - subject to only the weak interactions and the much weaker force of gravity
  - the ghostliest of subatomic particles
* are the most abundant particles in nature
  - prevalent objects in the universe
  - only subatomic particles that can penetrate virtually all types of matter
  - tiny elementary particles produced in nuclear reactions
  - tiny, electrically neutral, sub-atomic particles
  - uncharged subatomic particles
* are very difficult to detect
  - produce and detect
* carry kinetic energy and nothing, or next to nothing else.
* come in three varieties, called the electron, muon and tau-type neutrinos.
* do exist, in huge numbers, but all known neutrinos have zero mass.
* escape and carry away the energy.
* fly out of star.
* go in and out of photons and are coded to do something.
* have at most a tiny mass and only feebly interact with ordinary matter.
* have no charge and extremely small masses compared to other subatomic particles
  - similar shortcomings
  - the potential to map the heavens in a new and unique way
  - very low mass and no measurable charge
* interact only by means of the 'weak' interactions, which are, indeed, quite weak
* interact only very weakly with matter
  - other matter
  - primarily through the weak force
* interact very weakly with matter and are therefore very hard to detect
  - weakly, especially at low energy
* interact with charged particles which can be detected
  - matter only through the weak interaction and hence are difficult to detect
* originate in nuclear reactions and in the core of stars such as our sun.
* oscillate among the flavours as they move
  - between different flavors in flight
  - from one type to another without physical explanation
* pass through all forms of matter almost without interacting
  - everything without stopping
* perpetually flow in all directions.
* pervade all space as ghostly relics of the big bang.
* play an important role for the dynamics of supernovae.
* provide an extremely long-range glimpse into the universe.
* represent the frontier of high energy physics.
* serve as a sensitive probe of the standard model.
* simply fly through matter like ghosts fly through walls.
* stream out from the sun's core and into space at the speed of light unaffected by the sun.
* travel through the Earth all the time without being detected.
* turn up all the time in nuclear reactions.
* usually pass through normal matter.
+ Speed of light: Light :: Astrophysics :: Relativity
  * According to special relativity, 'c' is the maximum speed at which all energy, matter, and physical information in the universe can travel. Neutrinos also travel at the speed of light. Palmer, Jason 2012.
  * Neutrinos are very difficult to detect. This means that they are unaffected by the electromagnetic force, too. Detectors built to find them only see 10-15 a year.