<< bakc to neutrinos
|Dr. Richard Helmer
Answers My Questions
Sudbury Neutrino Observatory
Sally: What's your main interest, as a physicist?
Richard: The properties of the neutrinos are the interesting thing to
me. Of course, this whole laboratory was built for what's called the
Solar Neutrino Problem. When people first started measuring neutrinos
that came from the sun, there were too few, fewer than they'd
calculated. What this experiment has shown is that neutrinos actually
transform their properties when they come from the sun. It was luck of
the draw. The fact that there were fewer neutrinos from the sun than
were calculated could have been because the calculations were wrong. We
would've found that out, and it would've been very interesting to a
whole different set of people than the ones who are interested in the
result we actually got. But before we started we didn't know. This
result is really in a fundamental sense the more interesting one,
because it tells you that neutrinos have a certain property, that has
implications for our whole understanding of the basic constituents of
S: I understand that one of the things that's been discovered is that
neutrinos flip between the three species, or that they change, or
R: Oscillate, yes.
S: And that somehow indicates that they have mass.
S: But I don't understand how it could indicate that.
R: Okay, that's one of these quantum mechanical phenomena so...
S: ...so I never will be able to understand?
R: Well, let me try. ...Our normal way of describing the world is
through what we take in through our senses, our eyes and our hands and
so on, and we call that classical physics, or classical mechanics.
Maybe back in high school you did things like sliding a ball down an
incline, or something like that. All that sort of thing is related to
classical mechanics. Around the turn of the 19th and 20th centuries,
people started seeing phenomena that didn't fit into that pattern.
People thought that they understood the world fairly well but when they
started looking in finer and finer detail they started seeing things
that they just could not explain in the classical way.
One of the things that was a popular pastime was to look at light
from discharges, hydrodgen discharges for instance, where you pass a
high voltage through a hydrogen gas and the gas glows red. So that was
all fine, you'd say well, the gas is heating up, or something like
that. But when you looked at it in finer and finer detail you'd see
that it wasn't a continuous red, it was a series of what we'd call line
spectra. The light was coming in specific wavelengths or specific
energies (same thing), instead of being a continuum of wavelenths.
S: Is this the particle/wavelength thing?
R: No, that's also part of quantum mechanics but from a different area.
The problem here was that you expected that the output from a hydrogen
atom would be continuous. This was beyond all the measurements that
anyone had ever done. So the question was how can this be? Why is this
not continuous output, but specific wavelengths, or specific energies.
And of course theoreticians went to work trying to understand this. One
way that they sort of hit upon by accident was: If the energies that
the atom can take on only have certain discrete values - and here you
see, in a certain light, that it's jumping between two of these energy
values - then that would explain the situation.
So that was pretty much the beginning of quantum mechanics: this idea
that things weren't necessarily continuous, but were discrete. They had
to be thought of as discrete in terms of classical physics. And then
around the mid 1920s people came up with a real theory, not just
explaining the fact that there were these line spectra but giving the
underlying reasons, a whole new way of understanding the universe came
into being with this theory. It was all based on postulates, it was not
based on anything you could measure. People said, "suppose that the
universe were built this way, what are the consequences?" And one of
the consequences was that there would be line spectra instead of
That was a discovery in physics, but now, for instance, biologists use
the same theory. They do things with DNA molecules that they can
understand based on quantum mechanics, but they can't understand based
on classical mechanics. So quantum mechanics has filtered through into
That's how it began: by looking at phenomena in more and more detail
and seeing things that couldn't be explained any other way.
One of the things that pops out of quantum mechanics is the idea that
not just neutrinos, but other fundamental particles in the universe
too, can change from one type to another. One of the requirements
within quantum mechanics for them to do that is that they have to have
Say two species oscillate back and forth, at least one of them has to
have a mass. And if they both have mass it can't be the same mass, they
have to be different.
S: We were talking to Doug Hallman yesterday, and he was saying that
the mass of the neutrino is determined now within a pretty big bracket.
R: Yes. From our measurements we can tell what the mass differences are
between the neutrinos that are oscillating from one to another, but we
can't tell what the masses are. The thing that matters is the mass
difference, but it doesn't matter what the mass actually is. It could
be 10 tons and 10 point 000001 tons or it could be - like we believe it
is - less than an electron volt. The two neutrinos that are oscillating
back and forth have a mass difference of about .01 electron volts.
There are other types of experiment that have a hope of getting at the
actual mass. Those are called beta decay experiments, where a
radioactive nucleus and kicks out an electron, and a neutrino comes out
of it, and the original nucleus is replaced by a different nucleus. In
principle, if you could measure all those things: the energies that the
beta and the neutrino come out with, and the energy that the residual
nucleaus has left over, then you could tell what the neutrino mass is.
Now the problem is in making that measurement of the beta and the
residual nucleus. The best hope that you have is a nucleus of tritium
which is hydrogen with two extra neutrons. It's very light, and that
makes it a little easier to make the measurement. Those are experiments
that are ongoing and have been for many years. Nobody has ever seen an
actual signal to be able to say, "this is what it is." But they can say
what the upper limit is, and the upper limit is very small, say an
electron volt or something like that. The new round of experiments is
trying to do much better than that. They just keep honing in on it.
S: Does this work at SNO help those experiments?
R: Yes, this helps in the sense that they know what they're aiming at
now. They know how low they have to be able to get, and if they get
that low they have to have seen a signal. That level is still a little
out of reach for them but the next round will push it down a little
further, and maybe they'll get lucky and the next round will be in the
S: What does the experiment look like? Is it an accelerator type of
R: No its a whole different type of experiment where you just prepare a
sample of tritium. Tritium, unfortunately, is a difficult nucleus to
contain because it's a gas. You capture tritium in other materials. The
complication is separating out what happens as the tritium goes into
those other materials. There's a trick to it, and trying to understand
that better and better is the name of the game in these experiments.
But people are continually making progress.
S: Is that what you work on?
R: No I actually work on a different thing...my other life...but it
also has to do with neutrinos. We know that they have a mass, and we
know that they oscillate. We have a certain range of the parameters
that are involved in describing that oscillation. We call it the mixing
parameters. What this experiment has done, and a couple of others using
similar instruments, is to narrow down the range of the mixing
parameters. One of the parameters that comes into this is a technical
thing called CP violation. What this means is that things that are
going one way in time aren't necessarily the same as things that are
going the other way.
R: Normally if you run a film backwards it all looks the same. If you'd
come from some other planet and didn't know which way things were going
you wouldn't be able to tell. But if there is a CP violation then you
would be able to tell. One of the interests in CP violation is that if
it actually happens, it may explain why everything in the universe ever
happened. Because we believe that when the universe began, it began in
a perfectly symmetrical state of matter and anti-matter, but if CP
violation occurs then there is a process by which you can end up with
more of one than the other.
S: Ah. that would be helpful to know, eh?
R: Yes. Of course that is the way the universe had ended up. So if our
speculation that the universe began in a symmetrical state is true,
then something had to happen to make it turn out this way.
I'm involved in the beginning of an experiment in Japan (T2K) that is
attempting to make a measurement of this CP violation. If its there.
on to beta decay >>