# Insights Neutrino masses and speed - Comments

1. Jul 21, 2015

### Staff: Mentor

2. Jul 22, 2015

### PWiz

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

Last edited: Jul 22, 2015
3. Jul 22, 2015

### Greg Bernhardt

Ditto, great work @mfb!

4. Jul 23, 2015

### vanhees71

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

5. Jul 23, 2015

### Orodruin

Staff Emeritus
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!

6. Jul 23, 2015

### Orodruin

Staff Emeritus
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.

7. Jul 24, 2015

### 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?

8. Jul 24, 2015

### Orodruin

Staff Emeritus
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 $\delta_{CP} = -\pi/2$.

9. Jul 24, 2015

### strangerep

Typo? Should that be "electron neutrino mass" ?

10. Jul 25, 2015

### Staff: Mentor

Oh sure. Fixed, thanks.

11. Oct 6, 2015

### Ygggdrasil

12. Oct 6, 2015

### ChrisVer

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.

13. Oct 6, 2015

### 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).

14. Oct 6, 2015

### ChrisVer

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

15. Oct 7, 2015

### 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.)

16. Oct 7, 2015

### 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 occure and you have transitions.
So the fact that we saw missing electron neutrinos from the sun was telling us neutrinos are massive :p

17. Oct 7, 2015

### Orodruin

Staff Emeritus
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.

18. Oct 7, 2015

### ShayanJ

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

19. Oct 7, 2015

### Orodruin

Staff Emeritus
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".

20. Oct 7, 2015

### ShayanJ

Would he share the prize if he was alive?