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One of the greatest mysteries of physics solved

By SIGFPE in News
Mon Jun 18, 2001 at 03:13:10 PM EST
Tags: Science (all tags)

Many years ago detailed models for the fusions reactions that drive the Sun were developed. These predicted that a certain flux of neutrinos would be produced as a by product. However when neutrino detectors were first built they detected a far lower flux than predicted by the models. Subsequent neutrino detectors have also revealed a major flux deficit that cannot be accounted for by experimental error. This discrepancy between theory and experiment has been a major thorn in the side of physicsts for many years now because they have been otherwise very confident about the stellar models they have been using. A deficit in the number of electron neutrinos produced means a major problem with fairly elementary particle physics.

However it looks like the mystery has been solved!

There are at least three types of neutrino: the electron neutrino, the muon neutrino and the tau neutrino. The models for stellar fusion predict a certain flux of electron neutrinos. Traditionally neutrino detection has been carried out using 'charged current' devices that detect only electron neutrinos and they have all detected a deficit.

At Sudbury Neutrino Observatory (SNO) an alternative 'elastic scattering' experiment is sensitive to all three types of neutrino. This observatory has apparently been detecting all three types. More importantly the flux is compatible with the hypothesis that neutrinos can change type.

So the explanation is simple: the missing flux in electron neutrinos can be explained by them being converted to the other two types of neutrino in flight from the Sun to the Earth.

Incidentally, this is incompatible with the Standard Model so the SNO results should provide excellent experimental material for researchers working on variations on this model.


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One of the greatest mysteries of physics solved | 16 comments (16 topical, editorial, 0 hidden)
You mean... (2.60 / 5) (#1)
by jd on Mon Jun 18, 2001 at 02:26:34 PM EST

...a bit like politicians? :)

Seriously, this looks like it'll finally solve what has been an excellent detective mystery for physicists. There has been plenty of "The Butler Did It!" from some otherwise reputable people in physics. Maybe if they watched less Agatha Christie and more Connections, this could have been solved sooner.

(At least one theory went along the lines of "the sun somehow eats the neutrinos, but still undergoes fusion at the same time".)

A (provisional!) well done to those concerned! (Well, they have to actually -check- the findings, but this sounds a highly plausable theory.)

Two questions? (3.80 / 5) (#2)
by DesiredUsername on Mon Jun 18, 2001 at 02:28:19 PM EST

1) What causes neutrinos to change type?

2) Why do they change non-randomly (net change away from electron)?

Play 囲碁
Maybe... (4.80 / 5) (#9)
by SIGFPE on Mon Jun 18, 2001 at 03:23:03 PM EST

One model goes like this:

You have three types of neutrino. At any moment in time you can describe the state of a neutrino by three numbers forming a unit vector which tell you how much like each of the three types the neutrino is. So if it was an electron neutrino you might represent it by (1 0 0) or if it was an equal superposition of muon and tau neutrinos it might have a unit vector proportional to (0 1 1). When you actually come to observe a neutrino to see which of the three types it is the probability of getting each time is the square of the absolute value of the corresponding component.

Now think about a gyroscope precessing. The direction of the axis is also described by a unit vector and the x, y and z components osciallte back and forth. If you start off aligned with the x axis eventually a bit of y and z might come in and then disappear is it oscillates back to x.

Well the equations of motion describing gyro precession and neutrino type oscillation may be very similar. There is no special preference to move away from electron - that's just what the fusion reactions start you off at. Eventually you might expect the neutrinos to settle down to a dynamic equilibrium with a constant proportion of each type oscillating back and forth.

So in summary the hypothesis is that the state drifts between mixtures of all three types just like the way the 3 components of the axis of a gyro oscillates.
[ Parent ]

follow-up question (3.66 / 3) (#11)
by speek on Mon Jun 18, 2001 at 05:53:14 PM EST

But, the theories about the sun still assume the sun is producing electron neutrinos, and that they change on the way to the earth, right? If so, wouldn't the resulting ratio's of neutrino types detected be other than an equal distribution? I mean, wouldn't you expect the ratio of the input neutrinos to have some effect on the ratio of the output?

al queda is kicking themsleves for not knowing about the levees
[ Parent ]

Yes, but it takes a while... (4.66 / 3) (#12)
by mcherm on Mon Jun 18, 2001 at 06:56:02 PM EST

I mean, wouldn't you expect the ratio of the input neutrinos to have some effect on the ratio of the output?

Yes... but how big of an effect? I'm really not sure about neutrinos, but I seem to remember that things are so dense and hot in there that photons generated in the core (where most of the fusion is going on) take on the order of hundreds or thousands of years to make their way to the surface. Now, for neutrinos, I'd expect it to be MUCH better (after all, they hardly interact) but there still might be enough interaction to slow them down some and give time to change.

Besides, 4 light minutes is a long time (well... distance, but distance and time have the same units anyway) on an atomic scale, so perhaps it's large in comparison to the neutrino conversion time.

Of course keep in mind here that I have only an undergraduate degree in physics and even that is seriously outdated, so I probably have no idea what I'm talking about.

