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MARKUS KLUTE: Welcome
back to 8.701.

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So in this video, we want to
look at experimental studies

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of neutrino oscillations.

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The first question is, where
do we get the neutrinos?

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How do we produce the neutrinos?

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The answer is, there's
numerous sources for neutrinos.

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You might be lucky and find
them in supernova explosions.

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Or if we're really
trying hard, we

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can observe them as
relics of the Big Bang.

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There is a lot of neutrinos
as a relic of the Big

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Bang around us.

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Problem is that they
have very low energies

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and are difficult to observe.

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Easier-- so is the use of
neutrinos in the-- generated

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in cosmic ray showers.

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There's a lot of neutrinos
coming from the sun.

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Beams, beamlines--
accelerators can

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be used to smash
particles into a material,

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and then in the decay product,
produce also neutrinos.

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And also, reactors.

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Nuclear reactors can be
used as neutrino sources.

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By the way,
neutrinos can also be

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used in order to monitor
the nuclear activity

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around the globe.

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OK.

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Studies of neutrino
oscillations.

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So we can make this table here
and ask ourselves, what kind

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of-- the experimental
parameters are

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the length, the energy, and the
sensitivity to a specific mass

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range.

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So for the solar
neutrinos, you know

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the distance between
the Earth and the sun

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is pretty much fixed
to first order.

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The energy of the
neutrinos coming out

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is in the order of 1 meV.

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We are going to
look at the table.

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And so the mass range you can
probe is 10 to the minus 10

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in delta m squared.

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For atmospheric
neutrinos, they're

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produced in the
upper atmosphere, 10

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to the 4, 10 to 7 meters.

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Energies can range--
have a large range,

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let's say 10 to the
2 to 10 to the 5 meV.

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And then reactors,
typically meV range.

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It's kind of the nuclear range
for the neutrino energies.

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And the range is
given by how much

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space do you have around or
away from a nuclear reactor.

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Similarly for accelerators.

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You build an accelerator or
use an existing accelerator,

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and then you build your
detectors, maybe close to it,

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and maybe another one far away.

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And that's limited by
the size of our planet

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or wherever you want to
build your detectors.

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Energy ranges there
depends on the energy

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range of the accelerator.

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And that is in the order of
10 to the 3, 10 to the 4 meV.

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So you see that it's actually
rather a straightforward study.

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Also, it's interesting
to see-- and we'll

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see this next-- what kind
of flavor of neutrinos,

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and whether or not we
can study neutrinos

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or antineutrinos with other
experiment is important.

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Let's go through this.

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So it's been a little bit
of a history in how this all

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occurred.

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So the first question is, what
happens to the solar neutrinos?

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So solar neutrinos
are basically produced

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in the core of the sun,
together with light.

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It turns out that
the light of the sun

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takes about 10,000 years
to come out of the sun,

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while the neutrinos
come out immediately.

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So when first experiments tried
to observe solar neutrinos,

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they had to theoretically
estimate how many neutrinos

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to expect, and they saw less.

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And so one explanation would
have been, or could have been,

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or was, maybe something
happened at the core of the sun

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and we just haven't seen it
yet, because the light which

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come out of the sun has a
delay of up to 10,000 years.

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That didn't turn
out to be the case.

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So here is the spectrum
of the neutrino energies

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and the specific sources
of neutrinos from the sun.

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In our nuclear
physics discussion,

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we'll get to the point that we
understand how the neutrino--

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how the sun produces energy,
and then some of this

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becomes more clear.

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The story to take
away at this point

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is that there are certain--

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there's several processes in
the sun producing neutrinos.

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And they all come with
their characteristic energy

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distribution.

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But the bottom line is you
find meV scale neutrinos

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from the sun.

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There's a soup of
electron neutrinos.

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They start interacting
with the sun.

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And there's a little bit of
a flavor evolution within--

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when they go through
the material of the sun.

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But, you know, what
you want to really do

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is look for disappearance
in detectors

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which are sensitive
to electron neutrinos.

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And that has been done in
a number of experiments.

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Most famous may be the
Davis experiments which

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had a big tank of chlorine.

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And in the interaction,
you were looking

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for finding argon
in your detector,

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and you just every now
and then went in there

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and saw how much argon
was actually produced.

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And it turned out that those
experiments, all of them,

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found a reduced number of
neutrinos, reduced with respect

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to the theoretical expectation.

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So far so good.

