Perspectives for tests of neutrino mass generation at the GeV scale: Experimental reach versus theoretical predictions

Rasmus W. Rasmussen and Walter Winter

Phys. Rev. D 94, 073004 – Published 13 October 2016

Abstract

We discuss the parameter space reach of future experiments searching for heavy neutral leptons (HNLs). We consider the GeV seesaw model with three HNL generations and focus on two classes of models: generic assumptions (such as random mass matrices or the Casas-Ibarra parametrization) and flavor symmetry-generated models. We demonstrate that the generic approaches lead to comparable parameter space predictions, which tend to be at least partially within the reach of future experiments. On the other hand, specific flavor symmetry models yield more refined predictions, and some of these can be more clearly excluded. We also highlight the importance of measuring the flavor-dependent couplings of the HNLs as a model discriminator, and we clarify the impact of assumptions frequently used in the literature to show the parameter space reach for the active-sterile mixings.

Received 5 August 2016

DOI:https://doi.org/10.1103/PhysRevD.94.073004

Physics Subject Headings (PhySH)

Extensions of fermion sector Neutrino oscillations

Heavy neutrinos Majorana neutrinos Neutrinos Sterile neutrinos

Particles & Fields

Authors & Affiliations

Rasmus W. Rasmussen^* and Walter Winter^†

DESY, Platanenallee 6, 15738 Zeuthen, Germany

^*rasmus.westphal.rasmussen@desy.de
^†walter.winter@desy.de

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Vol. 94, Iss. 7 — 1 October 2016

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Images

Figure 1
Total active-sterile mixing predictions for the lightest (a) and heaviest (b) sterile neutrino for the Casas-Ibarra parametrization (red points) and random case (blue points), where one dot represents one model. “Assumption: None” means that no assumption has been imposed on the ratio of mixings with the individual flavors. As a consequence, no information on SHiP and DUNE sensitivities can be given based on present information (which would either require sensitivity to mixing with the tau flavor or direct sensitivity to the total mixing). We have taken the (optimistic) sensitivity bound expected from the FCC experiment [116], which is directly sensitive to the total mixing. For the discussion of the bounds, see the main text; there is no seesaw bound because it does not apply to three generations of sterile neutrinos at the GeV scale.
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Figure 2
Same as Fig. 1 with the lightest (a) and heaviest (b) sterile neutrino, where the experimental bounds are shown for the assumption $| U_{e I} |^{2} ∶ | U_{μ I} |^{2} ∶ | U_{τ I} |^{2} = 1 ∶ 16 ∶ 3.8$ [this is indicated by the statement “Assumption: Case 2” Eq. (9)]. We have taken the optimistic sensitivity bounds from FCC [116] and DUNE [117], whereas we have taken the case 2 [Eq. (9)] scenario as a sensitivity bound for the SHiP experiment [115].
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Figure 3
Figure similar to Fig. 2 for the predictions of the total active-sterile mixing of the lightest (a) and heaviest (b) sterile neutrino from different texture sets (color); see the figure legend. The assumption $| U_{e I} |^{2} ∶ | U_{μ I} |^{2} ∶ | U_{τ I} |^{2} = 1 ∶ 16 ∶ 3.8$ has been included for sensitivities and experimental bounds.
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Figure 4
Similar to Fig. 3 for texture 15 only where we focus on the complementary among the different experiments. The red (blue) points are the model predictions with the mixing $| U_{1} |^{2}$ (not) reachable by the DUNE experiment, as is obvious from subfigure (a). In subfigure (b), the corresponding mixing for the heaviest state is shown for the same models; some of the red points are within reach of other experiments, but others are not. This means that DUNE can exclude model predictions for the heaviest sterile neutrino which are out of reach by FCC and vice versa.
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Figure 5
The individual mixing elements $| U_{e I} |^{2}$ (a) and $| U_{μ I} |^{2}$ (b) for the lightest sterile neutrino for the Casas-Ibarra parametrization and random cases. The upper bound from direct search experiments is shown as well, whereas no lower bound exists for three generations of sterile neutrinos (since one mixing element can be very small if another is large to ensure the lifetime bound of the sterile neutrino relevant for BBN). The bounds have been translated from Refs. [115, 116, 117].
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Figure 6
Similar to Fig. 5 with the individual mixing elements $| U_{e 1} |^{2}$ (a) and $| U_{μ 1} |^{2}$ (b) for the lightest sterile neutrino, but showing the predictions for the different texture models.
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Physical Review D

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