Neutrino masses and speed - Comments

[mfb]

mfb submitted a new PF Insights post

Neutrino Masses and Speed

Continue reading the Original PF Insights Post.

 

[PWiz]

The neutrino said "nice post!". The neutrino read the insights post.
#causalityjokes

 

[Greg Bernhardt]

Ditto, great work @mfb!

 

[vanhees71]

Yes, great overview about neutrino masses, mixing, etc.

 

[Orodruin]

Latest news in the neutrino sector: The first T2K anti-neutrino data was presented at the EPS-HEP conference this morning (see slides here).

Summary: T2K observes 3 $\bar\nu_e$ candidates. The expectation (including background) for $\delta_{CP} = -\pi/2$ and normal mass ordering is 3.7, with the expectation for any other combination of the neutrino parameters being larger. On its own, this is not yet enough to rule out the no-oscillation hypothesis in the anti-neutrino run, but the result is suggestively close to the expectation of the previous hint on $\delta_{CP}$ from the combination of T2K neutrino results with reactor neutrino data. Exciting times!

 

[Orodruin]

Another update: OPERA now has 5 $\nu_\tau$ events, finally reaching the $5\sigma$ level for the appearance of $\nu_\tau$s in a $\nu_\mu$ beam. It does not do much in terms of determining the oscillation parameters, but it is an important consistency check.

 

[ChrisVer]

@Orodruin With 3 anti-nu events, how can this be an exciting time?
Or is it going near the expectation for δcp=-π/2 and normal hierarchy?

 

[Orodruin]

@Orodruin With 3 anti-nu events, how can this be an exciting time?
Or is it going near the expectation for δcp=-π/2 and normal hierarchy?

Well, first of all, T2K is getting antineutrino data. Getting both neutrino and antineutrino data is important in looking for CP violation (naturally). Second, the hint for maximal CP violation from T2K neutrino data is (assuming standard oscillations) an up-fluctuation regardless of where you are in parameter space. This is further accentuated by the fact that there is some tension with reactor data. Until the antineutrino run, essentially all there was was an up-fluctuation which was most likely in the case of maximal CP violation. Now there is a number of antineutrino events which are not statistically significant on their own, but suggestively twinkling their eyebrows at the previous best fit. We will know more over the coming years as T2K accumulates more data in the antineutrino run.

Also, more events would have been less exciting. It is the fact that there are hardly any events which is the signature trademark of $\delta_{CP} = -\pi/2$.

 

[strangerep]

Current status
The current best direct upper limit on the electron mass comes from tritium beta decay: the mass has to below 2 eV.[2] Combined with mixing experiments, all three masses have to below 2 eV.

Typo? Should that be "electron neutrino mass" ?

 

[mfb]

Typo? Should that be "electron neutrino mass" ?

Oh sure. Fixed, thanks.

 

[Ygggdrasil]

Kajita and McDonald just won the Nobel prize in physics for their discovery of neutrino oscillations, which showed that neutrinos have mass. http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html

 

[ChrisVer]

Kajita and McDonald just won the Nobel prize in physics for their discovery of neutrino oscillations, which showed that neutrinos have mass. http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html

I think they didn't know to whom to award the prize... afterall the solar neutrino problem was the most important indication for neutrino oscillations and the nobel prize was awarded because of that.

 

[vanhees71]

This prize had already been given some years ago:

http://www.nobelprize.org/nobel_prizes/physics/laureates/2002/

I think this year's prize is very well deserved. It's among the most interesting discoveries in fundamental particle physics of the recent years (including the discovery of the Higgs in 2012 with Nobel prizes in 2013).

 

[ChrisVer]

This prize had already been given some years ago:

That's why personally I didn't find the prize of this year any more interesting than the one (worthfully) awarded in 2002.

 

[Oceans_Unmoving]

I'm having trouble understanding why oscillations prove that neutrinos have mass. Why couldn't a massless particle oscillate?
(I know nothing about particle physics, just heard about this prize today, and wanted to find out more.)

 

[ChrisVer]

Neutrino flavors (electron, muon and tau) are not mass eigenstates, but rather a mixture of mass eigenstates ν_1 , ν_2 & ν_3. As a result when you want to let the flavor neutrinos to propagate in time via the Hamiltonian (recall from QM that the state evolution with time is equal to the Hamiltonian operator acting on the state) you get for example:
$$nu_e(t)= e^{iHt} \nu_e (0) $$
now since the Hamiltonian is giving the energy when acting on mass eigenstates you're better to write the RHS of the equation in mass eigenstates and then you will obtain an exponential having the mass squared of the i-th eigenstate.
Finally as known from QM the probability of a transition from electron neutrino for example to muon is the square of the amplitude of those 2- that square brings in an additional mass squared of the j-th eigenstate and together with the first you get an o)cillation pattern for the probability governed by also the difference of the squared masses.
If they were zero (massless neutrinos), or I think degenerate (I'm not sure about this but I just put it to be checked) the oscillations would not occur and an electron neutrino would propagate as an electron neutrino forever. If there is any difference to the masses squared, oscillations occur and you have transitions.
So the fact that we saw missing electron neutrinos from the sun was telling us neutrinos are massive :p

 

[Orodruin]

I think they didn't know to whom to award the prize... afterall the solar neutrino problem was the most important indication for neutrino oscillations and the nobel prize was awarded because of that.

The 2002 prize was awarded for the detection techniques used in neutrino astronomy, not for any oscillation measurement. The Super-Kamiokande atmospheric neutrino result is widely considered the first evidence for actual neutrino oscillations, the solar neutrino problem could have had other explanations. By measuring the total flux of neutrinos of any flavour and comparing with the electron neutrino flux, SNO was able to confirm that neutrinos do change flavour on the way from the Sun.

