# Neutrino rest mass

1. Jun 7, 2004

### TeV

Back in 1960's ,one could frequently hear opinions/claims that neutrino rest mass is 0.Not a very small one ,but exactly=0.Therefore,a nutrino would just like photons propagate at light velocity through the space.
What has changed in a meantime (since decade -two), except better insight to Standard model ,that physics community completely abandoned such opinion and many are in search for a small ,but yet nonzero rest mass of the particle?

2. Jun 7, 2004

### arivero

I read/heard there is still an intermediate possibility: neutrinos getting mass due to renormalisation -radiative corrections-, but zero in the mass matrix.

The change of opinion comes, surprisingly, from experimental input.

3. Jun 7, 2004

### jnorman

it would seem that zero rest mass would either make the neutrino some type of boson, or a new type of particle alltogether, which would seemingly violate the standard model. this debate has gone on for a while, and it used to be a standing joke at one of my early favorite forums (ask dr neutrino) that someone would regularly inquire as to the mass of a neutrino. any way you look at it, neutrinos are weird little critters...but then again, what in the subatomic realm isnt wierd as hell anyway?

4. Jun 7, 2004

### demosthenes

http://physicsweb.org/article/world/11/7/3/1

That gives a report of the SuperKamiokande experiment, in which the mass of a neutrino was found to be around 0.1eV (whatever an eV is)

http://cupp.oulu.fi/neutrino/nd-mass.html

and that provides some info on the masses of various types of neutrino. Not sure how scientifically valid it is (as you probably guessed from my lack of knowledge about the eV, I'm not the most scientifically knowlegeable person around). Fascinating stuff, though.

5. Jun 7, 2004

### DrChinese

Good point, TeV. Now that the tide has turned on the neutrino's mass due to the (admittedly skimpy but tangible) evidence, what does theory say about why the mass does not hold it back from moving at c? And are there other particles with similar attributes?

6. Jun 7, 2004

### jnorman

well now, we dont really have any evidence that neutrinos actually propagate at C, do we? and no, there is no evidence of any particle having non-zero mass being able to propagate at C.

7. Jun 7, 2004

### DrChinese

The neutrinos from SN 1987A are strong evidence that neutrinos propagate at c. They arrived here just a few hours before the photons (which do travel at c) from the supernova. The photons were delayed due to scattering against the contents of the star, while the neutrinos basically don't scatter against anything other than a black hole (in which case we dont see them either).

8. Jun 8, 2004

### Olias

I believe the resolution to the problem is still being speculated, and in a 'twist-of-fate' kind of way Neutrino's are thought to have a oscillation pattern to their detection?

Here for instance is a good explination:http://www.ps.uci.edu/~superk/oscillation.html

Some early posts:http://www.phys.hawaii.edu/~jgl/nuosc_story.html

And a link of current sites:http://www.hep.anl.gov/ndk/longbnews/0405.html

Last edited: Jun 8, 2004
9. Jun 8, 2004

### TeV

Not surprising.In mass matrix their contribution is so minute that it can be freely neglected,while in the decays and other related processes they still carry out observable momentum & energy.
My impression is that change of view on neutrinos rest masses has its' origin in extension of minimal standard model (evolving of the Standard model).Especially,after recent attempts to introduce Higgs field mechanism to explain how particles acquire masses.
Also,it seems cosmologists like the idea of nonzero rest mass of these particles that could play significant role in the problem of "missing mass" of the Universe.

10. Jun 8, 2004

Staff Emeritus
I believe the original reason for assuming neutrinos have mass was the solar neutrino deficit. An explanation was given that the three flavors of neutrino interchanged identities on the way from the sun, so that detectors only able to see electron neutrinos would only see a third as many as there should be, which agreed with the experiments.

But in order to interchange, the neutrinos had to have time on their flight from the Sun to the Earth, and relativity required that they be massive in order to do that, since of course a massless particle experiences no proper time on its flight.

11. Jun 8, 2004

### TeV

Yes,neutrino oscillations.That's a very good reason to suspect neutrinos have nonzero rest mass.However, I guess ,in 60's and 70's they could have good enough detectors to observe the solar neutrino deficit.And yet,many respectable refferences from this time (including University of Bekeley Physics Course 1965/67) mentioned nutrinos' rest mass is identical to photons (=0).
So,I conclude that theory of extended minimal SM and beyond it,with possibilities like violation of lepton number and similar oddities gave tool to the prevailing opinion today.

12. Jun 8, 2004

### LURCH

Although the Solar neutrino deficit may have been detected at that time, the explanation (that neutrinos ocsilate) had not been proven. Now that it is widely accepted as proven that they do ocsilate, this ocsilation requires energy, and if they have energy, they have mass. Or so the general reasoning goes.

