“Perspectives in Neutrino Physics” by Guido Altarelli
Here again to tell you about the concluding talk of this wonderful conference. As Guido stated, this was not a summary talk, but a collections of personal opinions, comments and insights of this “Titan” of physics.
The conference has been extremely interesting thanks to the numerous experimental talks with news and hints on different topics. Among the others, however, probably the most discussed one has been the “revival” of sterile neutrinos. Indeed in the past edition of this conference, the MiniBooNE results seemed to disagree with the LSND anomaly. After that something happened (more statistics, better analyses, new values of the reactor fluxes) and suddenly the hypothesis of the sterile neutrinos finds several supporters.
HOWEVER… How strong are the indications?
Let’s start discussing the MiniBooNE results. A strong indication of a new oscillation is in the peak of signal in the anti-ν_e appearance. BUT…: there are several points at low energy that have not been identified yet. This generates some doubts and therefore it is necessary to clarify this issue.
Furthermore, moving to the new measurements of the reactor fluxes, he systematic errors have not been discussed and they could be even larger than the estimated!!! Remember that these new values had a relevant impact on the results presented during the conference.
Taking in consideration the Gallium Anomaly, it should be remembered that it depends on the assumed cross section! Moreover, it does not really agree with the reactor angle: while the oscillation frequencies are very close to each other, the angle does not.
Not only terrestrial experiments open the possibility at sterile neutrinos, but they are allowed (and also supported) by Cosmology and the strong constrain come from BBN, N_s < 1.2 (@95% CL).
ON THE OTHER HAND… what does theory say? They are not necessary at all. Moreover, the related phenomenology makes the description of the lepton sector more complicated. But, maybe, they could be a remnant of some hidden sector and in this case it would be a great discovery!!
In this conference, we had the opportunity to listen to the results of two independent analyses, by Giunti and Schwetz. Both of them agree on the fact that 3+1 scheme is disfavoured (no CPV and tension between appearance and disappearance). The 3+2 scheme and the 3+1+NSI could be slightly better. In the 3+2 case, the would be two eV neutrinos.
The scenario arising from experiments and phenomenological fit is quite complicated and deserves a better analysis and further investigations. Carlo Rubbia presented a proposal for a new experiment at CERN, that would be “a dream experiment for sterile neutrinos” BUT… do we really need new experiments? Maybe no, or at least now they are badly needed. Indeed MiniBooNE will present new results in the summer.
Continuing discussing this topic, it has been opened the possibility that CPV could be responsible for some of the tensions, BUT… the difference between ν’s and anti-ν’s reported by MINOS could probably go away! On the contrary, it is not supported by SK, that gets equal neutrino and antineutrino mixing favored by the data.
By the way, discussing SK, it is impressive the precision that they reached in the last years (τ/B > 1.9×10^33)
=========> the 3ν standard scenario continues to be the reference framework.
In this scenario, there are still several open questions. In primes, the determination of the reactor angle: nowadays it seems that a positive value is suggested, but it depends on the treatment of the SBL reactor data. Thus we need to do some further progress in this direction.
André Rubbia presented the first T2K results at this conference and they appear extremely interesting and encouraging, but the T2K running was interrupted by the earthquake. “We hope it can be repaired fast and that the situation is Japan will improve very soon for all the population”.
However, in 3 years there should be a great improvement on the reactor angle
More questions, however, need attention:
_the absolute scale of neutrino mass
_shift from maximality of the atmospheric angle
_the sign of the atmospheric mass squared difference
_the CPV in the neutrino oscillations
_the Majorana/Dirac nature of neutrinos
A dedicated comment should be reserved to this last topic and to the 0ν2β-decay. A lot of effort is devoted to find a signal of 0ν2β (and a lot of it in the Italian territory) and a detection would be a proof of L non conservation.
This is of course linked to the very promising leptogenesis, that could give informations on the high energy parameters. While an equally important non conservation, that one of the leptonic flavour, is under investigation with very nice results at MEG. Also from this side, we expect interesting and decisive results very soon.
LFV would be a strong evidences of new physic and a complementary indication could come from Dark Matter searches at LHC
Up to now, there are important lessons on neutrino masses and mixing:
_neutrinos are massive (at least two)
_probably the masses are small because they are Majorana
_their masses are inversely proportional to the large scale of Lepton number violation
_L violation scale could be linked to the mass of the RH neutrinos, close to 10^(14-15) GeV, suggesting SUSY GUT scenarios
_decays of RH neutrinos with CP&L violation can produce B-L asymmetry and therefore can explain baryogenesis
_detecting 0ν2β would prove the Majorana nature of neutrinos and L non conservation.
BUT… neutrinos are not a significant component of DM
BUT… different classes of models are still possible
It is an interesting remark that the Cabibbo angle is a common parameter arising both in the quark and in the lepton sectors. Where? Well, the ratio among the solar and the atmospheric mass squared differences is defined as the parameter r∼1/30 and its square root is indeed very close to the Cabbibo angle:
for a hierarchical spectrum
This suggests the same hierarchy parameters for quarks, leptons and neutrinos. This is connected with the possible approaches, that have been implemented in the last 10 years:
-Tri-Bimaximal mixing, or similar mixing patterns, as a well-defined starting point
-Lepton-Quark complementarity as indication of a common origin among lepton and quark mixings.
There are two neutrino mixing patterns that well approximate the experimental values of the mixings: the first one is the very well-known Tri-Bimaximal scheme and the second is a new income, the Golden Ration pattern.
There has been a lot of activity in trying to reproduce these neutrino patterns in the context of flavour models, in which global discrete non-Abelian symmetries are added to the gauge group of the SM.
There is a vast literature dealing with the TB patter, but only few papers on the GR one. The common feature of models, dealing with these patterns, is that the corrections that usually arise are quite small, let’s say Cabibbo^2. The general result is that the reactor angle is given of this order of magnitude and therefore very small, below the future expected sensitivity.
The alternative approach, considering quark-lepton complementary, suggested instead that a very large value of the solar angle, even if in completely disagreement with the data, could be a reasonable starting point if the corrections are large, let’s say Cabibbo. This scenario corresponds to a third interesting mixing pattern called Bimaximal scheme. In the few models constructed with this pattern, the reactor angle arises very close to its present upper bound and therefore it will be tested very soon in the experiments.
A third possibility could be the correct description of nature: maybe all these “discrete” mixing patters are accidents and then anarchical approach, lopsided model, continuous flavour symmetries, could provide better descriptions.
In all this discussion, however, we cannot forget quarks. Indeed while neutrinos seem to prefer a discrete symmetry, quarks can be easily and elegantly described by a continuous symmetry. This is a non-trivial discussion, but any interesting answer or even any illuminating strategy has appeared.
_neutrino mixing angles are large except the reactor, that is small
_the experimental values are compatible with “discrete” schemes as Tri-Bimaximal, Golden Ratio or Bimaximal patterns.
_Maybe this points to discrete symmetries.
_In principle, there is no contradiction between large neutrino mixing angles and small quark ones
_Quarks, however, seem to not support discrete flavory symmetries
_Natural GUT’s describing quarks and leptons with “discrete” schemes are difficult to construct, in particulate for SO(10)
I conclude reporting the last slide of the talk and I add my personal acknowledgements to all the other organizers of the conference.