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) (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); Uncharged (neutral); Very low in mass; and Very low in mass; and Elusive! (scarcely interacting with other matter) Elusive! (scarcely interacting with other matter) Wolfgang Pauli’s prediction, therefore: Wolfgang Pauli’s prediction, therefore:Neutrinos!

Origins They are produced in large numbers:  in spontaneous radioactive decays (as from Uranium in the rocks). So they are coming up out of the ground.  in nuclear reactions (in the Sun and stars). So 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 C 2 Cl 4 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!) Cl 37 + ν  Ar 37 + 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 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 the reactions in the sun (the source of the neutrinos) we understand neutrinos themselves we understand neutrinos themselves

The Disturbing Result Only about 40% as many neutrinos were detected as had been predicted. Why? 1. Was the experiment not to be trusted? 2. Were we wrong about the sun? Does it produce fewer neutrinos than we thought? 3. 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! 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. Strong magnetic fields? Not plausible. Rapid rotation in the interior? Ditto. Rapid rotation in the interior? Ditto.

Moreover, Astronomers are Confident! 1. Decades of analysis always leads to the same answers. 2. We have improved our earth-based laboratory measurements of how nuclei interact, and still get the same final answers. 3. 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 ν e ]

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.” If neutrinos change from one form to another as they travel, this would explain the “solar neutrino problem.”

Catch! 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. There are better locations (e.g. deeper mines!) 2. There are more efficient detectors (better at capturing neutrinos than Cl atoms are). 3. We can now detect neutrinos one by one, as they arrive, not just as an accumulated number after a month or two. 4. We can tell what direction they come from (confirm that they originate in the sun!) 5. 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!) 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? D 2 O H 2 O D 2 O H 2 O

~10% Heavier (extra neutrons!)

A Challenging Construction Project

During the ConstructionPhase

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. - 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 directional time-resolved time-resolved sensitive! 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 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

AMANDA / Ice-Cube