Experiment Shows Neutrinos Have Mass

Tuzanor writes: "Physicists have found the most convincing evidence yet that neutrinos, subatomic particles that were thought to have no mass whatsoever, actually do have a very tiny mass after all. The story is at Yahoo!" We mentioned the experiment yesterday, but this is big news. The New York Times has a thorough article on the whole experiment and its meaning.

Comments

Re:Not just that they have mass...

[Sheridan]

smitty825 quoth:

If they have mass, then we must include that mass in all calculations, but for some reason they don't want to :-)

The current standard model does not predict the masses of neutrinos, but its equations are simpler if neutrinos have no mass.

That's like saying calculating the velocity of an object is easier to calcuate if we don't count friction!

Just because physically observed particles have mass, it is not necessarily required that the theory has particle masses in its bare Lagrangian form from which the perturbation theory Feynman rules are determined. (And I'm not talking about the Standard Model's Higgs Mechanism for mass generation by spontaneous symmetry breaking - which is another thing altogether...)

Non-perturbative calculations using the Schwinger-Dyson equations, Ward identities and renormalisability constraints show that masses can be generated dynamically through interactions of massless fields.

Some (8-10 year old) references can be found via this HEPDATA query. Note that this is not talking directly about neutrinos, but rather about generating masses for electrons in a simplified version of QED in which electrons start out massless.

There are almost certainly some newer papers that you could find either at HEPDATA or SPIRES.

(Full Disclosure: Mike Pennington was my Ph.D. Supervisor, although I didn't work in the non-perturbative SD equations field myself except for a short while at the start)
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Re:what about the velocity?

[gorgon]

Well, the mass of neutrinos is so small and they interact with other matter so sparing that its almost impossible to tell the difference between their speed and c. The typical total energy of a neutrino is going to be at a thousand times (and probably a lot more) times its rest mass. A particle with a total energy 1000 times its rest mass has a speed of about .9999995 c, which as you can imagine, is pretty hard to differentiate from 1.0 c .

So, neutrinos don't travel at c, but its pretty darn close.

--
I hope we shall crush in its birth the aristocracy of our monied corporations ...

Neutrino IMAGINARY rest mass shown a decade ago

[Kevin S. Van Horn]

I find these indirect indications of neutrino mass quite amusing, as they seem to be tiptoeing around some odd results found about a decade ago. I originally read about this in a science fact column by physicist John G. Cramer in Analog magazine. Some experiments were run that measured the square of the neutrino rest mass. The initial experiments had an error bar that overlapped zero, but was mostly negative. Later experiments had an error bar that lay entirely below zero. That is, within experimental error, they had measured a negative squared rest mass for the neutrino, implying an imaginary rest mass for the neutrino, which would mean that neutrinos are in fact tachyons. (Tachyons are hypothetical particles that can only go faster than light, and the higher their energy, the slower they go, so that high-energy tachyons approach the speed of light from above.)

Cramer quotes an anonymous source as saying that if the sign of these numbers had been reversed (positive instead of negative), there would have been a big press conference announcing that they had shown the neutrino to have a nonzero rest mass.

I sent email to Cramer maybe five years or so later, asking what had happened with these results. He told me that nothing had happened; there has been no followup, and nobody has shown them to be wrong.

The super-Kamiokande experiment seems to have been carefully designed to show nonzero rest mass for at least one kind of neutrino while yielding no information on the actual value of the squared rest mass (in particular, its sign.) This experiment measured only the difference in squared rest masses between two types of neutrinos. (If this difference is nonzero, then one of the two neutrino types must have a nonzero squared rest mass.) It is consistent with either a positive or negative squared rest mass.

This latest result also carefully avoids the issue of the actual value (and sign) of the squared rest mass. It appears that everybody wants to get their Nobel for showing that the neutrino has a nonzero rest mass, but nobody wants to be labeled as a crank for presenting data that would indicate the neutrino has an imaginary rest mass!

Re:Neutrino IMAGINARY rest mass shown a decade ago

[daknapp]

The initial experiments had an error bar that overlapped zero, but was mostly negative. Later experiments had an error bar that lay entirely below zero. That is, within experimental error, they had measured a negative squared rest mass for the neutrino, implying an imaginary rest mass for the neutrino, which would mean that neutrinos are in fact tachyons.

As one of the authors of the result to which you refer, I can authoritatively say that Cramer managed to get it almost completely wrong.

In fact, there was a paper written by Stephenson that showed that the result could not come from tachyonic neutrinos. In that case, we would have seen a completely different signature.

The most likely physical explanation for the result would have been another unknown particle. Lobashev still believes in that, but I think the evidence has accumulated that there is no significant excess at the endpoint of the tritium spectrum.

Of course, it is worth pointing out that nobody has ever found any error in the original data, and we spent something like 2 years trying to find problems before we published it in the first place!

what about the velocity?

[Mr. Slippery]

So this this mean that:

• neutinos have a non-zero rest mass, and previous ideas that they travel at c are incorrect,
• neutinos have a non-zero rest mass, and travel at c, screwing up special relativity, or
• neutinos are some special case, they somehow have a mass but not rest mass

Please, some physics geek tell us how to resolve "neutrinos has mass" with "neutrinos travel at c.

Tom Swiss | the infamous tms | http://www.infamous.net/

See SNO's homepage for more

[ErfC]

The Sudbury Neutrino Observatory homepage has their own article about the results. The full paper that they submitted to Physical Review Letters is also avilable online.

-Erf C.

Re:Not just that they have mass...

[ErfC]

Furthermore, it is this transformation that proves that they have mass.