13. Jun 8, 2004

### kurious

Do electron neutrinos coming from the sun interact with photons and change their mass?

14. Jun 8, 2004

Staff Emeritus
I don't think so. The oscillation theory is now quite well developed. The wave functions of the electron, mu, and tau neutrinos are in a matrix which allows them to swap.

15. Jun 8, 2004

### DrChinese

So how do they end up moving so fast if they have any mass? They must be moving very, very close to c on the lower limit of their velocity, based on SN1987A. Or c itself on the upper end.

In other words, it seems like a new mechanism must allow this particle to have both rest mass and a velocity near c. I guess if the mass was small enough, the neutrino could be accelerated to .999999c and we might not notice that it was moving a bit less than c. Does anyone know a bit about this? I looked at some of the references above, and they talk primarily about the mixing angles for oscillations.

16. Jun 9, 2004

### Nereid

Staff Emeritus
Would make a good case study on how science actually works, in the real world.

The solar neutrino deficit (or problem) was indeed observed way back in the late 60s. However, the experiment was difficult, the signal in the data weak, and full confidence in solar models lacking. So the simpler thing for textbook writers was to stick with the view from the then accepted theory. As the decades rolled by the experiments got better (more neutrino energy regimes observed, different detection reactions, etc), the solar models got way better (partly due to better data about the Sun), and the neutrino collision cross-sections got nailed down with decent precision. Or, to say it another way, all the alternative explanations of the data (no real signal, poorly understood Sun, wonky neutrino cross-sections, etc) were eliminated, leaving only 'we misunderstood the physics of the neutrino'. But, without the dogged persistence of Davis and Bahcall, would we have had to wait another three decades to have solved this little puzzle?

Dr Chinese: if you plug in an estimate of neutrino mass and observed energy, for SN1987 neutrinos, or solar ones, you will see how close to c they must be travelling.
Make that past tense! As data from large scale structure surveys and observations of the CMBR came in, and as more computer power was used on cosmological models, it became clear that the universe doesn't have a particularly large "HDM" component (HDM = hot dark matter, ie matter which doesn't interact with ordinary matter (much) and is flying around at near-light speeds); the leading candidate for any HDM - if it had been necessary - was non-zero mass neutrinos.

IIRC, the latest results from WMAP indicate HDM comprises ~<4% (?) of baryonic matter , and if you assume they're neutrinos (and only three flavours), then you get an upper estimate of the mass of the neutrino!

17. Jun 9, 2004

### kurious

I don't think so. The oscillation theory is now quite well developed. The wave functions of the electron, mu, and tau neutrinos are in a matrix which allows them to swap.

KURIOUS:

What is the theoretical justification for this matrix?
If it couldn't be derived in advance of "oscillations" being discovered, it could just
be "curve-fitting."

18. Jun 9, 2004

### bkfizz02

Kurious:
The matix that leads to the idea of oscillations is the full Hamiltonian. When we talk about different flavors of leptons (electron, mu, tau), we are implicitly talking about the eigenstates of the weak hamiltonian. If you consider the full Hamiltonian and the idea that there at least two non-zero mass neutrino eigenstates, it can be shown that the weak eigenstates are not eigenstates of the full hamiltonian.

In other words, the justification for the matrix is that you are changing from one basis in which the eigenstates are the flavors into another basis in which the eigenstates have specific masses. In the old view, the masses were 0, the mass differences were thus 0, and all off-diagonal matrix elements were also 0. An electron neutrino is an electron neutrino is an electon neutrino.

Now, an electron neutrino can be viewed as a linear combination of mass eigenstates m1, m2, m3. As the wave function evolves according to the full hamiltonian, these components evolve with different rates. Thus future measurements of an electron neutrino could yield muon or tau neutrinos in the flavor basis.

I think the fact that some (at least 2) types of neutrinos have non-zero mass is pretty well established.

Additionally, it is my understanding that there were some doubts about the neutrino deficiency in 60's experiments because the predicted flux from the standard solar model had a nasty Temperature^4 term that really depended on how well we knew the core temperature of the sun. Since then, the Temp & SSM has been more rigorously checked and we can confidently say that there was indeed a neutrino deficiency (of course because the detector was sensitive only to electron neutrinos).

19. Jun 9, 2004

### Norman

This is misleading... what is the ratio of the neutrino-blackhole cross section to the... lets say... neutrino-nucleus cross section? I know the neutrino-nucleus cross section is on the order of $10^{-38} cm^2$. But if neutrinos don't scatter off anything but blackholes how do we detect them?

20. Jun 9, 2004