Neutrino Astronomy
Newton’s Third Law
If they push hard,
these two will move
directly away
from one another.
Neutrinos
Consider “beta-decay”
In empty space, a ‘free neutron’ decays
spontaneously
neutron  proton + electron
(β)
But Something Doesn’t Add Up!
- problems with the total energy and momentum
(they don’t move ‘directly away’ from each other)
Important Note
This discrepancy was discovered in the
1930’s in experiments carried out in
physics laboratories on Earth.
It does not depend on nuclear reactions in
the sun.
Solutions? (~1930)
1.
Should we discard the conservation
laws? Maybe they don’t work on the
scale of atoms? Not an attractive
proposition…
2.
Alternatively, can we assume that a third
particle, as yet undetected, is produced
in the decay – one which carries the
‘missing’ momentum?
The Proposed Solution
Who Ordered That?
The particle must be

Uncharged (neutral);

Very low in mass; and

Elusive! (scarcely interacting with other matter)
Wolfgang Pauli’s prediction, therefore:
Neutrinos!
Origins
They are produced in large numbers:

in spontaneous radioactive decays (as from

in nuclear reactions (in the Sun and stars). So
Uranium in the rocks). So they are coming up
out of the ground.
they are flowing in from space.
Remember the P-P Cycle!
[note the neutrinos, in red]
Lots and Lots!
Not rare!
If our calculations are right,
the sun bombards us with
about 100 billion neutrinos
per square centimetre
(your thumbnail!) per
second.
The Problem
They are hard to capture! Almost all go right
through us – and the entire Earth!
(Indeed, a neutrino could travel though light years
of lead with a good chance of avoiding any
interactions.)
But They Provide a
‘Solar Thermometer’
Let’s Back Up a Bit
They were hypothesized in the 1930’s (to
save the conservation laws).
But how do you prove they exist?
A Clever Idea
Nuclear Reactors also create them in abundance,
and we can turn the supply on and off!
So, put a specialised detector (capable of
capturing just a tiny fraction of them) near a
reactor.
1959: success!
Fred Reines: Nobel Prize, 1995
(for a discovery made in 1959!)
Now Find Solar Neutrinos
One problem: we can’t turn the sun on and off.
(Even at night, we are flooded with neutrinos.
And they pass right through the Earth!)
We also have to eliminate other sources of ‘noise’
– things that will trigger our detector.
Solar Neutrinos: First Attempts
A Remarkable Technique
A tank containing 100,000 gallons of C2Cl4 cleaning
fluid! (Note that this liquid contains Chlorine
atoms.)
Located deep in a mine!
Flood the chamber with water before starting! (to
block radiation from the surrounding rocks)
Why in a Mine?
To block cosmic rays!
How It Worked
VERY RARELY, a neutrino hits a chlorine nucleus,
converting it to a radioactive isotope of Argon (a gas!)
Cl37 + ν  Ar37 + e[In this reaction, a neutron in the Cl nucleus spits out an electron,
turning into a proton in the process.]
So individual atoms of Argon accumulate in the cleaning
fluid in the tank.
How Much Argon?
After a month or two, collect the argon gas that has
accumulated in the tank, and determine the number -maybe a hundred atoms or so.
(Davis collected the Ar by bubbling Helium gas through the
tank.)
The Ar atoms produced are radioactive, so easily
‘counted.’
Success! Solar neutrinos were detected!
Ray Davis Nobel Prize 2002
How Much Argon Do We Expect?
The expected number of Argon atoms can be calculated
assuming that:



we understand the experiment, and know what tiny
fraction of the neutrinos will get caught
we understand the reactions in the sun (the source of
the neutrinos)
we understand neutrinos themselves
The Disturbing Result
Only about 40% as many neutrinos were detected
as had been predicted. Why?
1.
2.
3.
Was the experiment not to be trusted?
Were we wrong about the sun? Does it produce fewer
neutrinos than we thought?
Were we wrong about fundamental particle physics? Do
neutrinos behave in some unexpected way?
I. Can the Experiment be Trusted?
Yes!! Other experiments followed, using different
techniques, and they found the same result: too few!
II. Do We Understand the Sun?
Maybe it produces fewer neutrinos than predicted?
How could it? Well, suppose it is cooler in the
core. In that case, there will be reduced
reactions, and it will produce fewer neutrinos.
Or maybe our basic calculations of nuclear
reactions need to be refined.
This is not an Attractive Option!
If it is cooler, what holds it up?

Strong magnetic fields? Not plausible.

Rapid rotation in the interior? Ditto.
Moreover, Astronomers are Confident!
1.
2.
3.
Decades of analysis always leads to the same
answers.
We have improved our earth-based laboratory
measurements of how nuclei interact, and still
get the same final answers.
Moreover, Helioseismology (the study of
‘sunquakes’) -- also confirms this. (Next topic!)
III. Do We Understand Neutrinos?
Experiments on Earth revealed, many years
ago, that there are actually three (3) kinds
of neutrinos. These are called ‘flavours.’
(This discovery was NOT a result of studies
of the sun, by the way.)
The Sun Produces Only One Kind
[the so-called electron neutrinos, denoted νe]
Logically Enough
The Davis experiment was designed to capture
exactly the sort of neutrinos the Sun creates. So
were most of the subsequent experiments!
(Davis was not guilty of some silly oversight!)
But if neutrinos change flavours, the experiment
will miss some of them as they pass by!
Neutrino ‘Mixing’
Indeed, some modern theories predicted
‘neutrino mixing.’
If neutrinos change from one form to
another as they travel, this would explain
the “solar neutrino problem.”
Catch!
Sports Analogy
Throw a baseball to a friend, but discover
that it spontaneously turns into a football
or a basketball en route.
If he’s looking only for baseballs, he will
miss the others entirely.
If There’s ‘Mixing’: Incomplete Pass!
[the experiment doesn’t register the changed neutrinos]
This Has Profound Implications
If neutrinos do behave in this way:

our ‘standard model’ of particle physics is wrong.

