MARKUS KLUTE: Welcome back to 8.701.
So this is our last video in the chapter on neutrino physics.
And we'll talk about mass scales and the nature of the neutrino
particle very briefly.
When we think about how we can measure the neutrino masses,
there's a number of methods which come to mind.
The first one is to just look out into the universe
and try to understand how much matter in total
could come from as a source of neutrinos.
And one has to make assumptions about the model,
the cosmological models at hand.
But if I accept those potential biases or model dependencies,
one finds that there's a potential reach
of this kind of measurements of 20 to 50 MeV, millielectron
volts.
And the current best limits are in the order
of 0.1 to 1 electron volt.
A second source, and I'll talk more about this later,
is the study of neutrinoless beta decay--
double beta decays.
Here, the current best limits are on the order of 0.2
to 0.4 electron volts.
And there's a chance to reach 20 to 50 millielectron volts.
This kind of measurement will also answer the question
whether or not the neutrino is a direct particle or a Majorana
particle as we discussed in earlier lectures.
And then there is the more classical approach
of measuring the mass of an neutrino
from the end point spectrum of beta decays.
And so here the current best limit
is from the Kartrin experiment.
And I talk about it in the next slide.
And it's in the order of one electron volt.
And there's a potential reach to go down to 40 millielectron
volt. So currently the range of limits
is in the order of 1 electron volt or a bit better,
and when we'll be able to go down
to limits in the order of 20 to 50 millielectron volts.
So here is a cartoon of how those measurements are
being conducted.
One starts with tritium.
And it uses beta decay.
And this lecture overall is a good first entry
into the nuclear physics program where we discuss beta decays
and other nuclear decays in more detail.
What we find here is that you find an electron
and the neutrino-- antineutrino in this case--
being emitted.
And so the name of the game is now
to measure the electron energy as precisely as possible,
and then find a sensitivity off the neutrino mass in the end
point spectrum.
And those small differences here in the end point spectrum then
that leads to understanding of the mass of the neutrino
because the total energy in the collision
needs to be preserved.
And so the entire story here is about how precisely can we
measure the energy of the electron
in order to infer the neutrino mass in that.
And so the latest results came out last year
from the Kartrin experiment and shows
that the result is consistent with a neutrino mass of 0,
and that we can set an upper limit at 90% confidence level.
That electron neutrino is of mass of 1.1 electron
volt. Just as a reminder, we measure the mass
of the electron neutrino in this decay, which
is the sum of the individual components, mass
eigenstate, which make up the electron neutrino.
To just have historical context in this discussion here,
we find that this latest result is
an improvement of the order of factor of 2
compared to previous result by other experiments, which
had a very similar job to measure the electron
energy in beta decays, in the end point
spectrum of beta decays.
There's a new approach, which has
been proposed by Joe Formaggio here from MIT,
which changes the way the electron energy is
being measured.
So the idea is to have the decay happen in magnetic fields,
and use the cyclotron radiation of single electrons.
So the advantage here is that one
doesn't have to move the electrons somehow
into a spectrometer, but can immediately measure
the energy of the electron.
And the measurement of the energy
then turns into a measurement of the frequency
and basically measures the cyclotron frequency
of the electron circling around in a magnetic field.
And so it turns out that one moves the measurement
of the energy of the electron into a measurement
of a frequency.
And thus frequency can be measured
with very, very high precision.
So there's some hope that this kind of measurement
lead to very, very precise results
of the energy of the electron and with that the mass
of the neutrino.
So the last slide here is now how can we
figure out whether or not the neutrino has
Dirac or Majorana nature.
And this can be done, or the high sensitivity
comes from so-called neutrinoless double
beta decays.
So one starts with nuclear decays
where two electrons are emitted, but no neutrino.
And so this requires that in this process
there's a transition which includes
the neutrino where the neutrino has to be its own antiparticle.
And that just means that the neutrino is of Majorana nature.
This is being done by measuring, again, the energy spectrum.
You typically have all kinds of background contributions,
but also backgrounds from double beta decays with two neutrinos.
So you see this spectrum here.
And then you look at the end point of this part
here and find that there is this peak, a precise sharp peak
of the energies of the two electrons.
The issue is that forecasting where this peak is
requires proper knowledge of the dynamics inside the nuclei
here.
And those measurements are being conducted.
There's many of them conducted in various nuclear transitions
or decays.
And they haven't yielded a positive result yet.
Research is still going on on this end.