-- Michael Chermside
[ Parent ]

Neutrino interactions (5.00 / 1) (#16)
by DoubleEdd on Tue Jun 19, 2001 at 03:27:46 PM EST

The neutrinos virtually all come straight out. The interaction is incredibly weak, and that is an understatement. The kind of absorber you need to have interactions with half the neutrinos passing through it is something like five light years of solid lead. An incredibly small fraction will interact on the way out, but the overwhelmingly vast majority come out without interacting, and indeed pass through the SNO and the rest of the planet without interacting. There isn't a significant slowdown on the way out.

You're right about photons though. It takes forever (for small values of forever) for them to get out to the surface of the sun.

[ Parent ]

Yes (4.33 / 3) (#13)
by SIGFPE on Mon Jun 18, 2001 at 07:41:41 PM EST

The input ratio is (1 0 0) and the output is a function of this. But without knowing the conversion rates I have no idea how to fill in any further figures. I did look up some papers about potential neutrino oscillation models but the SNO paper explicitly says its results are inconsistent with at least one model so I don't think it's worth pulling any figures out of there as they might be wildly off. However in all likelihood the conversion is exactly like gyro precession with a sine wave type pattern going on.
[ Parent ]
Nice... (2.00 / 3) (#3)
by univgeek on Mon Jun 18, 2001 at 02:40:29 PM EST

But it would help to have some more explanation of the results or theory, or direct links to where we can learn about the theory.

Arguing with an Electrical Engineer is liking wrestling with a pig in mud, after a while you realise the pig is enjoying it!

More about the theory... (2.75 / 4) (#4)
by SIGFPE on Mon Jun 18, 2001 at 02:51:12 PM EST

You can read the published paper itself if you look at the results link I posted in the story. It discusses how the rate of conversion is calculated, the error estimates and a whole lot of other stuff. I actually tried to write more details than is actually contained in the press release which just says "the neutrinos change type".
[ Parent ]
the actual detectors. (2.00 / 3) (#5)
by garlic on Mon Jun 18, 2001 at 03:03:21 PM EST

I may be confusing my physics particles, but I thought neutrinos didn't interact with mass. Because of this, neutrino detectors were huge vats of some liquid (i forget which) that were stuck in old abandonned mines. Is this recalled correctly? What am I missing?

HUSI challenge: post 4 troll diaries on husi without being outed as a Kuron, or having the diaries deleted or moved by admins.

the detectors... (4.00 / 4) (#6)
by ana on Mon Jun 18, 2001 at 03:15:59 PM EST

Quoting from the paper (which you can get from the linked page):
SNO is an imaginv water Cerenkov detector ... It features 1000 metric tons of ultra-pure D2O contained in a 12 meter diameter spherical acrylic vessel. This sphere is surrounded by a shield of ultra-pure H2O contained in a 34 m high barrel-shaped cavity... A stainless steel structure 17.8 m in diameter dupports 9456 20-cm photomultiplier tubes with light concentrators...

Translation: big, honking, sucker.

Years go by; will I still be waiting
for somebody else to understand?
--Tori Amos

[ Parent ]

D2O (4.50 / 2) (#10)
by sigwinch on Mon Jun 18, 2001 at 03:59:13 PM EST

It features 1000 metric tons of ultra-pure D2O contained in a 12 meter diameter spherical acrylic vessel.

IIRC, the D2O is worth $300M and is on loan from a nuclear reactor operators consortium.

I don't want the world, I just want your half.
[ Parent ]

Note link (4.40 / 5) (#7)
by rusty on Mon Jun 18, 2001 at 03:19:24 PM EST

The SNO link explains what a neutrino detector is and how it works. They do interact with mass, but they're so small that they tend to miss nearly every thing you try to put in front of them. Luckily, the other effect of this property is that they can pass undisturbed through quite a lot of rock and other stuff, whereas cosmic rays and other things that could give false-positives in a detector can't. So they put detectors deep underground to filter out all that other noise.

The site linked above really explains it much better than I can though. :-)

Not the real rusty
[ Parent ]

It's not so much that they're small... (3.50 / 2) (#15)
by SIGFPE on Mon Jun 18, 2001 at 08:31:27 PM EST

...but that the force through which they interact is so weak. They can sail right through the heart of an atom's nucleus without so much as blinking. As far as we know an electron is point sized but it can't easily pass through a hunk of matter.
[ Parent ]
you're right (4.00 / 4) (#8)
by a humble lich on Mon Jun 18, 2001 at 03:22:15 PM EST

Neutrinos interact very weakly, so even though there is a huge flux of them containing a vast amount of energy, they don't do much. They are detected by looking at huge tanks of water. If a neutrino happens to collide with an electron from the water, it accelerates the electron to faster than the local speed of light. It is that shock wave which is seen. The tanks are deep underground to reduce the contamination from cosmic rays.

The Sudbury observatory mentioned in the story is 6800 feet underground and has a tank containing 1000 tonnes of heavy water

[ Parent ]

One correction (4.00 / 2) (#14)
by a humble lich on Mon Jun 18, 2001 at 07:52:12 PM EST

Looking at sno's web site I was almost right. It seems that most events are from the neutrino colliding with the deuterium nucleus which results in a high energy electron being emitted via the reaction

ne + n -> p + e

Neutrino-electron collisions also occure but they are only abouyt 5% of the total events. Details are at http://www.sno.phy.queensu.ca/sno/sno2.html#interactions

[ Parent ]

One of the greatest mysteries of physics solved | 16 comments (16 topical, 0 editorial, 0 hidden)
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