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On the trails of new neutrino properties

Final Report Summary - NUTRAILS (On the trails of new neutrino properties)

We know that the Standard Model of particle physics can not be the ultimate theory of fundamental interactions. Finding a more complete theory remains the most challenging task for elementary particle physics. Massive neutrinos constitute one of the strongest experimental evidences of such incompleteness. It follows that we need to enlarge our understanding of neutrinos if we want to gain knowledge on the description of particle physics. This view has been the inspiring idea of our project.

Neutrino physics is now more vibrant than ever, as a renewed flow of key data has just begun during the past two years. First evidence (2012) that the hitherto unknown value of the third neutrino-mixing angle is sizable raises concrete hopes of observing CP violation in the lepton sector, triggering new ideas for model building. Moreover, the entire neutrino community is intrigued by several experimental anomalies that hint at unexpected neutrino properties. This animated scenery has provided the ideal stage for the NuTrails project.

We have focused our investigations on both standard and non-standard neutrino properties. Concerning the standard properties, we have concentrated our attention on two of the last unknown neutrino properties, i.e. CP violation (CPV) and the neutrino mass-hierarchy (NMH). Any indication on these two properties would be vital both for guiding model builders and for helping experimentalists in defining strategies of future searches.

Notably, in our project we have provided the first hint in favor of maximal leptonic CP violation, exploiting the global combination of world neutrino data. If confirmed by future analyses, such a preference would have profound consequences also for leptogenesis, a popular mechanism of production of the baryon asymmetry of the Universe. Our indication will be tested at future experiments. Concerning the NMH, during the Marie Curie fellowship we have had an active role in the PINGU project, a new in-fill array of the IceCube experiment, devoted to the NMH identification. With the status of “associate member” of the IceCube/PINGU collaboration, the grantee has coauthored the first PINGU feasibility study as well as the Letter of Intent (LoI). In the LoI he has given contributions to the executive summary and to the theoretical introduction on the NMH.

Concerning non-standard neutrino properties, we have investigated light sterile neutrinos. These putative particles are at the center of attention of the scientific community. Several anomalies seem to point towards sterile neutrinos. In particular, a controversial finding, the so-called reactor anomaly, a deficit observed by all short-baseline (SBL) reactor experiments has given new impetus to sterile neutrino investigations. In this context, we have given important contributions.

A robust upper bound on the mixing of the electron neutrino with sterile states independent of the (controversial) reactor flux has been established. Notably, our upper bound has been adopted as a benchmark by the Borexino collaboration, in the context of the SOX project, whose main aim is to detect/exclude non-zero values of Ue4. The grantee has given substantial help in writing the LoI of the second phase of the Borexino experiment, specifically the part concerning the SOX project.

For the first time in the literature, we have obtained constraints on ultra-light (sub-eV) sterile neutrinos by using the three theta-13 dedicated reactor experiments (Daya Bay, Double Chooz and RENO). Also this result has been very influential. In fact, the Daya-Bay collaboration has taken very seriously our idea. Very recently it has presented its own analysis of ultra-light sterile neutrinos. The other two collaborations Double Chooz and RENO are also working in the same direction.

A further topic investigated by the grantee has been the impact of light sterile neutrinos on running long-baseline (LBL) accelerator experiments. Such studies have no counterparts in the literature and are very innovative. The role of sterile neutrinos in T2K has been considered within the 3+1 scheme involving one extra sterile species. The impact on the reconstruction of the CP-phase delta has been shown to be dramatic. A complete degeneracy between the standard CP-phase and the new non-standard CP-phases has been clearly evidenced. In addition, it has been shown that the slight tension currently present between the T2K and the reactor determinations of theta-13 may be alleviated by the presence of light sterile neutrinos.

The impact of sterile neutrinos in two other LBL experiments, ICARUS and OPERA, has been considered. We have evidenced that the effective 2-flavor approximation that was used by the two collaborations neglects a new interference term and that the correct inclusion of such a term leads to a substantial weakening of the upper bounds quoted by the two experiments.

The socio-economical impact of our activity, albeit indirect, is very high. Influencing high-level decisions of experimental collaborations is indeed a very effective pathway to making an impact, given the extremely high cost of the present experiments. In particular, our idea to constrain ultra-light sterile neutrinos by using (already existing) reactor experiments, originally devoted only to the search for theta-13, allowed us (and subsequently the Daya Bay collaboration itself) to probe new physics beyond the standard model at zero cost. The same considerations are valid for our studies involving LBL experiments. We envisage that T2K, ICARUS and OPERA (and other LBL experiments) will be influenced by our studies as well.