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MARKUS KLUTE: Welcome
back to 8.701.

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So this is our last video in
the chapter on neutrino physics.

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And we'll talk about mass scales
and the nature of the neutrino

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particle very briefly.

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When we think about how we can
measure the neutrino masses,

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there's a number of
methods which come to mind.

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The first one is to just
look out into the universe

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and try to understand
how much matter in total

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could come from as a
source of neutrinos.

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And one has to make
assumptions about the model,

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the cosmological models at hand.

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But if I accept those potential
biases or model dependencies,

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one finds that there's
a potential reach

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of this kind of measurements
of 20 to 50 MeV, millielectron

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volts.

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And the current best
limits are in the order

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of 0.1 to 1 electron volt.

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A second source, and I'll
talk more about this later,

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is the study of
neutrinoless beta decay--

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double beta decays.

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Here, the current best limits
are on the order of 0.2

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to 0.4 electron volts.

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And there's a chance to reach
20 to 50 millielectron volts.

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This kind of measurement
will also answer the question

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whether or not the neutrino is
a direct particle or a Majorana

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particle as we discussed
in earlier lectures.

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And then there is the
more classical approach

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of measuring the
mass of an neutrino

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from the end point
spectrum of beta decays.

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And so here the
current best limit

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is from the Kartrin experiment.

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And I talk about it
in the next slide.

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And it's in the order
of one electron volt.

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And there's a potential reach
to go down to 40 millielectron

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volt. So currently
the range of limits

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is in the order of 1 electron
volt or a bit better,

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and when we'll be
able to go down

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to limits in the order of 20
to 50 millielectron volts.

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So here is a cartoon of
how those measurements are

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being conducted.

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One starts with tritium.

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And it uses beta decay.

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And this lecture overall
is a good first entry

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into the nuclear physics program
where we discuss beta decays

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and other nuclear
decays in more detail.

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What we find here is
that you find an electron

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and the neutrino--
antineutrino in this case--

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being emitted.

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And so the name
of the game is now

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to measure the electron energy
as precisely as possible,

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and then find a sensitivity off
the neutrino mass in the end

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point spectrum.

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And those small differences here
in the end point spectrum then

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that leads to understanding
of the mass of the neutrino

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because the total
energy in the collision

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needs to be preserved.

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And so the entire story here
is about how precisely can we

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measure the energy
of the electron

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in order to infer the
neutrino mass in that.

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And so the latest results
came out last year

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from the Kartrin
experiment and shows

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that the result is consistent
with a neutrino mass of 0,

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and that we can set an upper
limit at 90% confidence level.

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That electron neutrino is
of mass of 1.1 electron

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volt. Just as a reminder,
we measure the mass

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of the electron neutrino
in this decay, which

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is the sum of the
individual components, mass

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eigenstate, which make
up the electron neutrino.

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To just have historical context
in this discussion here,

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we find that this
latest result is

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an improvement of the
order of factor of 2

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compared to previous result
by other experiments, which

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had a very similar job
to measure the electron

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energy in beta decays,
in the end point

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spectrum of beta decays.

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There's a new
approach, which has

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been proposed by Joe
Formaggio here from MIT,

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which changes the way
the electron energy is

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being measured.

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So the idea is to have the
decay happen in magnetic fields,

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and use the cyclotron
radiation of single electrons.

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So the advantage
here is that one

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doesn't have to move
the electrons somehow

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into a spectrometer, but
can immediately measure

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the energy of the electron.

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And the measurement
of the energy

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then turns into a
measurement of the frequency

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and basically measures
the cyclotron frequency

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of the electron circling
around in a magnetic field.

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And so it turns out that
one moves the measurement

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of the energy of the
electron into a measurement

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of a frequency.

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And thus frequency
can be measured

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with very, very high precision.

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So there's some hope that
this kind of measurement

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lead to very, very
precise results

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of the energy of the electron
and with that the mass

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of the neutrino.

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So the last slide
here is now how can we

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figure out whether or
not the neutrino has

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Dirac or Majorana nature.

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And this can be done,
or the high sensitivity

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comes from so-called
neutrinoless double

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beta decays.

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So one starts with
nuclear decays

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where two electrons are
emitted, but no neutrino.

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And so this requires
that in this process

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there's a transition
which includes

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the neutrino where the neutrino
has to be its own antiparticle.

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And that just means that the
neutrino is of Majorana nature.

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This is being done by measuring,
again, the energy spectrum.

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You typically have all kinds
of background contributions,

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but also backgrounds from double
beta decays with two neutrinos.

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So you see this spectrum here.

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And then you look at the
end point of this part

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here and find that there is
this peak, a precise sharp peak

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of the energies of
the two electrons.

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The issue is that forecasting
where this peak is

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requires proper knowledge of
the dynamics inside the nuclei

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here.

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And those measurements
are being conducted.

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There's many of them conducted
in various nuclear transitions

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or decays.

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And they haven't yielded
a positive result yet.

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Research is still
going on on this end.