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“Sterile Neutrino Fits” by Carlo Giunti

March 17, 2011

Hi everybody! Here again to post about sterile neutrinos…

To read about standard 3-ν oscillations just look at https://neutel11.wordpress.com/2011/03/15/overview-on-neutrino-phenomenology/ while here I am entering the most exotic topic of extra sterile neutrinos.

Sterile neutrinos have been theorized by Bruno Pontecorvo long time ago (as fairly remembered by Carlo Rubbia). Only more recently we thought that maybe he could be right and precisely after LSND data. LSND looks at anti-ν_μ -> anti-ν_e conversion and it could be understood in terms of a third oscillation frequency: Δm^2_LSND ≥ 0.2 eV^2 (that is extremely larger than the other two).

MiniBooNE experiment was expected to say something on this anomaly, working at larger distance and energy. There is an agreement between the two sets of data:

The scenario that arises at this point consists in the oscillation between the 3 active neutrinos and new states, where the mixings with these new states are very very small:

It is not the first time that we think at more than 3 neutrinos: the type I See-Saw mechanism indeed introduces several RH neutrinos. However, these states are extremely heavy (in general more than 10^10 GeV) and cannot be our new neutrinos.

What is defined as STERILE neutrino is a LIGHT ANTI-RH neutrino (notice that it is a LH state), with no standard model interactions. Active neutrinos can oscillate into sterile neutrinos and what should be looked for are 1) Disappearance of active neutrinos and 2) Indirect evidence through combined fit of data.

The subsequent question is how many sterile neutrinos are present?
We know from LEP that the Z decay into invisible tells information only on active neutrinos. On the other side, Cosmology can put constraints only on thermalized sterile neutrinos and therefore it depends on the type of interactions that were active at the time of the decoupling. From CMB and LSS there is a bound on the number of thermalized sterile neutrinos and on the sum of their masses:

At 95% CL, N_s=1.61+- 0.92 with a bound on the masses of m < 0.70 eV.

Assuming a scheme with 3 active neutrinos and only 1 sterile neutrino (3+1 scheme) it is possible to discuss SBL oscillation probabilities. It is interesting to note that in this scenario NO CP VIOLATION is allowed. This is interesting because of a tension between LSND+KARMEN+ MiniBooNE anti-ν_μ -> anti-ν_e data and MiniBooNE ν_μ -> ν_e data, suggesting CP Violation.

On the other hand, in schemes with 3 active neutrinos and either 2 sterile ones (3+2 scheme) or 1 sterile neutrino and non standard interactions (3+1+NSI), CPV is allowed (See the talk by Thomas Schwetz).

Furthermore, considering experiments on ν_e disappearance, ν_μ disappearance and ν_μ -> ν_e conversion, it is possible to put strong constraints on sin^2(2θ_eμ): the green line below!!

This underline a strong tension between anti-ν_μ -> anti-ν_e appearance and disappearance limits: that with the new reactor fluxes appears as

This tension, however is reduced when considering 3+2 or 3+1+NSI schemes.

An alternative is that maybe this is an indication that CPT is violated. To read about a brief exchange of ideas on it between Altarelli and Rubbia, just click on https://neutel11.wordpress.com/2011/03/16/titans-cross-swords-on-cpt-violation/

This CPT violation has already been proposed also during the presentation of MINOS results and by the yesterday paper by CDF collaboration in arXiv:1103.2782 [hep-ex].

At this point, one can think at considering ONLY the data from antineutrino oscillations (because without CPT these should be independent from neutrino oscillation data) and perform a fit of the data on the 3+1 scheme. The results are interesting, predicting a large SBL anti-ν_μ disappearance at 0.1<Δm^2<1eV.

An other complementary indication to the anomaly in LSND could come from the Gallium Anomaly (SBL ν_e disappearance) that shows a tension with the Reactor data (anti-ν_e disappearance). A combined fit is in the picture:

In this scenario there are two very welcome predictions for β-decay and 0ν2β-decay:
mβ∼0.14-1.63 eV
mββ∼0.02-0.35 eV

A work in progress is the interesting combined analysis on LSND and MiniBooNE + Gallium and Reactor Anomalies, thus stay tuned (to arXiv)…

(posted by Luca Merlo)

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