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The assumption was that--

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or there was no knowledge of
neutrino oscillations or mixing

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at this time.

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So that needed to be explained.

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And one way to explain--

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it's not just using the
charge interaction, which

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allows you to probe the
flavor of the neutrino,

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but also lose a neutral--

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the neutral
scattering, which then

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allows you to measure the
total number of neutrinos.

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And if you do this-- this was
done by the SNO experiment--

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you find that the total
number of neutrinos

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is in good agreement with
the theoretical expectation.

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Hence, those neutrinos
are not really lost,

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they're just more from
one flavor into the next.

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So this was the first
evidence for solar neutrinos

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to be oscillating.

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By now there's-- this first
experiment was Homestake.

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By now, there is a larger number
of solar neutrino experiments,

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and you see the long
time of neutrino studies.

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Different materials are
being used, different energy

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thresholds being
tested, different scale

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of the experiments, and
experiments become more

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sensitive the larger they are.

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And so this you can--

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something you can
see from this table.

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The next sort of neutrinos
is the ones which

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are produced in the atmosphere.

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So they are produced in
decays of pions and kaons

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and by the cosmic rays
interact with the atmosphere,

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or the Earth's atmosphere.

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And so you find,
for example, a pi

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plus decaying into a
muon and a muon neutrino.

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And then the muon
itself can decay

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into an electron, an
electron neutrino, and a muon

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antineutrino.

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So if you, for example, build
a ratio of muon/antimuon over

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electron/anti-electron
neutrinos,

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you find it should be around 2.

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You have two neutrinos--
muon neutrinos here,

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and an electron neutrino.

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And also, this wasn't
really observed.

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And you can see
here, as a function

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of the column of the
zenith, of looking up

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upwards towards the
atmosphere or downwards,

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you find that there is an effect
of this kind of oscillation.

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So the actual measurement
depends on the energy range.

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And you can see that
the muon neutrinos,

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the muon-like neutrinos,
they disappear.

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You see here in this very
clear plot the prediction

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without oscillation compared
to the experimental results,

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so you see the muon
neutrinos actually disappear.

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Moving on, accelerators
can be used.

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And the big accelerator on the
Earth at CERN or at Fermilab.

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The beamline at Fermilab is
called NuMI, Fermilab National

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Accelerator Laboratory, FNAL.

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Or CERN, or in Japan.

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Those are the big sources of
accelerator-driven neutrinos.

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And with those, there's big
detectors, typically a detector

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very close to the accelerator
and one further away.

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The close one probes
the total flux

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of the neutrino at the
experiment, and then the one

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which is away in order to probe
the effect of the neutrino

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oscillation in order to study
appearance or disappearance.

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And again here, you see
this is a long program.

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But it basically took off quite
a bit in the 2000s and after.

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So a lot of neutrino physics
happened in those years.

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A lot of information
about the neutrino

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was gathered in those years.

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And again here-- this is
from the T2K experiment--

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you see the comparison between
unoscillated predictions

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and oscillated using some
additional constraints

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about expectation of the
total flux of the neutrinos,

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and that compared to the data.

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And you see very
clearly that the--

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that the neutrinos
oscillate, that there

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is evidence of oscillation.

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All right.

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The last source are
reactor neutrinos.

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We'll talk about nuclear
physics starting from next week.

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Here neutrinos are
produced in nuclear fission

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of heavy isotopes, mainly
uranium and polonium.

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The flux can be calculated
in various ways,

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for example by knowing
the nuclear processes

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and the thermal power
produced in the reactor,

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or by just looking at
how much fuel is being--

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nuclear fuel is being used
by the reactor itself.

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What's being studied here is
the anti-electron neutrino

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disappearance.

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And what you do here is you use
this inverse beta decay, where

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you have a collision
or scattering

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of a anti-electron
neutrino with a proton,

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creating an electron-- a
positron and a neutron.

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And again, there's a
number of experiments.

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Basically, whenever you have
a large neutrino experiment,

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it can probe surrounding
nuclear reactors.

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There's many of them in France
and Japan, also in China.

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And they're being used
in those experiments.

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Again, you see that this topic
became really hot in the 2000s.

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And again, a lot of--
a lot has been learned.

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So this part here shows you
as a function of the energy--

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the length over the energy--

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so kilometer over meV--

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the oscillation, the
survival probability,

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meaning that you can actually
see directly the oscillation

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of the neutrinos.