Not only that, but the different neutrino flavours must have different masses in order to oscillate. The fact that they have mass at all is the most exciting bit, of course, but the fact that they're all different is pretty cool, too.

-Erf C.

Umm, yeah

[aozilla]

"On June 5, 1998, the Super-Kamiokande collaboration announced discovery of evidence for neutrino mass at the Neutrino '98 conference, held in Takayama, Japan."

poetic license?

[aozilla]

Neutrinos, they are small.
They have no charge, they have no mass.
They do not interact at all.
The Earth is just a silly ball

to them through which they simply pass
Like photons through a sheet of glass
Or dustmaids down a drafty hall.
They snub the most exquisite gas,
Insult the stallion in his stall,
Cold-shoulder steel and sounding brass
And pass, like tall and painless guillotines,

through you and me into the grass.
At night they enter Nepal
And pierce the lover and his lass

from underneath the bed.
You call it wonderful? I call it crass.

- John Updike

Dark matter v. anti matter

[gilroy]

Anti-matter: matter with opposite spin, helicity(? I think), and especially, charge. (Other quantum numbers, such as baryon number, are reversed, too.) Anti-matter particles are like mirror images of "normal" matter. As far as we know, no large sources of anti-matter occur in the Universe (although we can make some on Earth and some decay processes -- like the potassium in your banana -- create some).

"dark" matter: Matter that, for one reason or another, is not luminous. There is growing evidence that we cannot see all the matter in the Universe. The best evidence, IMHO, comes from studies of galaxy rotation, which show that galaxies are not rotating in a so-called "Keplerian" manner. A collection of particles orbiting a central mass should have a velocity that falls off (as 1/sqrt(r)) with distance. Saturn's rings do this, for example, which was how they were proven not to be solid.

It turns out that galaxies (which are rotating systems) do not obey this relation, as one might expect (since most of the luminous matter is contained near the center). This implies that there is something else "adding" mass as we travel out from the center of a galaxy. We can't see it, so it must be dark.

There are also cosmological arguments for dark matter. Most especially, there's a paramter (called Omega) which is the ratio of the Universe's density to "critical density". If Omega 1, the Universe will eventually collapse under the gravitational attraction of its elements. Observation of luminous matter indicates that Omega = 0.1. For a long time people had a bias that Omega should be exactly, leading to the claim that 90% of the matter must be "dark". Since that number agrees reasonably well with the one from galaxy rotations, people saw these as mutually supporting each other. (For the record, I find the cosmological evidence quite unconvincing.)

So, once you believe there is dark matter, you start to wonder what it's made of. In essence there are three classes of candidates:

1. Regular, but cold, baryonic matter. This could be gas clouds, failed stars, burnt-out embers, etc. After all, things only glow if they're hot enough. Observed stars, etc. ("luminous baryonic matter") seem to have Omega_luminous about 0.01. Limits from Big Bang nucleosynthesis (the formation of elements in the creation of the Universe) seem to limit all "normal" matter to Omega_baryonic = 0.1.
2. Neutrinos. We can estimate the flux of comsic neutrinos in the sky independently of their mass. Now that they've been shown to have some, we can estimate the total mass of neutrinos zipping around the Universe. According to the article, Omega_neutrinos is about 0.18.
3. WIMPs: weakly-interacting massive particles. Since they're weakling interacting, they'd be hard to detect. These would be new particles, so far undiscovered, and they would involve that highest of objects, new physics. Candidates include axions, supersymmetric partners, and other more esoteric items.

No matter what the "dark matter" is, it will likely consist of anti-matter conjugates as well.

Not just that they have mass...

[garett_spencley]

but the experiment also proves that electron neutrinos can also transform into other forms of neutrinos.

One of the biggest astronomical mysteries was why the sun was not producing anywhere near the predicted amount of electron neutrinos. This experiment proves that it is in fact producing them, but that 60% of them transform into other neutrinos before reaching the earth.

Furthermore, it is this transformation that proves that they have mass.

From the article:

But on Monday, representatives of the Sudbury Neutrino Observatory in Canada announced that neutrinos made by nuclear reactions in the sun's core change from one type to another during their 93-million-mile journey to Earth. And only particles with mass can change form.

--
Garett

Summary, for the non-physicists:

[mblase]

• A thirty-year-old theory about how fusion takes place in the center of our Sun has been validated.
• About 18 percent of all the "dark matter" in the universe may now be made up of neutrinos.
• The standard model held by particle physicists, which up until now assumed neutrinos were completely massless, will have to be revised.

Re:Summary, for the non-physicists:

[Dave Bailey]

A few quick physics for the non-physicists points:

• The Dark Matter problem gets far more fun than this: This result shows that between 0.1 and 18% of the critical density of the universe (i.e. what is needed so the universe just expands for ever - more than this and it will collapse...) can be neutrinos. What we can see (stars mainly), makes up 10% of this. The big question still is, what is the rest... (see here and references contained within for further information.)
• This result means neutrinos do have masses but we don't know what they are. We only know what the mass differences are (which determines the probability that they will have oscillated by the time they reach the detector) and that they must be small. What causes these masses to be small (new particles, extra dimensions...) is the next big question...
• If neutrinos have mass, then, according to special relativity, they can't travel at the speed of light. However, if they are very light (which they are) and have a much higher energy than their mass (which they do), they will travel very close to the speed of light.

Oh yes, maybe Canadians' don't make decent beer but at least they don't try to pass "lager" (weak fizzy pale stuff) off as beer. Give me an Old Peculier any day... ;)