Neutrino Mass Confirmed

biohack writes "BBC News reports that results from the MINOS experiment have confirmed that neutrinos have mass. To look for neutrino oscillations, scientists created muon neutrinos in a particle accelerator at the Fermi National Accelerator Laboratory (Fermilab). After passing through a particle detector at Fermilab, a high intensity beam of neutrinos travelled to another particle detector 724km (450 miles) away in a disused mine in Soudan, US. The set up established that fewer particles were being detected at the Soudan site than had been sent from Fermilab, which confirmed that some neutrinos changed their flavor on the way - an effect called neutrino flavor oscillation, which requires them to have mass. 'To put it simply, if they are heavy, it means that there is a lot more mass in the Universe than we thought there was,' said Professor Jenny Thomas from University College London."

Comments

Soudan, US

[Wyatt Earp]

Thats is sloppy on the BBC's part, they should have put the State in there. In this case it is Minnesota.

http://www.dnr.state.mn.us/state_parks/soudan_unde rground_mine/physicslab.html

Re:Soudan, US

[Bonker]

The US is a federation of 50 sovereign states (each with the size and economy to match), and saying "Foo City, US" would be like saying "Foo City, EU" (though Europe has the advantage of many languages to broaden the name space).

While this is true, it's somewhat misleading, especially to those will limited knowledge of U.S. history or government. Even many Americans don't understand the difference between as state and a province.

State governments in the U.S. function approximately equally to provincial governments in countries that are not federations. Most of them were not originally independant countries, but were instead provinces and territories that were sponsored into statehood.

A significant fraction of the United States were indeed independant countries at one point. ALL U.S. states have significantly more rights than any given province. Each has its own constitution and government, and, contrary to popular opinion, the states elect the President and Senators. The U.S. president is *not* elected by a popular vote. (Although there have been calls to change this.) A few, most notably Texas, still claim the right to secede from the Union, although no state has really had this right since the end of the American Civil War in the late 1800s.

The U.S. constitution sets up the states as individual entities, unlike provinces. They can each impose their own taxes and own laws. In fact, this is one of the major contentions in our government to this day. States can theoretically impose any law that the constitution doesn't reserve for the Federal government. This causes a lot of conflict and consternation since States are also required to respect contracts formed in other states, frequently under a different set of laws and regulations.

The conflict over gay marriage contracts is one of the more recent flaps this has caused.

States can also each maintain their own militias. Many states have 'State Troopers', who usually do the same kind of jobs as normal policemen, albeit with greatly expanded jurisdiction. A few states have 'State Guards', although they usually don't server a military purpose. They usually come to the fore during natural disasters and the like.

While the U.S. is an extremely tight federation-- the word 'Union' is very accurate-- it is still a federation. Each state is indeed its own nation.

Re:Already Known

[rewinn]

... as claimed in 1998 Scientific American article

Re:Dark Matter

[syntaxglitch]

http://en.wikipedia.org/wiki/Dark_Matter#Compositi on
http://en.wikipedia.org/wiki/Hot_dark_matter

...short answer is: yes it has been considered, but current models of neutrino formation suggest they can't account for all dark matter (or even a significant component of it).

Re:Pardon me, but. . .

[bcrowell]

A hundred years ago, physicists generally classified things like this:

• Matter has mass and is made of particles.
• Light has no mass and is made of waves.

Nowadays it's more like this:

• Fermions are wave-particles that have half-integer spin. Atoms are made of fermions.
• Bosons are wave-particles that have integer spins. Bosons are the things that carry forces.

All the familiar, everyday fermions have nonzero rest mass, and the only familiar, everyday boson -- the photon -- has zero rest mass. However, there are bosons that have nonzero rest mass (e.g., gluons), and it's also possible that there are fermions that have zero rest mass. (Experiments so far only measure the differences between masses of different types of neutrinos, so it's still possible that the electron's neutrino has zero mass.)

Implications regarding the Standard Model?

[TechnoGuyRob]

This is a very interesting conclusion. I am currently taking a modern physics II class at a college in my town, and I live 15 minutes away from Fermilab. In fact, our professor is a scientist at Fermilab that only comes in this term to teach our class. The interesting question, though, is (and I know it's small), what is the exact mass that they obtained (if any so far)? Of course, this would have to be given in eV (electron volts), but assuming it's very small (~E-3 eV) (EDIT: I just looked at the press release linked to at the end of this post, and indeed, it is on that scale!), this could prove to have some interesting conclusions. I actually found this passage in the article that explains it better than I could:

"In particle physics there is the Standard Model which describes how the fundamental building blocks of matter behave and interact with each other," explained Dr Falk Harris.

"And this model tells us that neutrinos should have no mass. So the fact that we have now got independent measurements of neutrinos saying that they must have mass, means that this Standard Model is going to have be revised or superseded by something else."

This is very interesting because of its possible re-affirmation of Wikipedia. I'm not going to take out my string theory book right now to see if calculations of a positive neutrino mass correspond to any viable conception in string theory, but a re-affirmation and eventual proof of string theory could spur as great of an innovation as the concept of an atom.

We'll have to wait and see, but for anyone who would like more information, Fermilab's website has an article about the discovery.

Re:Implications regarding the Standard Model?