Neutrinos must have some ‘rest-mass’ and travel at
slightly less than the speed of light.

The umpteen neutrinos in the universe (flowing out from
all the stars) might add up to a significant fraction of the
total mass of the observable universe
Question: How Do We Decide?
Answer: Design a better experiment, one
that will resolve this question by detecting
all three kinds of neutrinos.
… and, while we are at it, let’s improve it in
just as many other ways as we can.
How to Improve on the Davis Experiment
1.
2.
3.
4.
5.
There are better locations (e.g. deeper mines!)
There are more efficient detectors (better at capturing
neutrinos than Cl atoms are).
We can now detect neutrinos one by one, as they
arrive, not just as an accumulated number after a
month or two.
We can tell what direction they come from (confirm
that they originate in the sun!)
One particular experiment (SNO!) can detect all three
flavours of neutrinos. This is the key feature!
First, Kamiokande
and Super-K
Deep in a mine in
Japan.
Note the inward-looking
“fly eye” light detectors.
But still only sensitive to one flavour.
A Huge Water-Filled Volume
A Really
BIG
Tank!
(the whale is
shown only for
scale!)
How Neutrinos Announce Their Arrival
[just a tiny fraction of those passing through, of course]
Details to be explained in a bit.
The Sun in ‘Neutrino Light’
on each rare occasion when a neutrino is detected, put a
bright dot on a sky map to show where it came from
The vast majority of the detected neutrinos are
indeed coming from the direction of the sun!
The SNO Experiment
(Sudbury Neutrino Observatory)
Canada’s Advantage
A great location, deep underground. Well shielded
from cosmic rays!
Heavy water as a detector. (Particularly sensitive
to neutrinos – but still only a tiny fraction of them!)
Strong research support by government and
universities, and an excellent multi-national team
of physicists.
This experiment can detect all three flavours!
Down the Mine
Getting to the Office
(a working mine)
The Central
Element
A 10-metre
vessel, filled
with “heavy
water”
Why Heavy Water?
D2O
H2O
~10% Heavier (extra neutrons!)
A Challenging Construction Project
During the
Construction
Phase
The Vessel Surrounded
(by ~ 10,000 photomultiplier tubes)
The Vessel
and the ‘Eyes’
The Fly’s Eye
The Light Detectors
They work in complete darkness, and are
very sensitive.
They are looking
for tiny ‘flashes.’
But What Causes the Light?
The occasional neutrino interacts with the
heavy water in a way that causes a flash
of light.
Exactly how does it do that?
Einstein Said
“Nothing can travel faster than light…”
-- didn’t he?
Well, Not Quite!
Light slows down in various media!
in those circumstances, moving objects can
outrun light
Cerenkov
The Key Physics
When charged particles exceed the speed of light
in some medium, they can create a cone of
“Cerenkov radiation” (light!) in the forward
direction.
Imagine a tiny fast-moving car turning on its headlights!
Problem: Neutrinos are Uncharged
- so they don’t create Cerenkov radiation
themselves.
But they (very occasionally) ‘bump into’ nuclei
or electrons and set various pieces moving at
high speed!
This gives rise to a cone of light pointing in
roughly the direction the neutrino was going.
The Detection
Process

directional
time-resolved

sensitive!

The Canadian ‘Edge’
Access to $300,000,000 worth of heavy
water! (Loaned from AECL, Atomic Energy
of Canada Limited.)
The deuterons in the water (the nuclei in
the ‘heavy hydrogen’) are especially good
neutrino targets (although they still only
see a tiny fraction of them!)
Detecting the Neutrinos
[the details don’t matter, but please take note that
all flavours can be detected!]
Some Real Events
What We Discover
Astrophysicists were right: The sun is emitting just
as many neutrinos as our theory told us!
The earlier experiments missed more than half of
them because of neutrino mixing.
(Some baseballs had turned into footballs or
basketballs on the way, so to speak)
The Profound Implications
1. We do understand the sun – tremendously well!
2. Neutrino mixing does take place!
Point 2 means that we need nothing less than a
complete revision in our understanding of some
of the fundamentals of particle physics!
Widely Heralded!
Visit http://sno.phy.queensu.ca/
By the way, this experiment is now over.
The heavy water has been returned to
AECL.
Nobel Prize 2015:
Art McDonald
From SNO to SNOLAB:
More Chambers and Labs
Other neutrino experiments, plus searches for the
mysterious ‘dark matter’ (We meet this later!)
A Guided Underground Tour
http://www.astro.queensu.ca/~hanes/Movies/SNOLAB-Welcome.mp4
AMANDA / Ice-Cube