One curious thing to mention is that the solar neutrino flavour conversion mechanism is not really oscillations per se, at least not for the high energy boron-8 neutrinos, but rather resonant matter enhanced flavour conversion. Essentially all boron-8 neutrinos arriving at Earth do so in the second mass eigenstate.

 

[ShayanJ]

I know that Bruno Pontecorvo is dead now and people can't give him any prize, but why no mention of his name?

 

[Orodruin]

I know that Bruno Pontecorvo is dead now and people can't give him any prize, but why no mention of his name?

He was mentioned in the detailed presentation by the Royal Academy, but this is not something that media picks up on. It is not usually the case to mention other people in the short motivation which in this case was "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

 

[ShayanJ]

He was mentioned in the detailed presentation by the Royal Academy, but this is not something that media picks up on. It is not usually the case to mention other people in the short motivation which in this case was "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

Would he share the prize if he was alive?

 

[Orodruin]

Would he share the prize if he was alive?

I think that is a hypothetical question that only the Nobel committee could answer if anyone - and even if they knew the answer they would not tell anyone. The first step would have been to have someone nominating him, but I think that would be a minor hurdle.

 

[vanhees71]

I think so, at least he should, because he was among the first (if not the first) having the idea of neutrino oscillations. To stress it again, I think the Nobel for SuperKamiokanda and SNO is long deserved (not to say overdue). I don't understand, how ChrisVer can doubt this. It's among the most exciting discoveries in HEP within about the last 20 years. Of course, there was the discovery of the $\tau$ neutrino in 2000 and of the Higgs boson in 2012 (leading to the well-deserved Nobel for Higgs and Englert in 2013), but this was not that surprising, because it's Standard Model Physics. The neutrino oscillations are really something new. The prize to Davis et al. was of course also well deserved and this indeed did not cover the discovery of the neutrino oscillations themselves, but I think that the understanding of where the solar-neutrino deficit came from was a prerequisite to give the prize to Davis et al in 2002, because before there was still some doubt about the validity of the measurements. With the SNO results, showing that with neutrino oscillations taken into account the solar-neutrino deficit was completely explained. It's only a bit sad that Bahcall who worked on the solar-neutrino models for years and was found right after all this effort, couldn't share the 2002 prize anymore.

I hate it, if Nobel prizes are somehow discredited, because one of the unique things with these prizes (at least the ones for the "hard sciences") is that they are almost always very well deserved and justified. There are very little examples coming to my mind, where one can argue about that. The only example that comes to my mind is the omission of Lise Meitner and Strassmann in the prize for nuclear fission, given to Otto Hahn alone in 1945. Of course, the prize was well deserved but should have been shared with the key players in the discovery.

Sometimes you read stupid comments on why the one or the other prize was not deserved for the work honored, even in some serious newspapers. Some years ago the Frankfurter Allgemeine Zeitung (FAZ) in Germany wrote something against Abrikosov's Nobel for the theory of superconductors (Abrikosov lattices), which argued that this is just "textbook physics". At the time, I wrote a letter to the editor pointing out how stupid this is, because they should have thought, how this discoveries could come into the textbooks in the first place. It's because of Abrikosov's work, and thus it's more an argument to justify to give him the prize than arguing against it!

 

[Orodruin]

The only example that comes to my mind is the omission of Lise Meitner and Strassmann in the prize for nuclear fission, given to Otto Hahn alone in 1945. Of course, the prize was well deserved but should have been shared with the key players in the discovery.

Some of our Italian colleagues would probably also mention the omission of Cabibbo from the prize shared by Kobayashi and Maskawa (receiving 1/4 each with the other half to Nambu). That motivation was very carefully worded to explicitly target the extension of the mixing matrix to three states with the realisation that one CP phase no longer can be removed, leading to CP violation.

 

[vanhees71]

True! I found this also not very just. Here the dilemma is the restriction to maximally 3 people to get the prize. This is also the problem with the big collaborations in experimental HEP. Another omission is DESY for the discovery of gluons. There you also need good politics pushing single people out of the collaborations. CERN was doing better than DESY with Rubbia who got the prize for the discovery of the W- and Z-bosons (and van der Meer for stochastic cooling). I think the gluons are worth a Nobel prize as the W- and Z-bosons...

 

[mfb]

This is also the problem with the big collaborations in experimental HEP.

It is possible to award the prize to organizations. It has never been done (apart from the Peace prize), but for experimental high-energy physics it would certainly be useful. There was a third spot for the Higgs discovery...

 

[vanhees71]

I thought the prizes for the "hard sciences" can only be awarded to individuals (up to three people)?

 

[snorkack]

Neutrino oscillations only show that two neutrinos have mass.

 

[mfb]

I thought the prizes for the "hard sciences" can only be awarded to individuals (up to three people)?

Up to three organizations (or a mix) would be possible as well. It has never been done for the "hard sciences", but it is possible.

 

[arivero]

Some of our Italian colleagues would probably also mention the omission of Cabibbo from the prize shared by Kobayashi and Maskawa (receiving 1/4 each with the other half to Nambu). That motivation was very carefully worded to explicitly target the extension of the mixing matrix to three states with the realisation that one CP phase no longer can be removed, leading to CP violation.

Not easy wording indeed. Any reference to prediction of a third generation also should met with other italian claimant, Zichichi. And I always wondered if there were other claimants too for Cabibbo angle... surely not, but who knois... On other hand it seems -I had never looked at it- that Cabibbo's idea immediately generated extensions to include CP, adding other phases and bigger groups, see eg Morales et al in 1965 http://cds.cern.ch/record/345977/files/CM-P00057116.pdf, and himself in 1964, http://www.sciencedirect.com/science/article/pii/0031916364911382 so it would seem that the real thing in KM is to have the right context in the quark model.