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“Overview on Neutrino Phenomenology” by Eligio Lisi

March 15, 2011

Hi there,

first of all, welcome to the workshop in this marvellous city and building… and let me sympathize with all the participants that every morning come to Venice from the near cities, in particular from Padova… all of you are excused if you are a bit late (not too much) in the “early” morning…

I am posting on the talk “Phenomenology of neutrino physics” by Eligio Lisi. He gave an overview on the Standard 3-neutrino oscillations and beyond.


Following the Standard analysis, we have tree active oscillating neutrinos, whose frequencies are Δm^2 (atmospheric) and δm^2 (solar) and the three mixing angles are θ_23 (atmospheric), θ_12 (solar) and θ_13 (reactor). To fix the ideas the atmospheric angle is close to the maximal value, the solar one is large but not maximal, while the reactor angle is tiny and maybe vanishing.

Interesting new evaluation of actual reactor fluxes produces changes on the present picture: this allows extra ν_e “disappearance” in CHOOZ and KamLAND with consequent larger values for θ_12 and θ_23 (shift of 0.007 for both of them).

On θ_13, there have been several improvements in the last years:
@ NO-VE 2008, first hint for positive sin^2θ_13;
@ NEUTEL 2009, overall significance of about 2σ’s for a non-vanishing θ_13;
TODAY @NEUTEL 2011, update with the “new” reactor fluxes, sin^2θ_13∼0.021±0.009, to be compared with the old result of sin^2θ_13∼0.014±0.008.
On this topic, there will be a dedicated talk by Schwetz (tomorrow afternoon).

A further improvement from the last NEUTEL is that the tension between solar and KamLAND has turned into an agreement, but for more updates we have to wait for the current solar neutrino experiment and KamLAND results and for refinements of the solar models. By the way, no global fit result can substitute the direct search of non-vanishing reactor angle.

From MINOS and atmospheric data, the error on Δm^2 is now comparable to the value of δm^2 and therefore the usual approximation in LBL and ATM data analyses of δm^2=0 should be better justified or even avoided.

Considering the Standard analysis with SK-3 data, a non-maximal value for the atmospheric angle seems to be suggested: the effect of sign(sin^2θ_23-1/2) is comparable with statistical and systematical errors.

Global analyses are now not limited to 3ν oscillation parameter estimates. A relevant outcome concerns geo-neutrino rates from thorium and uranium decays. There is a 6-sigma evidence for the flux and sensitivity to the Th/U ratio.

Apart from the achieved knowledge, there is a lot of unknowns: the exact value of the reactor angle and the CP phase, the sign of Δm^2, the absolute neutrino mass scale, the (Majorana/Dirac) neutrino nature and the flavour origin of the neutrino mixings and masses.


Non oscillation topic: Neutrinoless double beta decay.

If the Klapdor et al. claim was confirmed by GERDA and CUORE, then there would be a tension with the cosmological bounds on the sum of the neutrino masses. Some possible solutions could be that either the Nuclear Matrix Elements are wrong or the Standard Cosmological model is wrong or finally some new physics is present and contributes, such as Heavy ν’s, KK modes, RH currents, SUSY.

Concentrating on the latter possibility of a new contribution to mββ, it would be difficult to identify the responsible new physics: for example considering 4 mechanisms and 4 nuclei, the current spread in Nuclear Matrix Elements prevents robust discriminations.

We need a better understanding of ν-related nuclear processes!


Sterile Neutrinos

The mismatch between theoretical and experimental determinations of reactor fluxes seems to indicate oscillations into sterile neutrinos. Before doing any claim of a new oscillation frequency we need to prove that fission beta-decay are under control and furthermore we should remember that the reactor fluxes are anchored to one beta-decay experiment, ILL. What about few percent deviations from the “anchor point”?

A second challenge is that we should consider the possibility of new degeneracies: considering one extra sterile neutrino, there could be a degeneracy among θ_13 and θ_14 in the solar+KamLAND data, but not in the SBL data…

Stay tuned for new posts…

(posted by Luca Merlo)

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One Comment leave one →
  1. Francesco Vissani permalink
    March 16, 2011 8:10 am

    Hi, some remarks on neutrinoless double beta decay.

    Even if the light Majorana neutrinos would contribute to this decay, they are just virtual particles, and thus are invisible to the detectors. In fact, all we see experimentally is the violation of the lepton number.

    For this reason, I agree that it is a legitimate attempt to postulate that some other mechanism could be contribute or even also or even dominate the neutrinoless double beta decay transition. E.g., Heavy ν’s, KK modes, RH currents, SUSY, mentioned in the talk.

    However, in this case it is not clear to me why one should narrow down the investigation of such theoretical possibilities to the neutrinoless double beta decay, encountering the limitations described in the talk.

    In my personal opinion, one should take the speculations seriously. One should complement neutrinoless double beta decay by direct search for these particle at colliders and indirect search for lepton flavor violating processes.

    Negative results would provide useful bounds; if positive results were instead obtained, not only we could give meaning to all existing data (as discussed in the talk) but would presumably have profound impact on the entire field of high energy physics.

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