What are neutrinos?

1
What are neutrinos? Neutrinos are elementary
particles, like electrons, protons, and neutrons.
They however, are not present in the nucleus of
an atom. First hypothesized by Wolfgang Pauli,
neutrinos were once thought to be massless and
travel at the speed of light, like photons. We
now know that they have a very small rest mass
and travel at very nearly the speed of light.
These particles come in three flavors electron
neutrinos, tau neutrinos, and muon neutrinos
Every second, about 70 billion solar neutrinos
pass through every square centimeter on Earth
that faces the sun. Neutrinos have such weak
interactions with ordinary matter that virtually
all of these pass through the earth without
interacting with any other particles.
What is neutrino oscillation? Through quantum
mechanics we know that neutrinos have a certain
probability to shift from on flavor to
another. As a neutrino travels through space the
chance that it will change increases.
Wolfgang Pauli
Why is this important? Weak Interaction Neutrin
os are electrically neutral particles so they are
unaffected by the electromagnetic force and the
strong nuclear force. This means that the only
two forces that act on neutrinos are gravity and
the weak nuclear force and because the neutrinos
have such small masses gravity has a virtually
negligible affect on them. This means that by
looking at neutrino interactions we have a window
into the weak nuclear force by itself. Dark
Matter Neutrino oscillation also allows us to
account for some of the dark matter in the
universe. Dark matter is matter that does not
emit light on other electromagnetic waves, and is
very difficult to detect because of this. If we
can precisely estimate the mass of each flavor of
neutrino we should be able to see what percent
of dark matter comes from neutrinos. This
knowledge will provide some clues to exactly
what else could make up dark matter.
Neutrino Oscillation
This is the path the Neutrinos travel from
Illinois to Minnesota
What Do I do? When the Neutrinos are fired from
Fermilab in Chicago to the MINOS Far detector in
Soudan, Minnesota the far detector records energy
readings on incoming particles. A computer
program locates the few events per day that
might be neutrino interactions. This information
is then recorded in a graphical format, I can
then look at the graphs and determine if they
are Charged Current or Neutral Current
interactions. I am also looking at the data for
each Charged Current event to see if we can fine
tune the computer screening process and
eliminate the human component in analyzing and
categorizing neutrino interactions.
Discriminating Charged Current and Neutral
Current events It is important that we determine
what kind of interaction the neutrinos are
undergoing when they undergo neutrino
oscillation on their way to the MINOS Far
detector. In a neutral current (NC) interaction
the neutrino interacts with another particle and
transfers some of its energy. This is not
useful for our purposes because no information
on the flavor of neutrino is left behind. In
order to see what kind of neutrino interacted we
need a charged current (CC) interaction. When
this happens the neutrino turns into its partner
lepton an electron, tau, or muon. The particle
accelerator can only produce muon and tau type
neutrinos. Charged Current tau interactions
occur at energies not relevant to the MINOS
experiment, so the oscillation of muon neutrinos
to tau neutrinos is measured by a reduction in
the number of CC interactions.
Fermilab in Chicago, Illinois
What do events look like? Below, there are three
different neutrino events represented in a
graphical format -An easily identifiable CC
event -A more difficult to identify CC event -A
NC event
MINOS Far Detector in Soudan, MInnesota
Note the two tracks that are somewhat
concealed by the shower energy that make this a
less obvious CC event.
Note the two straight tracks following the
shower of energy that make this clearly a CC
event.
Note how similar this is to the less obvious CC
event but it is a NC event because there are too
many possible tracks and most of the energy is
in the shower.
Identifying neutrinos CC vs NC Muon neutrino
charged current interactions tend to leave one or
two straight tracks that are easy to follow.
When there is less energy in the charged current
interaction it is much more difficult to see
these tracks because they are much shorter.
There is also a splash of energy that is
released at the beginning of the interaction
which can obscure the lower energy CC tracks.
NC interactions have much more diffuse energy
readings and look much different than high energy
CC interactions. It gets difficult to tell the
difference between CC and NC when the CC
interactions have less energy and shorter tracks
that don't travel beyond their shower pattern.