[honkycat]

Basically, at this point, they've got measurements of the difference between the masses of the three flavors of neutrinos. They also have an upper bound of, IIRC, around 0.7eV (not from this experiment) for the absolute value of the neutrino mass. The delta sets the lower bound (if one flavor were at zero mass, the heavier ones must be at least the delta heavier).

The mass deltas are known as squared values -- the sign is unknown, so there's the question of overall mass scale plus the ordering of the various flavors.

Re:This is new?

[bcrowell]

Yes, this is a confirmation of something that had already been shown by one experiment.

The experiment was similar and involved muon neutrinos changing flavors to electron neutrinos in a large particle accelerator.
No, it wasn't an accelerator, and the experiment wasn't similar.

The real question is how many eV are the combined masses of the three flavors? The answer to that question portends much for the state of the universe.
No, not really. Not unless the mass of the electron's neutrino is surprisingly large compared to the mass differences among the different types of neutrinos.

Re:explanation about oscillation/mass relationship

[Anonymous Coward]

Okay, as a particle physicist, I learned about this in terms of the Hamiltonian evolution of a wavefunction, and some analogy to neutral kaons, and a page of math. But thats not what you wanted to hear.

A physicist on the recent Nova special "The Ghost Particle" (Maybe it was Boris Kayser) had a nice explanation. If neutrinos have no mass, then they travel at the speed of light. If they travel at the speed of light, then they would not experience "time". Since changing flavor is a process that takes time, or duration, or something like that (this previous clause is maybe a non-trivial thing to say), then if neutrinos change flavor, they must experience time, so they must travel slower than the speed of light, so they must have some mass.

A Sad note

[stox]

This may be one of the last discoveries at Fermilab. As it stands now, Fermilab, SLAC, and Brookhaven's future is in severe doubt.

http://www.sciam.com/article.cfm?chanID=sa006&arti cleID=00080A6A-C9C7-1419-89C783414B7F0101&colID=2

Umm...a couple of corrections

[kf6auf]

Bosons don't necessarily carry forces; in fact not all atoms are fermions. For example, the Helium-4 and Carbon-12 nuclei is a boson. See wikipedia. Bosons are best defined as having integer spin and being capable of sharing the same quantum state while fermions have half-integer spin and obey the Pauil Exclusion Principle (cannot share the same quantum state). A composite particle of an even number of fermions (2 protons + 2 neutrons) is a boson (helium nucleus) but an odd number of fermions is always a fermion.

I also believe that physicists have determined that the electron neutrino has a mass of about 1meV-1eV (from a slide I saw in lecture a couple days ago).

In addition, physicists divide fermions into quarks and leptons, which are supersets of the elementary particles that make up nucleons and electrons.

Re:*shakes head*

[honkycat]

They have two detectors. One very near to the source, one very far away. The near source measures many more hits than the far source does. Thus, they know they're being produced in larger quantities than they're being received in. Compared to a model of the test configuration assuming no oscillation, there are about 33% too few hits on the far detector as compared to the near. This amounts to a 4 or 5 sigma detection of the missing neutrinos (in other words, there is approximtely a 0.7%-1.8% chance that this is due to a statistical coincidence). It's typically at 2 or 3 sigma that you start making a confident announcement of a discovery, so a 4 or 5 sigma confirmation of an already reported result is very, very strong evidence.

They don't yet have enough data to rule out some alternative explanations. At this point, though, neutrino oscillation (and mass) would really be the simplest, least "out there" explanation. These experimenters would like nothing more than to find that even the oscillation theories don't explain the data. That would open a whole new field of inquiry and possibly lead to Nobel Prizes.

If you're techincally inclined, read about the Minos results straight from the horses' mouths.

The seminar talks go into a fair bit of detail about their data analysis, which included "blind analysis." In other words, they kept a significant (and unknown until the end) fraction of their data secret from those doing the analysis. Using the other fraction, they went through their testing procedures -- figuring out how to detect false events, how to deal with various , etc -- using a limited piece of the data. Once they were confident that they had done everything correctly, they opened the whole data set and ran their procedure without changing it.

This protected them from tainting their data by, e.g., throwing out data points that didn't match expectations. That is a common problem, even among good scientists. It's very easy to subconsciously make decisions that bias your results toward the expected answer.

Anyway, I am a physicist, and I think you should believe these guys. Everything I've seen indicates they've done a good, careful job with the experiment.

simple explanation

[alexander m]

have a look at this. it's the transcript from the BBC's recent "horizon" show, called "project poltergeist", which is on precisely this topic (neutrinos having mass). very neatly explains to a lay audience what the mystery is, and also answers exactly your specific question. it's not a long read, maybe 10mins max, and as it's the transcript to the show it leads you through the topic in a well thought out manner http://www.bbc.co.uk/science/horizon/2004/polterge isttrans.shtml and the short answer to your question is as follows: in order to undergo neutrino oscillation, the neutrino must be capable of change. to be capable of change it must experience a personal sense of time. if it was travelling at the speed of light, it would have no sense of time. objects with mass cannot travel at the speed of light (infinite energy required for objects with mass to do this). therefore, as we experimentally can confirm neutrino oscillation, we are also confirming that neutrinos have a sense of time, which implies they are not travelling at the speed of light, which implies they have mass. hope that clears it up -- on a side-note my first degree was actually in astrophysics, at University College London (UCL), where the article's quoted scientist comes from... didn't have her for any of my lecures though ;)