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MARKUS KLUTE: Welcome
back to 8.701.

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In this chapter, we will
talk about neutrinos.

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And we'll start the discussion
with a relatively simple

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introduction.

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How does a neutrino look
in the standard model,

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and how does it interact?

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We have discussed the
neutrino already quite a bit.

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So this is more
or less a summary.

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In the standard model,
the neutrino is massless.

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It's a massless particle.

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And it interacts with
a weak direction.

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So it interacts with w
bosons and with z bosons.

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And specifically,
it does not interact

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with photons of gluons.

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If we look at the Lagrangian or
try to write down the current,

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we find that there is a
charged current via the w.

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And there is a neutral
current, we have the z boson.

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It's quite interesting to
think about those two currents

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a little bit more.

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So in case of a charged
current, for example,

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I have an incoming neutrino,
we can determine the flavor,

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the kind of neutrino we have.

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We are detecting the
flavor of the lepton.

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So if, for example, identify
an electron in this interaction

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in the interaction,
the initial neutrino

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was an electron neutrino.

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While for the neutral current,
when we have some sort

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of interaction happening,
we cannot identify directly

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the neutrino.

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Hence, we cannot find the
flavor of the neutrino.

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You can just measure the sum
of all flavors of neutrinos

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in the neutral current.

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On that story, neutrinos
have three flavors.

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They come in electron flavor,
muon favor, or tau flavor.

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Neutrinos are left-handed.

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Anti-neutrinos are right-handed.

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So that's the story.

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That's how neutrinos
are characterized

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in the standard model.

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In the standard model, in
the framework we set up,

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we can calculate cross-sections,
scattering cross-sections.

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And here, we are looking in
the neutrino nuclei scattering.

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Again, we can split this up in
the charged current and neutral

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current discussion.

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But they go very
much in parallel.

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So we have elastic scattering.

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Or in the case of
the charged current,

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we talk about
quasi-elastic scattering.

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Then we have an
incoming neutrino,

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let's say a muon neutrino,
hitting a neutron,

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producing a muon and a proton.

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It's called quasi-elastic,
because we do not

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break up the target,
but we change its kind.

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So we change from a neutron
to a proton, in this case.

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While, for the
elastic scattering,

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the neutron just stays intact.

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We can also have nuclear
resonance production,

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where we hits the nuclei.

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And then inside the nuclei,
we could use a [INAUDIBLE],,

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like a neutral or a
charged [? pion. ?]

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And also, that's possible in
the neutral current exchange.

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And then we have
deep-inelastic scattering,

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where we hit a nuclei
or nucleon that hard,

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that we'd start breaking it up.

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And in this case, we
scatter off the quark,

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and we produce a new quark in
the charged current interaction

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in the same quark in the
neutral current interaction.

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So this is no different from
the stories we had before.

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The intriguing part about
studying neutrino scattering

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is that we do know that we
have weak interactions being--

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we use a dominant force
of being the process.

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While, if we use
photons to interact,

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or we have electrons
being [INAUDIBLE],,

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then we can have a mixture
of weak and electromagnetic

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interaction.

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Another important
takeaway from this slide

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is that, when we
calculate cross-section,

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we find that a linear
increase in the cross-section

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of the function of the energy.

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So while cross-sections
for neutrinos are small,

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at higher energy source
cross-sections scale linearly.

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As we can discuss this for
neutrino scattering with nuclei

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and nucleons, we can also
look at neutrino scattering

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with electrons directly.

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There's a lot of electrons
in the metal around us.

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And the neutrino [INAUDIBLE]
with this metal that can

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interact with electrons, too.

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And they can live in the charged
current interaction of muon

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in an electron neutrino and in
a neutral current interaction

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in electron in the [INAUDIBLE].

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Also, here, you see
cross-section, total

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cross-section scaled
with [INAUDIBLE] energy.

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Good.

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It's the first introduction
to neutrino physics.

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We'll go into more detail in
the following presentations.